Chemical Reactions

Study revision notes for Chemical Reactions

Chemical Reactions Study Pack

1. Introduction / Essential Question

Essential Question

How can we tell when a chemical reaction has happened, and how can evidence help us explain what happened to matter and energy?

Introduction / Hook

Think about a bike chain turning rusty, a candle burning, a cake rising in the oven, or an antacid tablet fizzing in water. These are all connected by one big idea: matter can change into new substances. In science, a change that makes new substances is called a chemical reaction.

Chemical reactions are happening around you all the time. They help your body release energy from food, allow plants to make sugar, make fireworks colorful, clean polluted water, cook eggs, power batteries, and recycle materials in nature.

In this study pack, you will investigate:

how chemical reactions are different from physical changes
how scientists use evidence to decide whether a reaction occurred
what happens to atoms during a chemical reaction
why mass is conserved, even when a gas forms
how energy can be absorbed or released
how variables can affect the speed or results of a reaction
how chemical reactions matter in everyday life, engineering, medicine, food, and the environment

As you read, keep asking:

What do I observe?
What evidence supports my explanation?
Could there be another explanation?
How could a scientist test this?

2. Key Vocabulary / Definitions

Required Science Vocabulary

Term	Student-Friendly Definition	Example
Hypothesis	A testable explanation or prediction based on observations.	“If the water is warmer, then the tablet will react faster.”
Variable	A factor that can change in an investigation.	Temperature, amount of vinegar, type of metal.
Evidence	Observations or data used to support a scientific idea.	Bubbles formed, temperature changed, mass data.
System	The part of the universe being studied.	A sealed bag containing baking soda and vinegar.
Energy	The ability to cause change or do work.	Heat released by burning fuel.
Matter	Anything that has mass and takes up space.	Air, water, sugar, iron, carbon dioxide.

Topic-Specific Vocabulary

Term	Definition	Why It Matters
Chemical reaction	A process in which atoms are rearranged to form new substances.	Explains changes such as burning, rusting, and cooking.
Chemical change	A change that produces one or more new substances.	Different from melting, cutting, or dissolving.
Physical change	A change in size, shape, state, or appearance that does not make a new substance.	Ice melting is physical because it is still water.
Reactant	A starting substance in a chemical reaction.	Vinegar and baking soda are reactants.
Product	A new substance formed by a chemical reaction.	Carbon dioxide gas is a product when vinegar reacts with baking soda.
Atom	The smallest basic unit of an element that keeps that element’s identity.	Oxygen atoms are part of water and carbon dioxide.
Molecule	Two or more atoms bonded together.	Water is a molecule made of hydrogen and oxygen atoms.
Element	A pure substance made of only one kind of atom.	Oxygen, carbon, iron, copper.
Compound	A substance made of atoms of two or more different elements chemically bonded.	Water, carbon dioxide, table salt.
Chemical bond	A force that holds atoms together in molecules or compounds.	Bonds hold hydrogen and oxygen together in water.
Conservation of mass	Matter is not created or destroyed in a chemical reaction; atoms are rearranged.	Total mass before and after a reaction stays the same in a closed system.
Closed system	A system where matter cannot enter or leave.	A sealed plastic bag during a reaction.
Open system	A system where matter can enter or leave.	An open cup where gas can escape.
Precipitate	A solid that forms when two liquids react.	A cloudy solid may appear when certain solutions are mixed.
Gas production	Formation of a gas during a reaction.	Bubbles or fizzing can show a gas is forming.
Temperature change	Warming or cooling that may show energy is released or absorbed.	Some hand warmers release heat.
Endothermic reaction	A reaction that absorbs energy from its surroundings.	Some cold packs become cold when chemicals mix.
Exothermic reaction	A reaction that releases energy to its surroundings.	Burning wood releases heat and light.
Combustion	A reaction with oxygen that releases energy, often as heat and light.	Burning natural gas on a stove.
Oxidation	A reaction involving oxygen or electron transfer; in middle school, often seen as rusting.	Iron reacting with oxygen and water to form rust.
Reaction rate	How fast reactants turn into products.	A crushed tablet may react faster than a whole tablet.
Catalyst	A substance that speeds up a reaction without being used up.	Enzymes in your body act like catalysts.
Coefficient	A number used in a chemical equation to show how many particles or molecules are involved.	In 2H₂O, the 2 means two water molecules.
Chemical equation	A model that uses symbols and formulas to show reactants and products.	Hydrogen + oxygen → water.

3. Core Science Concepts

3.1 What Is a Chemical Reaction?

A chemical reaction happens when atoms in substances are rearranged to make new substances. The starting substances are called reactants. The substances formed are called products.

General pattern:

Reactants → Products

Example:

Vinegar + baking soda → carbon dioxide gas + other products

The important idea is that atoms are not disappearing. They are being rearranged. The same atoms that were present before the reaction are still present after the reaction, but they are connected in new ways.

