KS3 Chemistry Basics Study Pack

Key Knowledge: What Chemistry Studies and Why It Matters

Chemistry is the study of substances, their properties, and how they change. A substance is a type of matter with particular properties. For example, water, oxygen, iron, sugar, salt, copper, and carbon dioxide are all substances. Chemists investigate what substances are like, how they behave, how they can be separated, and how they react with other substances.

Chemistry matters because substances are everywhere. The air you breathe contains gases. Food changes when it is cooked. Iron railings can rust. Soap and detergents help remove dirt. Medicines, fuels, plastics, paints, batteries, fertilisers, and cleaning products all involve chemistry. In school chemistry, you learn how to describe changes carefully and how to use evidence instead of guesses.

A material is what an object is made from. A window may be made from glass, a saucepan from metal, and a jumper from wool or polyester. A mixture contains two or more substances that are not chemically joined together. Air is a mixture of gases, salt water is a mixture of salt and water, and soil is a mixture of many different solids and liquids.

In chemistry, a reaction is a change in which new substances are made. The substances at the start are called reactants. The new substances made are called products. For example, when magnesium burns in oxygen, magnesium and oxygen are the reactants, and magnesium oxide is the product.

Not every change is a chemical reaction. Some changes only alter the form, state, shape, or position of a substance. These are physical changes. Melting ice, freezing water, cutting paper, crushing chalk, and dissolving sugar in water are physical changes because no new substance is made.

Good chemistry depends on careful observations. An observation is something you notice or measure directly, such as colour, temperature, mass, bubbles, or whether a solid forms. An inference is an explanation based on observations. For example, “bubbles formed” is an observation. “A gas was produced by a chemical reaction” is an inference, and it needs evidence.

Key Vocabulary for Chemistry Basics

Word	Meaning	Example
Chemistry	The study of substances, their properties, and how they change.	Studying rusting, burning, dissolving, and changes of state.
Substance	A type of matter with particular properties.	Water, oxygen, copper, salt.
Material	What an object is made from.	Glass, wood, plastic, steel.
Mixture	Two or more substances together but not chemically joined.	Salt water, air, soil.
Physical change	A change where no new substance is made.	Ice melting; sugar dissolving.
Chemical change	A change where new substances are made.	Burning paper; iron rusting.
Reaction	A chemical change where reactants form products.	Vinegar reacting with bicarbonate of soda.
Reactant	A starting substance in a chemical reaction.	Magnesium in a reaction with oxygen.
Product	A new substance made in a chemical reaction.	Magnesium oxide after magnesium burns.
State of matter	The physical form of a substance: solid, liquid, or gas.	Ice, liquid water, steam/water vapour.
Melting	A solid changing into a liquid.	Ice melting into water.
Freezing	A liquid changing into a solid.	Water freezing into ice.
Evaporation	A liquid changing into a gas at the surface, below boiling point.	A puddle drying.
Boiling	A liquid changing into a gas throughout the liquid at its boiling point.	Water boiling in a kettle.
Condensation	A gas cooling and changing into a liquid.	Water droplets on a cold mirror.
Sublimation	A solid changing directly into a gas.	Solid carbon dioxide changing into gas.
Hazard	Something that could cause harm.	A hot beaker or irritating chemical.
Risk	The chance of harm happening and how serious it could be.	Burning skin if hot glass is touched.
Control measure	An action that reduces risk.	Wearing goggles; using tongs.
Independent variable	The variable deliberately changed in an investigation.	Temperature of water in an evaporation test.
Dependent variable	The variable measured as the outcome.	Mass of water lost after 10 minutes.
Control variable	A variable kept the same to make a test fair.	Starting volume of water.
Accuracy	How close a measurement is to the true value.	Reading a measuring cylinder correctly.
Precision	How small the scale divisions are or how close repeated readings are.	Measuring to the nearest 1 cm3.
Repeatability	Getting similar results when the same person repeats the method.	Three similar evaporation times.
Reliability	Trustworthiness of results, often improved by repeats and checking anomalies.	A mean calculated from consistent repeats.
Anomaly	A result that does not fit the pattern.	One repeat is much higher than the others.

Physical Changes Explained with Particle Examples

A physical change changes the form, shape, state, or position of a substance but does not make a new substance. The particles are still the same type of particles after the change. They may be arranged differently, move differently, or be more spread out, but they have not become a different substance.

Examples of physical changes include:

• Ice melting into liquid water.
• Water freezing into ice.
• Water evaporating from salt water.
• Sugar dissolving in tea.
• Chalk being crushed into powder.
• Paper being cut into smaller pieces.
• Wax melting near a flame.
• Water vapour condensing on a cold window.

Reversibility is a useful clue. Many physical changes are easy to reverse. Melted water can be frozen again. Water vapour can condense back into liquid water. Salt can be separated from salt water by evaporation. However, reversibility is not a perfect test. Cutting paper is a physical change even though it is not easy to put the paper back exactly as it was. Some chemical changes may also be partly reversed using advanced methods, but that is beyond KS3.

In a change of state, the particles of the substance do not become new particles. For example, when ice melts, water particles in a solid arrangement gain energy and move more freely as a liquid. The substance is still water. When water evaporates, water particles leave the surface of the liquid and spread out as a gas. The particles are still water particles.

In a closed system, mass is conserved during a change of state. This means the total mass stays the same because no particles are lost. If 50 g of ice melts in a sealed container, there will be 50 g of liquid water. If some water evaporates from an open beaker, the mass measured in the beaker decreases because water particles have escaped into the air, not because the particles have disappeared.

Physical Changes versus Chemical Changes

Feature	Physical change	Chemical change
Definition	A change in form, state, shape, or position where no new substance is made.	A change where new substances are made.
Does a new substance form?	No. The particles are still the same substance.	Yes. Reactants form products.
Typical clues	Change of state, dissolving, crushing, cutting, changing shape.	Gas made, colour change, temperature change, light, smell, precipitate, difficult to reverse.
Examples	Melting ice, freezing water, dissolving sugar, crushing chalk, cutting paper.	Burning paper, rusting iron, cooking an egg, magnesium burning, vinegar reacting with bicarbonate of soda.
Usually easy to reverse?	Often, but not always.	Usually difficult to reverse, but reversibility alone is not a perfect test.

Chemical Changes Explained with Reactants and Products

A chemical change happens when new substances are made. Another name for a chemical change is a chemical reaction. The substances you start with are called reactants. The substances made are called products.

A simple word equation shows the reactants on the left and the products on the right:

reactants  ->  products
substances at the start  ->  new substances made

For example:

magnesium + oxygen -> magnesium oxide

Magnesium and oxygen are reactants. Magnesium oxide is the product. Magnesium is a shiny grey metal. Magnesium oxide is a white powder. The product has different properties from the reactants, which shows that a chemical change has taken place.

Another example is:

vinegar + bicarbonate of soda -> carbon dioxide + other products

The bubbles show that carbon dioxide gas is being produced. At KS3, you do not need to write balanced symbol equations for this topic. A clear word equation is enough if it correctly shows reactants and products.

Common chemical changes include burning, rusting, cooking, and some reactions between acids and carbonates. Burning is a chemical change because substances react with oxygen and new products form. Rusting is a chemical change because iron reacts with oxygen and water to form rust. Cooking an egg is a chemical change because the substances in the egg change into new forms that are difficult to reverse.

Evidence for Chemical Reactions

Scientists use evidence to decide whether a chemical reaction has happened. No single clue is perfect on its own, but several clues together can make a strong case.

Observation	What it might suggest	Why it is not always proof by itself
Bubbles or fizzing	A gas may have been produced in a reaction.	Boiling also makes bubbles, but that is a physical change.
Colour change	A new substance may have formed.	Mixing dyes or indicators can change colour without a full reaction.
Temperature increase	Energy may have been released in a reaction.	Heating from a flame also raises temperature.
Temperature decrease	Energy may have been taken in during a change.	Dissolving can sometimes cool a mixture without making a new substance.
Light produced	A reaction such as burning may be happening.	A lamp produces light using electrical energy.
New smell	New substances may have formed.	Smell is not always safe to test and can be misleading.
Solid forms from two liquids	A precipitate may have formed.	More evidence is needed to identify the substance.
Change is hard to reverse	A chemical reaction may have occurred.	Cutting paper is hard to reverse but is still physical.

When judging a change, ask:

1. What did I observe before, during, and after the change?
2. Was a new substance made?
3. Are there signs such as gas, colour change, temperature change, light, smell, or a new solid?
4. Could the evidence also be explained by a physical change?
5. What extra evidence would make the conclusion stronger?

Worked Example 1: Classifying Changes

Scenario A: Wax Melting

1. Observable change: Solid wax becomes liquid wax when heated.
2. Has a new substance formed? No. It is still wax.
3. Classification: Physical change.
4. Explanation: The wax particles gain energy and move more freely, but they remain wax particles. If cooled, the liquid wax can solidify again.

Scenario B: Paper Burning

1. Observable change: Paper gives off heat and light, smoke forms, and ash is left.
2. Has a new substance formed? Yes. Ash, smoke, and gases are new substances.
3. Classification: Chemical change.
4. Explanation: The paper reacts with oxygen during burning. The products have different properties from the original paper.

Scenario C: Sugar Dissolving

1. Observable change: Sugar crystals disappear into water and the liquid tastes sweet if it is food-safe. In a laboratory, chemicals must not be tasted.
2. Has a new substance formed? No. Sugar particles spread through the water.
3. Classification: Physical change.
4. Explanation: Dissolving sugar is a physical change because the sugar is still present and can be recovered by evaporating the water.

Scenario D: Iron Rusting

1. Observable change: Shiny grey iron slowly becomes coated with reddish-brown rust.
2. Has a new substance formed? Yes. Rust is a new substance.
3. Classification: Chemical change.
4. Explanation: Iron reacts with oxygen and water. The colour change and new material show a chemical reaction.

Scenario E: Water Condensing

1. Observable change: Droplets of liquid water appear on a cold surface.
2. Has a new substance formed? No. Water vapour has changed into liquid water.
3. Classification: Physical change.
4. Explanation: The water particles lose energy and move closer together, but they are still water particles.

States of Matter and Changes of State

Most substances can exist as solids, liquids, or gases depending on temperature and pressure. At KS3, you usually explain states of matter using the particle model. The particle model says that substances are made from tiny particles. The arrangement and movement of the particles explain many properties of solids, liquids, and gases.

Particle Arrangement Diagrams

Solid
particles close together in a regular arrangement

O O O O
O O O O
O O O O
O O O O

Liquid
particles close together but irregular

O O  O
 O O O
O  O O
  O O

Gas
particles far apart and moving freely

O              O

       O

             O
   O

States of Matter Table

State	Particle arrangement	Particle movement	Shape	Volume	Everyday example
Solid	Particles are close together in a fixed arrangement.	Particles vibrate in fixed positions.	Fixed shape.	Fixed volume.	Ice, a metal spoon, chalk.
Liquid	Particles are close together but irregular.	Particles move around each other.	Takes the shape of the container.	Fixed volume.	Water, cooking oil, milk.
Gas	Particles are far apart and randomly arranged.	Particles move quickly in all directions.	Fills the container.	No fixed volume; expands to fill space.	Air, oxygen, carbon dioxide, water vapour.