3.2 Chemical Change vs. Physical Change

Not every change is a chemical reaction. Some changes are only physical.

A physical change changes the appearance, size, shape, or state of matter, but it does not create a new substance.

Examples of physical changes:

ice melting into liquid water
tearing paper
dissolving sugar in water
crushing a can
cutting vegetables

A chemical change creates new substances with new properties.

Examples of chemical changes:

wood burning and forming ash, smoke, carbon dioxide, and water vapor
iron rusting
milk souring
eggs cooking
fireworks exploding

Comparison Grid

Feature	Physical Change	Chemical Change
New substance formed?	No	Yes
Atoms rearranged into new substances?	No	Yes
Often reversible?	Sometimes	Often difficult to reverse
Example	Water freezing	Wood burning
Evidence	Change in state, size, or shape	Gas, color change, temperature change, precipitate, odor, light

Thinking question: A piece of paper is cut into tiny pieces. Is that chemical or physical? What evidence supports your answer?

3.3 Evidence of Chemical Reactions

Scientists do not decide that a chemical reaction happened just because something “looks different.” They look for evidence. One clue alone may not be enough, so scientists often collect several types of evidence.

Common evidence includes:

Gas production: bubbles, fizzing, or pressure increase
Temperature change: warming or cooling without outside heating or cooling
Color change: a new color appears that is not just mixing
Light production: light is released, such as in glow sticks or fireworks
Formation of a precipitate: a solid forms when liquids are mixed
New odor: a new smell appears because a new substance forms
Change in properties: the product has different properties from the reactants

Important caution: Some clues can also happen during physical changes. For example, boiling water makes bubbles, but the bubbles are water vapor, not a new substance. That is why evidence must be interpreted carefully.

3.4 The Particle View: What Happens to Atoms?

During a chemical reaction, chemical bonds between atoms break and new bonds form. The atoms are rearranged into products.

The atoms themselves are conserved. This means the same kinds and numbers of atoms are present before and after the reaction.

scientificDiagram: Atoms Rearranging

Before reaction:

Reactants:

  A-B       C-D

Atoms present: A, B, C, D

During reaction:

Bonds break and atoms rearrange:

  A   B   C   D

After reaction:

Products:

  A-D       C-B

Atoms present: A, B, C, D

The atoms are the same, but the combinations are different.

3.5 Conservation of Mass

The law of conservation of mass says that matter is not created or destroyed in a chemical reaction. The total mass of the reactants equals the total mass of the products, as long as the system is closed.

If a reaction happens in an open container, a gas product might escape. If you only measure what remains in the container, the mass may seem to decrease. But the missing mass is not destroyed. It left the system as gas.

Closed System Example

If baking soda and vinegar react inside a sealed bag:

carbon dioxide gas forms
the bag inflates
no gas escapes
the total mass stays the same

Open System Example

If baking soda and vinegar react in an open cup:

carbon dioxide gas forms
bubbles rise and escape into the air
the mass of the cup and contents may decrease
the missing mass is in the air as gas

3.6 Energy in Chemical Reactions

Chemical reactions involve energy because chemical bonds store energy. When bonds break and new bonds form, energy may be absorbed or released.

An exothermic reaction releases energy to the surroundings. The surroundings may get warmer, or light may be produced.

Examples:

burning wood
many hand warmers
some glow sticks

An endothermic reaction absorbs energy from the surroundings. The surroundings may get cooler.

Examples:

some instant cold packs
some reactions between baking soda and citric acid
photosynthesis, which uses light energy

3.7 Reaction Rate

The reaction rate tells how fast a chemical reaction happens. Some reactions are very fast, like a firework exploding. Others are slow, like iron rusting.

Factors that can affect reaction rate include:

Temperature: Higher temperature often makes particles move faster and collide more often.
Surface area: Smaller pieces have more exposed surface area, which can speed up reactions.
Concentration: More particles in the same space can lead to more collisions.
Stirring or mixing: Helps reactants contact each other.
Catalysts: Speed up reactions without being used up.

Example: A crushed antacid tablet reacts faster in water than a whole tablet because more surface area touches the water.

3.8 Chemical Equations as Models

A chemical equation is a model of a reaction. It shows reactants on the left and products on the right.

Example in words:

Hydrogen + oxygen → water

Example using formulas:

2H₂ + O₂ → 2H₂O

This equation means:

hydrogen and oxygen are reactants
water is the product
the same atoms are present before and after
there are 4 hydrogen atoms and 2 oxygen atoms on each side

Middle school students do not need to memorize many equations, but it is useful to understand that equations show matter being rearranged.

3.9 Systems, Variables, and Evidence

When scientists study chemical reactions, they define the system. The system is the part being studied.

Example system:

a sealed bag containing vinegar and baking soda

Surroundings:

the classroom air, table, students, and room

Scientists also control variables.