Particles in a solid do move. They vibrate in fixed positions. They do not flow past each other like particles in a liquid, but they are not completely still.

Gases have mass. Gas particles are spread out and often invisible, but they are still matter. A balloon has a greater mass when it is filled with air than when it is empty, although the difference may be small.

Change of State Summary

solid --melting--> liquid --evaporation/boiling--> gas
solid <--freezing-- liquid <--condensation--------- gas

Changes of State Table

Process	Start state	End state	Heating or cooling?	Example
Melting	Solid	Liquid	Heating	Ice melting into water.
Freezing	Liquid	Solid	Cooling	Water freezing into ice.
Evaporation	Liquid	Gas	Heating from surroundings; can happen below boiling point	Wet clothes drying.
Boiling	Liquid	Gas	Heating to boiling point	Water boiling in a kettle.
Condensation	Gas	Liquid	Cooling	Droplets on a cold window.
Sublimation	Solid	Gas	Heating	Solid carbon dioxide changing directly to gas.

During melting and boiling, the temperature may stay steady for a while even though heating continues. The energy is being used to change the arrangement of particles, not to raise the temperature. You do not need GCSE-level detail about latent heat for this topic.

Evaporation, Boiling, and Condensation Compared

Evaporation is when a liquid changes into a gas at the surface of the liquid. It can happen below boiling point. This is why a puddle can dry on a mild day and wet clothes can dry without being heated to 100 degrees Celsius. Faster-moving particles near the surface can escape into the air.

Boiling is when a liquid changes into a gas throughout the liquid at its boiling point. Bubbles form inside the liquid and rise to the surface. For pure water at normal air pressure, the boiling point is about 100 degrees Celsius.

Condensation is when a gas cools and changes into a liquid. When warm water vapour touches a cold bathroom mirror, it loses energy and forms tiny liquid droplets. The droplets did not come through the mirror; they came from water vapour in the air.

Evaporation from the surface

        water particles escaping into air
             O        O
        O
   -------------------------
   O O O O O O O O O O O O   surface of liquid
    O O O O O O O O O O O
   -------------------------

Examples:

• A puddle dries because water evaporates from the surface.
• Clothes dry because water evaporates from the fabric.
• Droplets appear on a cold window because water vapour condenses.
• The water cycle includes evaporation from seas, lakes, and rivers, followed by condensation in clouds.

Worked Example 2: Interpreting Heating Data

A student heats water and records the temperature every minute.

Time in minutes	Temperature in degrees Celsius
0	20
1	35
2	51
3	66
4	82
5	96
6	100
7	100
8	100
9	100

Step 1: Identify the pattern. The temperature increases from 20 degrees Celsius to 100 degrees Celsius between 0 and 6 minutes.

Step 2: Identify the plateau. From 6 to 9 minutes, the temperature stays at 100 degrees Celsius.

Step 3: Explain what is happening. The water is boiling. Energy is still being transferred, but the temperature stays steady while liquid water changes into gas.

Step 4: State the boiling point from the data. The boiling point is 100 degrees Celsius.

Graph sketch:

Temperature (degrees Celsius)
100 |                         _________
 90 |                    ____/
 80 |               ____/
 70 |          ____/
 60 |       __/
 50 |    __/
 40 |  _/
 30 | /
 20 |/
    +--------------------------------
      0  1  2  3  4  5  6  7  8  9
                 Time (minutes)

Laboratory Safety Rules and Hazard Symbols

Laboratory safety rules protect eyes, skin, lungs, hair, clothing, and everyone in the room. Practical work must be supervised by a teacher. You should follow instructions exactly, ask if you are unsure, and report accidents or spills immediately.

Important safety rules:

• Wear safety goggles when instructed, especially when heating or using chemicals.
• Tie back long hair and keep loose clothing away from flames.
• Stand up when heating so you can move away safely if needed.
• Point test tubes away from yourself and others.
• Never taste chemicals in a laboratory.
• Only smell a chemical by wafting, and only if your teacher says it is safe.
• Keep the bench clear and bags under benches.
• Use tongs or a test tube holder for hot equipment.
• Put hot glassware on a heatproof mat and allow it to cool.
• Do not run, push, or distract others.
• Turn off gas taps and electrical equipment when not in use.
• Tell the teacher about broken glass, spills, burns, or splashes.

A hazard is something that could cause harm. A risk is the chance that harm will happen and how serious it could be. A control measure reduces the risk.

Hazard Symbols

Symbol name	Meaning	Example risk	Sensible control measure
Corrosive	Can damage skin, eyes, or materials.	Acid splashing into eyes.	Wear goggles; use small amounts; rinse spills as instructed.
Irritant or harmful	Can irritate skin, eyes, or lungs, or cause harm.	Irritating vapour or skin contact.	Avoid contact; use in a ventilated area; follow teacher instructions.
Flammable	Catches fire easily.	Liquid or vapour ignites near a flame.	Keep away from flames; use small amounts; replace lids.
Toxic	Can poison if swallowed, inhaled, or absorbed.	Breathing in poisonous vapour.	Do not inhale; use teacher-approved method; wash hands after use.
Explosive	May explode under certain conditions.	Pressure or heat causing sudden explosion.	Keep away from heat or impact; teacher handles it.
Oxidising	Can make fires burn more strongly.	Fire spreads faster.	Keep away from flammable materials.
Gas under pressure	Gas container may burst if heated.	Cylinder or canister bursts.	Keep away from heat; secure containers.
Environmental hazard	Can harm living things or water systems.	Chemical enters drains or ponds.	Dispose of waste as instructed.

The hazard symbol is not the same as the risk. A concentrated acid may be hazardous because it can burn skin. The risk depends on how it is used. A tiny amount used carefully with goggles has a lower risk than a large open bottle used carelessly.

Common Laboratory Equipment and Correct Uses

Choosing the right equipment makes practical work safer and more accurate.

Equipment name	Use	What it measures or holds	Safety note or common mistake
Beaker	Holding, mixing, or rough measuring of liquids.	Holds liquids; approximate volumes only.	Do not use when an accurate volume is needed.
Conical flask	Swirling liquids without spilling; collecting filtrate.	Holds liquids.	Do not heat strongly unless instructed.
Test tube	Holding or heating small amounts of liquid.	Small samples.	Point away from people when heating.
Boiling tube	Heating larger small samples than a test tube.	Small amounts of liquid or solid.	Use a holder and goggles.
Measuring cylinder	Measuring liquid volume.	Measures volume in cm3.	Read the bottom of the meniscus at eye level.
Thermometer	Measuring temperature.	Measures temperature in degrees Celsius.	Do not use as a stirring rod.
Bunsen burner	Heating substances.	Provides flame.	Use on a heatproof mat; tie hair back.
Tripod	Supports apparatus above a Bunsen burner.	Holds gauze and beaker/basin.	Make sure it is stable.
Gauze	Spreads heat and supports a beaker or evaporating basin.	Support, not a measuring tool.	It becomes very hot.
Heatproof mat	Protects the bench from heat.	Holds hot apparatus.	Hot items may still burn you.
Evaporating basin	Evaporating solvent to leave solute behind.	Holds solution during evaporation.	Do not overheat to dryness unless instructed.
Filter funnel	Holds filter paper during filtration.	Directs liquid into a flask.	Support it properly to avoid spills.
Filter paper	Separates insoluble solid from liquid.	Traps residue.	Fold correctly and do not overfill.
Spatula	Transfers small amounts of solid.	Small solid samples.	Do not use the same spatula in different chemicals without cleaning.
Stirring rod	Stirs mixtures and guides pouring.	Not a measuring tool.	Do not use a thermometer for stirring.
Tongs	Holds hot or hazardous solids/equipment.	Grip tool.	Check grip before lifting hot objects.
Safety goggles	Protect eyes.	Personal protective equipment.	Wear properly over eyes, not on forehead.
Balance	Measures mass.	Measures mass in grams.	Use a container or weighing boat; zero the balance first.

Worked Example 3: Choosing Equipment

Task: Measure 50 cm3 of water.

Best equipment: Measuring cylinder.

Reason: A measuring cylinder has a scale designed for measuring volume more accurately than a beaker. A beaker may show rough volumes, but it is not the best tool for accurate measurement.

Task: Heat a small amount of liquid safely.

Suitable equipment: Boiling tube or test tube, test tube holder, safety goggles, Bunsen burner, heatproof mat.

Reason: A boiling tube or test tube holds a small amount of liquid. A holder keeps hands away from heat. Goggles protect eyes. The tube must be pointed away from people and heated gently.

Required Practical Diagrams

Safe Heating Setup

          safety goggles worn by student
                 [GOGGLES]

             beaker with liquid
              ____________
             /            \
            /______________\
                 gauze
          =====================
              tripod legs
             /     |     \
            /      |      \
           /       |       \
              Bunsen burner
                  /\
                 /  \
                /____\
             heatproof mat
          ___________________

Evaporation Setup

          evaporating basin
          ________________
         /  solution      \
        /__________________\
                gauze
        =====================
             tripod
            /  |  \
           /   |   \
              Bunsen burner
                 /\
                /__\
          heatproof mat
        ___________________

Filtration Setup

            filter funnel
              \      /
               \    /
            filter paper
              \  /\  /
               \/  \/
            residue trapped
                 ||
                 || filtrate passes through
                 \/
            ______________
           /              \
          / conical flask  \
         /__________________\
             filtrate

Observation Skills: What to Record and How to Use Evidence

A careful chemist records observations before, during, and after a change. This helps you compare what has changed and decide whether the evidence suggests a physical change or chemical reaction.

You can safely observe:

• Colour.
• State: solid, liquid, or gas.
• Texture or appearance, such as powdery, shiny, cloudy, or crystalline.
• Whether bubbles form.
• Whether a new solid forms.
• Temperature using a thermometer.
• Mass using a balance.
• Time using a stopwatch.
• pH indicator colour if an indicator is used.
• Whether a substance dissolves, melts, boils, evaporates, or condenses.

You must not taste chemicals in a laboratory. You should not smell chemicals directly. If smelling is needed and the teacher says it is safe, use wafting: gently move a small amount of vapour towards your nose with your hand, keeping the container away from your face.

Observations and explanations are different:

Observation	Inference or explanation
The liquid changed from blue to green.	A new substance may have formed, or an indicator may have changed colour.
Bubbles appeared.	A gas may have been produced, or the liquid may be boiling.
The temperature rose from 20 degrees Celsius to 32 degrees Celsius.	Energy may have been released by a reaction.
A white solid appeared when two clear liquids were mixed.	A precipitate may have formed.

A strong conclusion uses evidence. Instead of writing “It reacted because it looked different”, write “The mixture fizzed, the temperature increased by 8 degrees Celsius, and a new smell was noticed by wafting. This suggests a chemical reaction because a gas and new substances may have formed.”

Working Scientifically in a Simple Chemistry Practical

In a fair test, only the independent variable is changed. The dependent variable is measured. Control variables are kept the same.

Investigation question	Independent variable	Dependent variable	Control variables
How does temperature affect evaporation rate?	Temperature of surroundings or water.	Time taken to evaporate, or mass of water lost.	Starting volume, container, surface area, type of liquid, measurement interval, room airflow.
How does surface area affect evaporation?	Surface area of container.	Mass of water lost after a fixed time.	Starting volume, temperature, type of liquid, time, room conditions.
How does stirring affect dissolving time?	Whether the liquid is stirred, or stirring speed.	Time taken to dissolve.	Mass of solute, volume of water, temperature, particle size.
Does heating affect how fast salt water evaporates?	Heating level or temperature.	Time taken to obtain salt crystals.	Volume of salt water, concentration, basin size, distance from flame.