Types of variables:

Independent variable: the factor changed on purpose
Dependent variable: the factor measured as the result
Controlled variables: factors kept the same

Example investigation:

Question: How does water temperature affect the reaction time of an antacid tablet?

Independent variable: water temperature
Dependent variable: time for tablet to stop fizzing
Controlled variables: same tablet brand, same amount of water, same cup size

4. Examples, Case Studies, and Real-World Applications

4.1 Cooking Food

Cooking often causes chemical reactions. When an egg is heated, proteins change shape and connect in new ways. The clear liquid egg white becomes white and solid. This is a chemical change because new structures and properties form.

Questions to consider:

What evidence shows the egg changed?
Is the change easy to reverse?
How is heating related to energy?

4.2 Rusting Metal

Rusting is a slow chemical reaction involving iron, oxygen, and water. The product is rust, which has different properties from iron.

Iron is strong and gray. Rust is reddish-brown, flaky, and weaker. Engineers try to prevent rust by painting metal, coating it, using stainless steel, or keeping it dry.

STEM connection:

Bridges, cars, bicycles, and ships must be protected from corrosion.
Engineers choose materials based on chemical properties and the environment.

4.3 Burning Fuel

Combustion reactions release energy. When fuels such as natural gas, gasoline, or wood burn, they react with oxygen and produce new substances such as carbon dioxide and water vapor.

Combustion can be useful because it provides energy, but it can also affect air quality and climate. Scientists and engineers work on cleaner energy sources, better engines, and ways to reduce pollution.

4.4 Batteries

Batteries use chemical reactions to produce electrical energy. Inside a battery, chemical reactions cause charged particles to move through a circuit. This can power a flashlight, phone, calculator, or remote control.

Inquiry question:

How could you test which battery lasts longer under the same conditions?

4.5 Medicine and the Human Body

Your body depends on chemical reactions. Digestion breaks large food molecules into smaller molecules. Cells use chemical reactions to release energy from food. Enzymes act as catalysts that help reactions happen at body temperature.

Examples:

saliva begins breaking down starch
stomach acid helps digest food
cells use glucose and oxygen to release energy

4.6 Environmental Cleanup

Chemical reactions can help clean water and soil. For example, some water treatment systems use reactions to remove harmful substances or kill microbes. Scientists must test carefully because adding chemicals can also create unwanted products.

Scenario:

A town has water with too much dissolved iron. Engineers test different treatment methods that cause iron compounds to form solids that can be filtered out.

Questions:

What evidence would show the treatment worked?
What data should engineers collect?
How could they make sure the treated water is safe?

4.7 Food Packaging and Spoilage

Food spoils because chemical reactions and microbial processes change the food. Packaging can slow these changes by limiting oxygen, light, moisture, or temperature changes.

Examples:

potato chips are sealed to reduce oxygen exposure
milk is refrigerated to slow reactions and microbial growth
some packages include oxygen absorbers

5. Tables and Data

dataTable: Evidence From Several Changes

Change Observed	Possible Type of Change	Evidence	Is It Definitely Chemical?
Ice melts on a plate	Physical	State changes from solid to liquid	No, it is still water
Baking soda and vinegar fizz	Chemical	Gas forms, bubbles appear	Likely chemical
Sugar dissolves in tea	Physical	Sugar particles spread through liquid	No, sugar is still sugar
Iron nail turns rusty	Chemical	New reddish-brown substance forms	Yes
Water boils	Physical	Gas bubbles form from water vapor	No, boiling is a state change
Two clear liquids form a cloudy solid	Chemical	Precipitate forms	Likely chemical
Wood burns	Chemical	Heat, light, smoke, ash, gas products	Yes

dataTable: Antacid Tablet Reaction Time

A student tests how water temperature affects the time for an antacid tablet to stop fizzing. The same tablet type and water amount are used each time.

Trial	Water Temperature (°C)	Time Until Fizzing Stops (s)
1	10	94
2	20	71
3	30	49
4	40	34
5	50	25

What patterns do you see?

As temperature increases, reaction time decreases.
A shorter reaction time means a faster reaction rate.
The data supports the idea that warmer water speeds up this reaction.

graph: Reaction Time vs. Temperature

Time
(s)
100 | *
 90 |
 80 |
 70 |      *
 60 |
 50 |            *
 40 |                  *
 30 |                        *
 20 |
 10 |
  0 +--------------------------------
      10     20     30     40     50
            Water Temperature (°C)

Interpretation:

The graph slopes downward.
This means the tablet reacts faster at higher temperatures.
A scientist would repeat trials to check reliability.

dataTable: Open vs. Closed System

A class reacts baking soda and vinegar in two setups.

Setup	Starting Mass (g)	Ending Mass (g)	Observation
Open cup	115.0	111.8	Bubbles formed and gas escaped
Sealed bag	115.0	115.0	Bag inflated but stayed sealed

Analysis:

In the open cup, gas left the system.
In the sealed bag, gas stayed inside the system.
The sealed bag better shows conservation of mass.

comparisonGrid: Reaction Rate Factors

Factor	How It Can Affect Reaction Rate	Example
Temperature	Warmer particles move faster and collide more often	Tablet fizzes faster in warm water
Surface area	More exposed surface allows more contact	Powder reacts faster than a large chunk
Concentration	More particles in the same volume can increase collisions	Stronger vinegar may react faster
Stirring	Brings reactants together	Stirring sugar and yeast mixture
Catalyst	Lowers the energy needed for reaction	Enzymes help digestion

6. Text / ASCII Diagrams and Visual Aids

flowDiagram: From Reactants to Products

Reactants
   |
   | particles collide
   v
Bonds break
   |
   | atoms rearrange
   v
New bonds form
   |
   v
Products with new properties

experimentSetup: Baking Soda and Vinegar in a Closed System

Before mixing:

        sealed plastic bag
      ______________________
     /                      \
    |   small cup vinegar    |
    |                        |
    |   baking soda in corner|
     \______________________/

After mixing:

        inflated sealed bag
      __________________________
     /                          \
    |  CO2 gas fills the bag     |
    |  liquid products remain    |
     \__________________________/

Observation focus:

Does gas form?
Does the bag inflate?
Does mass change if the bag remains sealed?

scientificDiagram: Open System vs. Closed System

OPEN SYSTEM

   gas escapes
      ↑ ↑ ↑
   __________
  | vinegar  |
  | + baking |
  | soda     |
  |__________|

Measured mass may decrease.

CLOSED SYSTEM

  ___________________
 / gas trapped inside\
| vinegar + baking    |
| soda react          |
 \___________________/

Measured mass stays the same.

infographic: Evidence of Chemical Reaction

CHEMICAL REACTION CLUES

[Gas forms]       bubbles, fizzing, pressure
[Heat/light]      energy released
[Cooling]         energy absorbed
[Color change]    new substance may form
[Precipitate]     solid forms from liquids
[New odor]        new gas or substance may form

Important: A clue is not proof by itself.
Use several pieces of evidence.

scenarioCard: Mystery Reaction

Scenario Card: The Mystery Cup

A student mixes two clear liquids.
Observations:
- The mixture turns cloudy.
- A white solid settles at the bottom.
- The cup feels slightly warmer.

Question:
What evidence suggests a chemical reaction occurred?

Possible evidence:
- A precipitate formed.
- Temperature changed.
- New substances may have formed.

Particle Model for Conservation of Atoms

Before:

H-H     O=O     H-H

Atoms before:
H H H H O O

After:

H-O-H       H-O-H

Atoms after:
H H H H O O

The atoms are conserved. They are arranged differently.

7. Interactive Thinking Tasks

Task 1: Notice and Wonder

Read the observations below.

A student places a shiny iron nail in a damp paper towel. After several days, reddish-brown flakes appear on the nail.

Write:

one thing you notice
one thing you wonder
one possible explanation
one piece of evidence you would collect next

Task 2: Chemical or Physical?

Sort each example into “chemical change,” “physical change,” or “need more evidence.”

butter melting in a pan
toast turning brown
glass breaking
a glow stick giving off light
salt dissolving in water
a fruit rotting
water vapor condensing on a cold window
a metal spoon bending
fireworks producing light, sound, and smoke

Explain your reasoning for at least three examples.

Task 3: Design a Fair Test

Question: Does crushing an antacid tablet affect how fast it reacts in water?

Plan a fair test:

Independent variable:
Dependent variable:
Controlled variables:
Data table headings:
Safety steps:
What evidence would support your conclusion?

Task 4: Claim-Evidence-Reasoning

Claim: A chemical reaction happened when two liquids were mixed.

Evidence:

The mixture became cloudy.
A yellow solid formed.
The temperature increased from 22°C to 28°C.

Reasoning:

Write 3-5 sentences explaining how the evidence supports the claim.

Task 5: Graph Interpretation

Use the antacid tablet graph from Section 5.

Answer:

What is the independent variable?
What is the dependent variable?
What pattern do you see?
What prediction would you make for 60°C water?
Why should scientists repeat trials?

8. Common Misconceptions

Misconception 1: “Bubbles always mean a chemical reaction.”

Correct thinking: Bubbles can show a gas is forming, but not always a new gas from a chemical reaction. Boiling water makes bubbles of water vapor, which is still water. Scientists ask what gas formed and whether a new substance was produced.

Misconception 2: “If mass decreases, matter disappeared.”

Correct thinking: Matter does not disappear in a chemical reaction. In an open system, gas may escape. In a closed system, the total mass stays the same.

Misconception 3: “All color changes prove chemical reactions.”

Correct thinking: A color change can be evidence, but it is not proof by itself. Mixing food coloring into water changes color but does not create a new substance. A color change plus other evidence may support a chemical reaction.

Misconception 4: “Chemical reactions destroy atoms.”