Accuracy means getting close to the true value. Use suitable measuring equipment, read scales correctly, and avoid parallax error by reading at eye level. Precision means using measurements with smaller scale divisions or repeated readings that are close together. A measuring cylinder marked every 1 cm3 is more precise than a beaker marked every 50 cm3.

Repeatability means the same method gives similar results when repeated by the same person using the same equipment. Reliability means results can be trusted. Repeats, means, careful control variables, and checking anomalies all improve reliability.

An anomaly is a result that does not fit the pattern. You should not ignore an anomaly without thinking. Check whether there was a mistake, repeat the measurement if possible, and explain how the method could be improved.

Writing a Simple Method

A good method should:

• Be clear enough for another student to follow.
• State the equipment used.
• Say what will be changed, measured, and kept the same.
• Include sensible volumes, times, and temperatures.
• Include repeats.
• Include safety precautions.

Example method for an evaporation investigation:

1. Measure 20 cm3 of water using a measuring cylinder.
2. Pour the water into an evaporating basin.
3. Place the basin in a safe location at the chosen temperature, as instructed by the teacher.
4. Measure the mass of the basin and water at the start.
5. Measure the mass again every 5 minutes for 20 minutes.
6. Repeat the investigation three times for each temperature.
7. Keep the starting volume, basin size, and measurement times the same.
8. Wear goggles and follow teacher instructions around hot equipment.

Worked Example 4: Completing a Simple Risk Assessment

Hazard	Possible harm	Control measure
Hot equipment	Burns to skin.	Use tongs or a holder; place hot items on a heatproof mat; allow to cool.
Eye-irritating chemical	Splash could irritate eyes.	Wear goggles; use small quantities; pour carefully.
Broken glass	Cuts to skin.	Keep glassware away from bench edges; tell the teacher if glass breaks.
Spill on floor	Slip or skin contact.	Report spills immediately; keep bags out of walkways; clean as instructed.

Real-World Examples at Home, School, and Outdoors

Chemistry basics appear in everyday life:

• Ice melts on a path during the day and refreezes overnight. These are physical changes because the substance is still water.
• Wet clothes dry because water evaporates from the fabric into the air.
• A cold bathroom mirror becomes covered in droplets because water vapour condenses on the cold surface.
• In a candle, wax melting is a physical change, but wax burning is a chemical change. Burning produces new substances such as gases and soot.
• Vinegar reacting with bicarbonate of soda produces bubbles of carbon dioxide gas. This is evidence of a chemical reaction.
• Iron gates, bikes, and tools can rust when exposed to oxygen and water. Rusting is a chemical change.
• Cooking an egg is a chemical change because new substances form and the change is difficult to reverse.
• Salt can be separated from salt water by evaporation. The water evaporates and salt crystals are left behind.
• Product labels on bleach, aerosols, and dishwasher tablets use hazard symbols to warn about possible harm. These products should only be used as directed by adults and should not be handled in school experiments unless a teacher has planned the activity safely.

Common Misconceptions and Corrections

Misconception	Correction
Melting is a chemical change.	Melting is a physical change. It changes state but does not make a new substance.
Freezing, evaporating, boiling, and condensing make new substances.	These are changes of state. The particles are still the same substance.
All changes that are hard to reverse are chemical changes.	Reversibility is a clue, not a rule. Cutting paper is physical but hard to reverse exactly.
All bubbles mean a chemical reaction has happened.	Bubbles can be gas made in a reaction, but boiling also makes bubbles during a physical change.
Dissolving is always a chemical reaction.	Dissolving salt or sugar in water is usually a physical change because the substances are still present.
A colour change always proves a chemical reaction.	Colour change is evidence, but more evidence may be needed.
Gases have no mass.	Gas particles have mass even when spread out and invisible.
Particles in a solid do not move at all.	Solid particles vibrate in fixed positions.
Evaporation and boiling are the same.	Evaporation happens at the surface and can happen below boiling point. Boiling happens throughout the liquid at its boiling point.
The hazard symbol is the same as the risk.	A hazard could cause harm. Risk depends on how likely and serious the harm is.
A beaker is the best tool for accurate volume measurement.	A measuring cylinder is better for measuring liquid volume accurately.
Observations and explanations are the same.	An observation is what is seen or measured. An explanation is an inference based on evidence.

Diagram Interpretation Tasks

Task 1: Match the Particle Diagrams

Diagram A
O O O O
O O O O
O O O O

Diagram B
O       O
     O
          O
  O

Diagram C
O O  O
 O O O
O  O

Questions:

1. Which diagram shows a solid?
2. Which diagram shows a liquid?
3. Which diagram shows a gas?
4. Explain how the particle arrangement helped you decide.
5. Which diagram shows particles that vibrate in fixed positions?

Task 2: Heating Setup

Look at the safe heating setup diagram earlier in the pack.

Questions:

1. Name the equipment that protects the bench.
2. Name the equipment that supports the beaker.
3. Why should goggles be worn?
4. Why should the student stand while heating?
5. Give one risk if the beaker is heated without gauze.

Task 3: Filtration

Questions:

1. What is the purpose of the filter paper?
2. What is the residue?
3. What is the filtrate?
4. Why is a conical flask suitable for collecting the filtrate?
5. Is filtration usually a physical separation or a chemical reaction? Explain.