Correct thinking: Chemical reactions rearrange atoms. Atoms are conserved. Bonds break and form, but the atoms are still there.

Misconception 5: “Physical changes are not important.”

Correct thinking: Physical changes matter too. Dissolving, melting, freezing, and breaking can affect how substances behave. Some physical changes can make chemical reactions faster by increasing surface area.

Misconception 6: “Reactants and products have the same properties.”

Correct thinking: Products usually have different properties from reactants. Sodium metal and chlorine gas are dangerous by themselves, but together they form sodium chloride, which is table salt.

Misconception 7: “Cold means no reaction is happening.”

Correct thinking: Some reactions absorb energy and make the surroundings cooler. These are endothermic reactions.

Misconception 8: “Natural chemicals are safe and human-made chemicals are dangerous.”

Correct thinking: Safety depends on the substance, amount, exposure, and use. Some natural substances are dangerous, and many human-made substances are useful and safe when used properly.

Misconception 9: “Fire is a substance.”

Correct thinking: Fire is not a material substance. Fire is a visible result of combustion, including hot gases, light, and energy release.

Misconception 10: “A chemical reaction always happens quickly.”

Correct thinking: Some reactions are fast, and some are slow. Rusting can take days, weeks, or years. Food spoilage can happen over hours or days.

9. Science Thinking Tips

Tip 1: Use Evidence, Not Just Impressions

Instead of saying, “It looked different,” describe what changed:

“A gas formed because bubbles appeared and the bag inflated.”
“The temperature rose from 21°C to 35°C.”
“A solid formed from two liquids.”

Specific observations are stronger evidence than general descriptions.

Tip 2: Separate Observation From Inference

An observation is what you directly notice or measure.

Example:

The liquid turned cloudy.
The thermometer reading increased.

An inference is an explanation based on evidence.

Example:

A new solid may have formed.
The reaction may be exothermic.

Good scientists clearly separate what they observed from what they think it means.

Tip 3: Use Claim-Evidence-Reasoning

Use this structure for scientific explanations:

Claim: Answer the question.
Evidence: Give observations or data.
Reasoning: Explain why the evidence supports the claim using science ideas.

Example:

Claim: A chemical reaction occurred.

Evidence: The mixture produced bubbles and the sealed bag inflated. The temperature also decreased from 22°C to 17°C.

Reasoning: Gas production and temperature change suggest that new substances formed and energy was absorbed. Since a chemical reaction rearranges atoms into new substances, the evidence supports the claim.

Tip 4: Compare Open and Closed Systems

When interpreting mass data, ask:

Could matter enter or leave the system?
Did a gas escape?
Was the container sealed?
What exactly was measured?

Mass conservation is easiest to observe in a closed system.

Tip 5: Read Graphs Carefully

When reading a graph:

Read the title.
Identify the x-axis and y-axis.
Check units.
Look for patterns.
Compare data points.
Use the pattern to make a reasonable prediction.

For reaction rate graphs, remember that shorter time usually means faster reaction.

Tip 6: Think About Variables

A fair test changes only one important variable at a time. If two things change at once, it is hard to know what caused the result.

Example:

Not fair:

warm water with crushed tablet
cold water with whole tablet

Fair:

warm water with whole tablet
cold water with whole tablet

Only temperature changes.

Tip 7: Use Vocabulary Precisely

Try not to say:

“The chemical disappeared.”
“The gas went away.”
“The reaction used up energy.”

Better:

“The reactants were converted into products.”
“The gas escaped from the open system.”
“Energy was transferred to or from the surroundings.”

10. Practice Questions

A. Quick Recall Questions

What is a chemical reaction?
What are reactants?
What are products?
What is one sign that a gas may have formed?
What does conservation of mass mean?
Why is a sealed bag useful when studying a reaction that makes gas?
What is the difference between a chemical change and a physical change?
What is a precipitate?
What does exothermic mean?
What does endothermic mean?
What is a variable?
What is evidence?
What is a catalyst?
Name one factor that can affect reaction rate.
Why is boiling water not usually considered a chemical reaction?

B. Multiple Choice Questions

Choose the best answer.

Which statement best describes a chemical reaction? A. Matter changes shape but stays the same substance
B. Atoms are rearranged to form new substances
C. Matter is destroyed and replaced by energy
D. A substance changes from solid to liquid
In a chemical reaction, the starting substances are called: A. Products
B. Reactants
C. Variables
D. Catalysts
The substances formed by a chemical reaction are called: A. Reactants
B. Products
C. Systems
D. Mixtures
Which is the best evidence that a chemical reaction may have occurred? A. A piece of paper is folded
B. Ice changes into liquid water
C. A solid forms when two clear liquids are mixed
D. A pencil is sharpened
Which example is most likely a physical change? A. Iron rusting
B. Wood burning
C. Water freezing
D. Milk souring
Which example is most likely a chemical change? A. Sugar dissolving in water
B. Glass breaking
C. Paper burning
D. Clay being shaped
A sealed bag gets larger when baking soda and vinegar react. What is the best explanation? A. New atoms were created
B. A gas formed and filled the bag
C. The bag absorbed the vinegar
D. The solid disappeared completely
In a closed system, the total mass after a chemical reaction should: A. Increase because products are new
B. Decrease because reactants are used up
C. Stay the same because atoms are conserved
D. Become zero because energy is released
A reaction causes the temperature of the surroundings to rise. The reaction is probably: A. Exothermic
B. Endothermic
C. Physical only
D. Not involving energy
A cold pack becomes colder when chemicals inside it mix. This is evidence that the process: A. Releases heat to the surroundings
B. Absorbs energy from the surroundings
C. Destroys matter
D. Creates atoms
Which factor often increases reaction rate? A. Lowering temperature
B. Decreasing surface area
C. Increasing temperature
D. Removing all reactants
Why does a crushed tablet usually react faster than a whole tablet? A. It has less mass of atoms
B. It has more exposed surface area
C. It becomes a different element
D. It contains no reactants
A catalyst is a substance that: A. Slows every reaction
B. Speeds up a reaction without being used up
C. Turns matter into energy
D. Prevents particles from colliding
Which is a controlled variable in a fair test? A. The factor changed on purpose
B. The factor measured as the result
C. A factor kept the same
D. The final answer
A student tests how temperature affects reaction time. What is the independent variable? A. Temperature
B. Reaction time
C. Type of graph
D. Student name
In the same investigation, what is the dependent variable? A. Cup color
B. Reaction time
C. Room number
D. Safety goggles
Which observation is most likely evidence of a precipitate? A. A solid appears after two liquids are mixed
B. A solid melts into a liquid
C. Steam rises from boiling water
D. Salt dissolves completely
Which statement about atoms in a chemical reaction is correct? A. Atoms vanish
B. Atoms are created from energy
C. Atoms are rearranged
D. Atoms become larger
Which situation is an open system? A. A sealed plastic bag
B. A closed jar with a tight lid
C. An open cup where gas can escape
D. A capped bottle
Why might mass seem to decrease in an open cup reaction? A. Gas escaped into the air
B. Atoms were destroyed
C. Products have no mass
D. The cup became invisible
Which chemical reaction is important in plants? A. Photosynthesis
B. Cutting leaves
C. Freezing water
D. Dissolving salt
Combustion usually involves a fuel reacting with: A. Nitrogen only
B. Oxygen
C. Sand
D. Helium
Which is an example of oxidation commonly seen in daily life? A. Ice melting
B. Iron rusting
C. Water condensing
D. Sugar dissolving
Which statement is the best scientific claim? A. “It was cool.”
B. “A chemical reaction occurred.”
C. “I liked the experiment.”
D. “The cup looked weird.”
Which is the best evidence statement? A. “Something happened.”
B. “The reaction was interesting.”
C. “The temperature increased from 20°C to 31°C.”
D. “It was probably science.”
Which statement is reasoning? A. “The liquid changed from clear to cloudy.”
B. “The thermometer read 28°C.”
C. “Gas production and temperature change suggest new substances formed.”
D. “The cup was plastic.”
A student says, “Bubbles prove a chemical reaction.” What is the best response? A. Correct, bubbles always prove a chemical reaction
B. Not always; boiling also makes bubbles without making a new substance
C. Bubbles mean atoms were destroyed
D. Bubbles only happen in solids
Which would make an antacid tablet investigation more reliable? A. Using a different tablet each time
B. Repeating trials and finding a pattern
C. Changing temperature and tablet size at the same time
D. Recording no measurements
What does a chemical equation model? A. The rearrangement of atoms from reactants to products
B. The color of the lab table
C. The age of the scientist
D. The cost of the materials
Which statement best explains why products can have different properties from reactants? A. Atoms are rearranged into new combinations
B. All atoms disappear during reactions
C. Products are always heavier than reactants
D. Reactants are never made of matter

C. Short Answer Questions

Explain why rusting is a chemical change.
A student mixes two liquids and sees bubbles. What additional evidence would help decide whether a chemical reaction occurred?
Why does a closed system help show conservation of mass?
Compare an exothermic reaction and an endothermic reaction.
Explain why melting chocolate is usually a physical change.
A reaction in an open cup starts with a mass of 80.0 g and ends with a mass of 76.5 g. Explain what may have happened.
Why should scientists repeat an experiment more than once?
How could increasing surface area affect reaction rate?
Explain how a catalyst helps a reaction.
A student claims that gas bubbles from boiling water show a chemical reaction. Do you agree? Explain.
What is the difference between observation and inference?
Why are products often different from reactants?
How can chemical reactions be useful in environmental cleanup?
What variables should be controlled when testing antacid tablet reaction rate?
Why is “the substance disappeared” usually not a strong scientific explanation?