Data and Practical Skills Tasks

Task A: Heating Water Results

Time in minutes	Temperature in degrees Celsius
0	18
1	32
2	47
3	62
4	78
5	91
6	99
7	100
8	100
9	100

Questions:

1. Describe the temperature pattern from 0 to 7 minutes.
2. What is the boiling point shown by the data?
3. Quote evidence from the table that boiling is happening.
4. Why does the temperature stay steady from 7 to 9 minutes?
5. Give one improvement to make the results more reliable.

Task B: Cooling and Condensation Scenario

A student places a cold metal tray above a beaker of warm water. After a few minutes, droplets form on the underside of the tray.

Questions:

1. Where did the water droplets come from?
2. Which change of state happened at the surface of the warm water?
3. Which change of state happened on the cold tray?
4. Explain why cooling caused droplets to form.
5. Is this a physical or chemical change? Explain.

Task C: Classifying Changes from Observations

Change	Observation	Physical or chemical?	Justification
Ice left in a cup	Solid becomes liquid
Vinegar added to bicarbonate of soda	Fizzing and bubbles
Sugar stirred into water	Crystals disappear
Magnesium heated in oxygen	Bright white light and white powder forms
Blue and yellow food colourings mixed	Green mixture forms

Questions:

1. Complete the physical or chemical column.
2. Complete the justification column.
3. Which example shows that colour change alone does not always prove a chemical reaction?
4. Which examples give stronger evidence for a chemical reaction? Explain why.

Task D: Evaporation Investigation with Repeats

A class investigates whether surface area affects evaporation. They place 20 cm3 of water in three different containers and measure the mass of water lost after 30 minutes. The room temperature is kept the same.

Container	Surface area	Repeat 1 mass lost in g	Repeat 2 mass lost in g	Repeat 3 mass lost in g	Mean mass lost in g
Narrow beaker	Small	2	2	3
Medium dish	Medium	5	6	5
Wide tray	Large	9	15	10

Questions:

1. Calculate the mean for each container.
2. Identify the anomaly.
3. Describe the pattern between surface area and evaporation.
4. State the independent variable.
5. State the dependent variable.
6. Give two control variables.
7. Suggest one improvement to make the results more reliable.

Task E: Safety Evaluation

Read the practical description:

A student heats a test tube of liquid while sitting down. The test tube points towards another student. The student is not wearing goggles. A bag is in the walkway. The student notices a small spill but does not tell the teacher.

Questions:

1. Identify at least three risks.
2. Write a safer alternative for each risky behaviour.
3. Which parts of the body are protected by goggles?
4. Why should spills be reported immediately?
5. Explain the difference between a hazard and a risk using one example from the description.

Task F: Equipment Selection

Choose the best equipment for each job and explain your choice.

Job	Best equipment	Reason
Measure 50 cm3 of water accurately
Heat a small amount of liquid
Separate sand from water
Evaporate water from salt water
Measure the temperature of a reacting mixture
Measure 5 g of salt

Worked Example 5: Variables in an Evaporation Investigation

Question: How does temperature affect the rate of evaporation of water?

Independent variable: Temperature of the surroundings or water.

Dependent variable: Time taken to evaporate, or mass of water lost after a fixed time.

Control variables:

• Starting volume of water.
• Type of liquid.
• Container size and surface area.
• Room airflow.
• Measurement interval.
• Starting temperature if comparing room conditions.

Fair test explanation: Only the temperature should be changed. If one container had more water or a larger surface area, it would not be clear whether temperature caused the difference.

Repeatability and reliability: Repeat each temperature at least three times. Calculate a mean. Check for anomalies and repeat any suspicious result if possible.

Exam-Style Questions

Multiple-Choice Questions

1. Which statement best describes a physical change? A. A change where atoms disappear. B. A change where no new substance is made. C. A change that always produces heat. D. A change that always cannot be reversed.

2. Which example is most likely a chemical change? A. Ice melting. B. Water evaporating. C. Paper burning. D. Sugar dissolving in water.

3. In a word equation, reactants are written: A. On the right of the arrow. B. Above the arrow. C. On the left of the arrow. D. Only underneath the products.

4. Which observation is evidence that a gas may have formed in a reaction? A. The container is made of glass. B. Bubbles appear when two substances are mixed. C. The balance is switched on. D. The liquid is measured in cm3.

5. Why is bubbling alone not proof of a chemical reaction? A. Bubbles never contain gas. B. Boiling can make bubbles during a physical change. C. Bubbles only happen in solids. D. Bubbles always mean condensation.

6. Which statement about solids is correct? A. The particles do not move at all. B. The particles vibrate in fixed positions. C. The particles are far apart and move freely. D. The particles fill any container.

7. Evaporation differs from boiling because evaporation: A. Happens only at the surface and can happen below boiling point. B. Happens throughout the liquid at the boiling point. C. Always makes a new substance. D. Only happens to solids.

8. Which equipment should be used to measure 50 cm3 of water most accurately? A. Beaker. B. Measuring cylinder. C. Evaporating basin. D. Filter funnel.

9. A hazard is: A. The result at the end of an experiment. B. Something that could cause harm. C. The value you measure. D. A repeated reading.

10. In an investigation, the dependent variable is: A. The variable deliberately changed. B. The variable measured as the outcome. C. A variable that is ignored. D. The equipment used for heating.

Fill-in-the-Blank Questions

Use these words: reactants, products, evaporation, condensation, physical, chemical, hazard, risk, control, anomaly.

1. A change where no new substance is made is a __________ change.
2. A change where new substances are made is a __________ change.
3. The substances at the start of a reaction are called __________.
4. The substances made in a reaction are called __________.
5. __________ is when a liquid changes into a gas at the surface.
6. __________ is when a gas cools and changes into a liquid.
7. A __________ is something that could cause harm.
8. A __________ is the chance of harm happening and how serious it could be.
9. Variables kept the same are __________ variables.
10. A result that does not fit the pattern is an __________.