D. Data Analysis Questions

Use the antacid tablet data table:

Water Temperature (°C)	Time Until Fizzing Stops (s)
10	94
20	71
30	49
40	34
50	25

What is the independent variable?
What is the dependent variable?
What pattern do you see in the data?
At which temperature was the reaction fastest?
Predict the reaction time at 60°C. Explain your reasoning.
Why might it be unsafe or unfair to keep increasing the water temperature?
Write a claim supported by the data.
What controlled variables would make this investigation fair?

E. Experiment Analysis Questions

A class mixes baking soda and vinegar in an open cup and in a sealed bag.

Setup	Starting Mass (g)	Ending Mass (g)
Open cup	115.0	111.8
Sealed bag	115.0	115.0

Which setup better shows conservation of mass?
Why did the open cup ending mass decrease?
What evidence shows gas formed?
What is the system in the sealed bag setup?
If the bag leaked, how might the results change?
Write a short CER explanation about conservation of mass using this data.

F. Longer Written / Reasoning Questions

A student says, “When wood burns, the wood disappears.” Write a scientific response explaining what actually happens to the matter.
Design an investigation to test how temperature affects the rate of a chemical reaction. Include the question, hypothesis, variables, method summary, safety steps, and the type of data you would collect.
Compare physical changes and chemical changes. Include at least two examples of each and explain how evidence helps you tell them apart.
A town wants to reduce rust damage on metal bridges. Explain the chemical process involved in rusting and suggest two engineering solutions. Explain why each solution could help.
Use Claim-Evidence-Reasoning to explain whether a reaction occurred in this scenario: Two clear liquids are mixed. The mixture turns yellow, a solid forms, and the temperature rises by 6°C.

G. Discussion Prompts

Why do you think engineers care about chemical reactions when designing buildings, vehicles, and bridges?
How can chemical reactions be both helpful and harmful?
Should all chemical reactions that release pollution be banned, or should society weigh benefits and risks? Explain your reasoning.
How could chemical reactions help solve environmental problems?
What is one chemical reaction you noticed this week? What evidence did you observe?

H. STEM Extension Challenge

Design a safer, slower fizzing container.

Your goal:

Create a plan for a small container that can safely hold a fizzing reaction without bursting.

Constraints:

It must allow pressure to escape safely.
It must keep liquid from spilling.
It must be easy to observe.
It must use low-cost classroom materials.

Include:

labeled design sketch using text
materials list
safety features
variables you would test
data you would collect

Starter sketch:

        vent hole with filter
              |
       _______v_______
      /               \
     | clear container |
     | reaction cup    |
     | absorbent base  |
      \_______________/

11. Answer Key

A. Quick Recall Answers

A chemical reaction is a process where atoms are rearranged to form new substances.
Reactants are the starting substances in a chemical reaction.
Products are the new substances formed in a chemical reaction.
Bubbles, fizzing, pressure increase, or an inflated bag can suggest gas formed.
Conservation of mass means matter is not created or destroyed; atoms are conserved.
A sealed bag keeps gas from escaping, so total mass can be measured more accurately.
A physical change does not form a new substance; a chemical change does.
A precipitate is a solid that forms when substances in liquids react.
Exothermic means energy is released to the surroundings.
Endothermic means energy is absorbed from the surroundings.
A variable is a factor that can change in an investigation.
Evidence is data or observations that support a scientific explanation.
A catalyst speeds up a reaction without being used up.
Temperature, surface area, concentration, stirring, or a catalyst.
Boiling water changes state, but the substance is still water.

B. Multiple Choice Answers

C. Short Answer Suggested Answers

Rusting is a chemical change because iron reacts with oxygen and water to form rust, a new substance with different properties.
Additional evidence could include temperature change, a new odor, a color change, a precipitate, or identifying the gas produced.
A closed system prevents matter, especially gas, from entering or leaving, so total mass can be compared before and after.
An exothermic reaction releases energy and may warm the surroundings. An endothermic reaction absorbs energy and may cool the surroundings.
Melting chocolate changes its state and shape, but it is still chocolate, so it is usually physical.
A gas may have formed and escaped from the open cup, lowering the measured mass.
Repeating experiments helps scientists check reliability and reduce the effect of random errors.
Increasing surface area lets more particles contact each other, which can increase reaction rate.
A catalyst provides an easier pathway for a reaction, speeding it up without being used up.
No. Boiling makes bubbles of water vapor, but water remains water, so it is a physical change.
An observation is directly noticed or measured. An inference is an explanation based on observations.
Products have atoms arranged in new ways, so they can have different properties.
Chemical reactions can remove pollutants, form filterable solids, break down harmful substances, or disinfect water.
Same tablet type, same tablet mass, same water amount, same cup, same stirring method, and same timing method.
Matter does not simply disappear; it changes form, becomes part of products, or may leave the system as gas.