Short-Answer Questions

1. Explain why melting ice is a physical change.
2. Explain why cooking an egg is a chemical change.
3. Give two pieces of evidence that may suggest a chemical reaction has happened.
4. Why should you not taste chemicals in a laboratory?
5. What is the difference between an observation and an inference?
6. Why is a measuring cylinder better than a beaker for measuring volume?
7. Explain how condensation forms on a cold window.
8. Write a word equation for magnesium reacting with oxygen.
9. Describe one control measure for hot equipment.
10. Explain why repeat readings can improve reliability.

Table-Completion Question

Complete the table.

Process	Start state	End state	New substance made?
Melting	Solid
Freezing		Solid
Evaporation	Liquid
Condensation		Liquid
Burning	Often solid, liquid, or gas fuel	New products

Practical Planning Question

A student wants to investigate whether warmer water evaporates faster than cooler water.

1. State the independent variable.
2. State the dependent variable.
3. Give three control variables.
4. Describe a simple method.
5. Name two pieces of measuring equipment.
6. Describe one hazard and one control measure.
7. Explain why repeats are useful.

Longer 6-8 Mark Question

A student heats salt water in an evaporating basin to obtain salt crystals.

Describe the method, name the equipment, identify hazards and control measures, and explain which changes are physical. Include ideas about evaporation, safety, observations, and evidence.

Model Answers

Multiple-Choice Answers

1. B. A physical change does not make a new substance.
2. C. Burning paper makes new substances such as ash and gases.
3. C. Reactants are written on the left of the arrow.
4. B. Bubbles may show that a gas is being produced.
5. B. Boiling makes bubbles during a physical change.
6. B. Solid particles vibrate in fixed positions.
7. A. Evaporation happens at the surface and can happen below boiling point.
8. B. A measuring cylinder is designed to measure liquid volume more accurately.
9. B. A hazard is something that could cause harm.
10. B. The dependent variable is measured as the outcome.

Fill-in-the-Blank Answers

1. physical
2. chemical
3. reactants
4. products
5. evaporation
6. condensation
7. hazard
8. risk
9. control
10. anomaly

Short-Answer Model Answers

1. Melting ice is a physical change because solid water changes into liquid water but no new substance is made. The particles are still water particles.

2. Cooking an egg is a chemical change because new substances form in the egg. The cooked egg has different properties and the change is difficult to reverse.

3. Evidence for a chemical reaction may include gas production, colour change, temperature change, light, smell, or a new solid forming. No single clue is perfect by itself.

4. Chemicals should not be tasted because they may be harmful, toxic, corrosive, or contaminated. Laboratory safety rules protect the mouth, throat, and body from harm.

5. An observation is what is seen or measured, such as “bubbles formed”. An inference is an explanation based on evidence, such as “a gas was produced in a reaction”.

6. A measuring cylinder is better because it has a scale designed for measuring liquid volume accurately. A beaker is mainly for holding or mixing liquids and gives only rough volumes.

7. Condensation forms on a cold window when water vapour in the air touches the cold glass, loses energy, and changes into liquid water droplets.

8. magnesium + oxygen -> magnesium oxide

9. A control measure for hot equipment is to use tongs or a test tube holder, place hot items on a heatproof mat, and allow them to cool before touching.

10. Repeat readings help identify anomalies and make the results more reliable. If repeats are similar, the result is more trustworthy.

Table-Completion Model Answer

Process	Start state	End state	New substance made?
Melting	Solid	Liquid	No
Freezing	Liquid	Solid	No
Evaporation	Liquid	Gas	No
Condensation	Gas	Liquid	No
Burning	Often solid, liquid, or gas fuel	New products	Yes

Practical Planning Model Answer

1. Independent variable: Temperature of the water or surroundings.
2. Dependent variable: Time taken to evaporate, or mass of water lost after a fixed time.
3. Control variables: Starting volume of water, container size, surface area, type of liquid, room airflow, and measurement interval.
4. Method: Measure the same volume of water into identical containers. Place each container at a different teacher-approved temperature. Measure the starting mass. After a fixed time, measure the final mass. Repeat each temperature three times and calculate a mean.
5. Measuring equipment: Measuring cylinder, balance, thermometer, stopwatch.
6. Hazard and control: Hot water or hot equipment could burn skin. Use goggles, tongs or heatproof gloves if instructed, a heatproof mat, and teacher supervision.
7. Repeats are useful because they help spot anomalies and make the mean more reliable.

Longer 6-8 Mark Model Answer

To obtain salt crystals from salt water, measure a suitable volume of salt water using a measuring cylinder and pour it into an evaporating basin. Place the basin on gauze supported by a tripod above a Bunsen burner on a heatproof mat. Wear goggles and heat gently under teacher supervision. The water evaporates from the solution, leaving salt behind. If crystals start to appear, stop heating as instructed and allow the basin to cool safely.

The equipment includes a measuring cylinder, evaporating basin, tripod, gauze, Bunsen burner, heatproof mat, and safety goggles. Hot equipment is a hazard because it can burn skin, so tongs should be used and the basin should cool before being touched. The flame is a hazard, so long hair should be tied back and loose clothing kept away. Salt water may spit if overheated, so goggles protect the eyes and heating should be gentle.

The changes are physical because no new substance is made. The water changes state from liquid to gas during evaporation. The salt remains as salt and can form crystals when the water is removed. Observations might include steam or water vapour leaving the solution, the volume decreasing, and solid salt crystals appearing. These observations show separation by evaporation, not a chemical reaction.