D. Data Analysis Answers

Water temperature.
Time until fizzing stops.
As temperature increases, reaction time decreases.
50°C, because it had the shortest time.
A reasonable prediction might be less than 25 seconds, perhaps around 18-22 seconds, if the pattern continues. The exact value would need testing.
Very hot water could be unsafe, and extreme temperatures might change the materials or make the test less comparable.
Claim: Increasing water temperature increases the reaction rate of the antacid tablet. Evidence: The time decreased from 94 s at 10°C to 25 s at 50°C.
Tablet type and size, amount of water, container, stirring, timing method, and tablet condition should be controlled.

E. Experiment Analysis Answers

The sealed bag better shows conservation of mass.
Carbon dioxide gas likely escaped from the open cup.
Bubbles, fizzing, or inflation of the sealed bag show gas formed.
The system is the sealed bag and everything inside it.
If the bag leaked, gas could escape and the ending mass might decrease.
Sample CER: Claim: Mass was conserved in the sealed bag reaction. Evidence: The starting mass and ending mass were both 115.0 g. Reasoning: In a closed system, matter cannot escape. The gas formed stayed inside the bag, so the atoms were still part of the measured system.

12. Model Answers / Suggested Responses

Longer Response 1: Burning Wood

When wood burns, the wood does not simply disappear. Burning is a chemical reaction called combustion. The wood reacts with oxygen in the air and forms new substances, including gases such as carbon dioxide and water vapor, along with ash and smoke particles. In an open fire, some products escape into the air, so the leftover ash has much less mass than the original wood. However, the atoms were not destroyed. They were rearranged into new products, and many of those products moved into the surroundings.

Strong answer features:

Identifies burning as combustion
Explains that oxygen is a reactant
Mentions products such as ash, carbon dioxide, and water vapor
Explains why mass seems to decrease in an open system
Uses conservation of mass

Longer Response 2: Temperature and Reaction Rate Investigation

Question: How does water temperature affect the reaction rate of an antacid tablet?

Hypothesis: If the water temperature increases, then the tablet will stop fizzing faster because warmer particles move faster and collide more often.

Variables:

Independent variable: water temperature
Dependent variable: time until fizzing stops
Controlled variables: same amount of water, same tablet type, same tablet size, same container, same timing method

Method summary:

Place equal amounts of water at different temperatures into identical cups. Add one identical antacid tablet to each cup. Start a timer when the tablet touches the water and stop when fizzing ends. Repeat each temperature at least three times and calculate the average time.

Safety:

Wear goggles, avoid very hot water, clean spills, and do not taste materials.

Data:

Collect temperature in degrees Celsius and reaction time in seconds. A graph can show how reaction time changes with temperature.

Longer Response 3: Physical and Chemical Changes

Physical changes do not create new substances. Examples include ice melting and glass breaking. In both cases, the material is still the same substance even though its state, shape, or size changed. Chemical changes create new substances with new properties. Examples include iron rusting and wood burning. Rust is different from iron, and burning wood produces ash, gases, heat, and light. Evidence helps scientists tell the difference. Gas production, temperature change, precipitate formation, and new substances with new properties can support the idea that a chemical reaction occurred.

Longer Response 4: Rust Damage on Bridges

Rusting is a chemical reaction where iron reacts with oxygen and water to form rust. Rust is weaker and flaky, so it can damage structures over time. One engineering solution is painting the metal. Paint forms a barrier that keeps oxygen and water away from the iron. Another solution is using stainless steel or a protective coating, because these materials resist corrosion better than plain iron. Engineers must consider cost, strength, weather, safety, and how often repairs will be needed.

Longer Response 5: CER for Mystery Liquids

Claim: A chemical reaction occurred when the two clear liquids were mixed.

Evidence: The mixture turned yellow, a solid formed, and the temperature increased by 6°C.

Reasoning: A color change can suggest that a new substance formed, and a solid forming from two liquids is evidence of a precipitate. The temperature increase shows energy was released to the surroundings, which is common in exothermic reactions. Since chemical reactions produce new substances and involve energy changes, the evidence supports the claim that a reaction occurred.

Final Revision Checklist

□ I can define key vocabulary, including reactant, product, evidence, variable, system, matter, and energy.
□ I can explain the difference between a physical change and a chemical change.
□ I can identify evidence that may show a chemical reaction occurred.
□ I can explain why one clue alone may not prove a chemical reaction.
□ I can describe how atoms are rearranged during a chemical reaction.
□ I can explain conservation of mass in a closed system.
□ I can explain why mass may seem to decrease in an open system.
□ I can compare exothermic and endothermic reactions.
□ I can identify factors that affect reaction rate.
□ I can read a data table and graph about reaction rate.
□ I can identify independent, dependent, and controlled variables.
□ I can write a Claim-Evidence-Reasoning explanation.
□ I can explain real-world examples such as rusting, burning, cooking, batteries, and digestion.
□ I can identify common misconceptions about chemical reactions.
□ I attempted the practice questions.
□ I reviewed the answer key and model answers.

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