Model Answers for Data and Practical Skills Tasks

Task A Answers

1. The temperature increases from 18 degrees Celsius to 100 degrees Celsius between 0 and 7 minutes.
2. The boiling point shown is 100 degrees Celsius.
3. The temperature stays at 100 degrees Celsius from 7 to 9 minutes, which is a plateau.
4. The temperature stays steady because energy is being used to change liquid water into gas during boiling.
5. An improvement would be to repeat the experiment, record temperature more often, stir gently if instructed, or use the same thermometer correctly each time.

Task B Answers

1. The droplets came from water vapour that evaporated from the warm water and then condensed.
2. Evaporation happened at the surface of the warm water.
3. Condensation happened on the cold tray.
4. Cooling caused water vapour particles to lose energy and move closer together as liquid droplets.
5. It is a physical change because the substance is still water and no new substance is made.

Task C Answers

Change	Observation	Physical or chemical?	Justification
Ice left in a cup	Solid becomes liquid	Physical	Melting changes state but does not make a new substance.
Vinegar added to bicarbonate of soda	Fizzing and bubbles	Chemical	Bubbles suggest carbon dioxide gas is produced as a new substance.
Sugar stirred into water	Crystals disappear	Physical	Sugar dissolves and spreads through the water; no new substance is made.
Magnesium heated in oxygen	Bright white light and white powder forms	Chemical	Magnesium oxide forms as a new product.
Blue and yellow food colourings mixed	Green mixture forms	Physical	The colours mix, but this alone does not prove a new substance formed.

The food colouring example shows that colour change alone does not always prove a reaction. The vinegar and bicarbonate reaction and the magnesium reaction give stronger evidence because gas, light, and a new solid product are involved.

Task D Answers

Means:

• Narrow beaker: (2 + 2 + 3) / 3 = 7 / 3 = about 2.3 g.
• Medium dish: (5 + 6 + 5) / 3 = 16 / 3 = about 5.3 g.
• Wide tray: repeat 2, 15 g, is likely an anomaly. Including it gives (9 + 15 + 10) / 3 = 34 / 3 = about 11.3 g. If the anomaly is excluded after checking, the mean of 9 g and 10 g is 9.5 g.

The anomaly is 15 g for the wide tray because it is much higher than the other repeats. The pattern is that larger surface area increases evaporation, shown by greater mass lost. The independent variable is surface area. The dependent variable is mass of water lost after 30 minutes. Control variables include starting volume, type of liquid, room temperature, time, and container material. An improvement would be to repeat the wide tray result, use more repeats, and make sure containers are kept in the same conditions.

Task E Answers

Risks include:

• Heating while sitting down makes it harder to move away safely.
• Pointing the test tube at another student could cause hot liquid to splash onto them.
• Not wearing goggles risks eye injury.
• A bag in the walkway is a trip hazard.
• Not reporting a spill could lead to slipping or unsafe chemical contact.

Safer alternatives:

• Stand while heating.
• Point test tubes away from people.
• Wear goggles properly.
• Keep bags under benches.
• Report spills to the teacher immediately.

Goggles protect the eyes. Spills should be reported because they may cause slips, skin contact, contamination, or damage to surfaces. A hot test tube is a hazard because it could burn skin; the risk is higher if it is pointed at someone or held carelessly.

Task F Answers

Job	Best equipment	Reason
Measure 50 cm3 of water accurately	Measuring cylinder	It measures volume more accurately than a beaker.
Heat a small amount of liquid	Test tube or boiling tube, holder, Bunsen burner, goggles	It is suitable for small samples and can be heated safely with care.
Separate sand from water	Filter funnel and filter paper	Insoluble sand is trapped as residue while water passes through as filtrate.
Evaporate water from salt water	Evaporating basin, tripod, gauze, Bunsen burner, heatproof mat	The water evaporates and salt remains.
Measure the temperature of a reacting mixture	Thermometer	It measures temperature in degrees Celsius.
Measure 5 g of salt	Balance and weighing boat	A balance measures mass in grams.

Revision Checklist

Use this checklist to review the topic.

• I can define chemistry as the study of substances, their properties, and how they change.
• I can explain the difference between a substance, material, and mixture.
• I can define physical change and give examples.
• I can define chemical change and give examples.
• I can explain that reactants are starting substances and products are new substances made.
• I can write a simple word equation with reactants on the left and products on the right.
• I can explain why melting, freezing, evaporation, boiling, and condensation are physical changes.
• I can compare solids, liquids, and gases using particle arrangement and movement.
• I can explain evaporation as a surface process that can happen below boiling point.
• I can explain boiling as a change that happens throughout a liquid at its boiling point.
• I can explain condensation as gas cooling and changing into liquid.
• I can use evidence such as bubbles, colour change, temperature change, light, smell, and precipitate formation carefully.
• I can explain why one observation alone may not prove a chemical reaction.
• I can distinguish observations from inferences.
• I can describe safe behaviour around chemicals, flames, heating, glassware, electrical equipment, and spills.
• I can explain the difference between a hazard, a risk, and a control measure.
• I can interpret common school laboratory hazard symbols.
• I can choose suitable equipment for measuring, heating, filtering, evaporating, observing, and weighing.
• I can identify independent, dependent, and control variables in simple chemistry investigations.
• I can explain fair testing, accuracy, precision, repeatability, reliability, anomalies, and improvements.
• I can calculate a simple mean from repeat results.
• I can interpret heating data and identify a boiling plateau.
• I can evaluate a practical method for safety and fairness.
• I can explain real-world examples such as rusting, cooking, drying clothes, condensation, and salt separation.

Final Quick Review

Chemistry is about substances and how they change. Physical changes do not make new substances, even if the substance changes state, shape, or position. Chemical changes make new substances called products from starting substances called reactants. Particle ideas help explain solids, liquids, gases, and changes of state. Laboratory work must be safe, supervised, and based on careful observations. Good scientific conclusions use evidence, consider alternative explanations, and explain how reliable the results are.