Investigating Physical and Chemical Changes

Aim

To investigate the difference between physical and chemical changes by observing changes in matter and identifying whether new substances are formed.

Learning Outcome

• Define physical and chemical changes.
• Identify evidence of chemical reactions.
• Distinguish between physical and chemical changes.
• Explain how chemical bonds are involved in chemical reactions.
• Classify changes in matter correctly.
• Identify reactants and products.
• Explain whether a new substance has formed.

Time Allocation

45 to 60 minutes.

Introduction

Matter is constantly changing around us. Examples include melting ice, burning wood, rusting metal, baking food, dissolving sugar and cooking eggs.

In science, changes in matter are classified as either physical changes or chemical changes.

Scientific Background

Physical Changes

A physical change changes the appearance, shape, size or state of a substance.

No new substance forms during a physical change and chemical bonds are not broken or formed.

The substance remains the same type of matter. Many physical changes can be reversed.

Examples of Physical Changes

• Melting ice
• Freezing water
• Cutting paper
• Dissolving sugar in water
• Crushing chalk
• Cutting hair

Chemical Changes

A chemical change occurs when chemical bonds break and new bonds form between atoms.

This rearrangement of atoms forms one or more new substances with different properties.

Chemical changes usually cannot easily be reversed.

Examples of Chemical Changes

• Burning wood
• Rusting iron
• Baking bread
• Frying an egg
• Fireworks exploding
• Vinegar reacting with baking soda

Reactants and Products

Term	Definition
Reactants	Substances present before a chemical reaction.
Products	New substances formed during a chemical reaction.

Indicators of a Chemical Change

The following observations may indicate that a chemical reaction has taken place:

Important Definitions

Term	Definition
Matter	Anything that has mass and occupies space.
Physical Change	A change where no new substance forms.
Chemical Change	A change where new substances form.
Chemical Bond	A force holding atoms together.
Reversible Change	A change that can be undone.
Irreversible Change	A change that cannot easily be undone.
Precipitate	An insoluble solid formed during a chemical reaction.

Physical Change vs Chemical Change

Feature	Physical Change	Chemical Change
New substance formed?	No	Yes
Chemical bonds broken or formed?	No	Yes
Chemical identity changes?	No	Yes
Usually reversible?	Often	Usually not
Energy change?	Small or none	Often noticeable
Example	Melting ice	Burning magnesium

Hypothesis

If a change only affects the form, shape or state of a substance, then it is a physical change.

If a new substance forms and chemical bonds break or form, then it is a chemical change.

Investigation Question

How can physical and chemical changes be identified using observations made during different reactions and processes?

Variables

Independent Variable

Type of change investigated.

Dependent Variable

Observations recorded during each change.

Controlled Variables

• Same observation method.
• Same apparatus where possible.
• Same safety procedures.
• Same heating method.

Apparatus

• Ice cubes
• Beakers
• Salt
• Water
• Glass stirring rod
• Magnesium ribbon
• Vinegar
• Baking soda
• Test tubes or small beakers
• Bunsen burner or spirit burner
• Tongs
• Heatproof mat
• Safety goggles

Safety Precautions

Important Notes

Experiment Part A: Melting Ice

Aim

To observe a physical change involving a change of state.

Method

1. Place an ice cube into a beaker.
2. Allow the ice cube to melt naturally.
3. Observe the change carefully.
4. Record whether a new substance forms.

Observation: The ice changes from a solid to a liquid.

Classification: Physical change.

Explanation: The substance remains water throughout the process. Only the state changes.

Experiment Part B: Dissolving Salt in Water

Aim

To investigate dissolving as a physical change.

Method

1. Pour water into a beaker.
2. Add a small amount of salt.
3. Stir using a glass rod.
4. Observe the solution formed.
5. Record whether a new substance forms.

Observation: The salt dissolves and forms a clear salt solution.

Classification: Physical change.

Explanation: The salt particles disperse throughout the water, but no new substance forms. The salt can be recovered by evaporation.

Experiment Part C: Vinegar and Baking Soda

Aim

To observe a chemical change.

Method

1. Pour a small amount of vinegar into a beaker or test tube.
2. Add a small amount of baking soda.
3. Observe the reaction carefully.
4. Record any bubbling, fizzing or temperature changes.

Observation: Fizzing and bubbling occur as gas is released.

Classification: Chemical change.

Explanation: A chemical reaction occurs and new substances form, including carbon dioxide gas.

Experiment Part D: Burning Magnesium Ribbon

Aim

To observe a chemical reaction involving combustion.

Method

1. Hold a short piece of magnesium ribbon using tongs.
2. Heat the magnesium ribbon in a flame.
3. Observe the reaction carefully.
4. Allow the product to cool.
5. Observe the white powder produced.

Observation: The magnesium burns with a bright white flame and forms a white powder.

Classification: Chemical change.

Explanation: Magnesium reacts with oxygen in the air to form magnesium oxide.

Results Table

Investigation	Observation	New Substance Formed?	Physical or Chemical Change?
Melting ice
Dissolving salt
Vinegar and baking soda
Burning magnesium

Sample Results

Investigation	Observation	New Substance Formed?	Physical or Chemical Change?
Melting ice	Ice melted into water.	No	Physical
Dissolving salt	Salt dissolved completely.	No	Physical
Vinegar and baking soda	Bubbling and fizzing occurred.	Yes	Chemical
Burning magnesium	Bright flame and white powder formed.	Yes	Chemical

What Learners Should Observe

• Physical changes do not form new substances.
• Chemical changes form new substances.
• Some chemical reactions produce gases.
• Some reactions release energy as heat or light.
• Dissolving is not necessarily a chemical reaction.

Analysis of Results

The investigation shows that physical and chemical changes differ according to whether new substances form.

During physical changes, the type of matter remains the same and chemical bonds are not broken or formed.

During chemical changes, atoms rearrange, chemical bonds break and form, and new substances with different properties are produced.

Accuracy and Reliability

Improving Accuracy

• Observe carefully before and after each change.
• Use clean apparatus.
• Use small measured quantities.
• Record observations immediately.

Improving Reliability

• Repeat the investigation.
• Compare results with other groups.
• Use the same procedures each time.

Sources of Experimental Error

• Contaminated apparatus.
• Incorrect observations.
• Using too much chemical.
• Missing brief observations.
• Overheating substances.
• Confusion between dissolving and reacting.

Conclusion

The investigation successfully distinguished between physical and chemical changes.

Physical changes changed the appearance or state of substances without forming new substances.

Chemical changes formed new substances through the breaking and forming of chemical bonds.

The investigation confirmed that melting ice and dissolving salt are physical changes, while vinegar reacting with baking soda and burning magnesium are chemical changes.

Questions for Learners

1. What is a physical change?
2. What is a chemical change?
3. What is the main difference between physical and chemical changes?
4. What are reactants?
5. What are products?
6. Which investigation produced a gas?
7. Why is dissolving salt considered a physical change?
8. What evidence showed that burning magnesium was a chemical change?
9. Which indicators of chemical change were observed?
10. Why is melting ice not a chemical reaction?
11. What new substance formed when magnesium burned?
12. Which changes were reversible?

Everyday Examples

Physical Changes	Chemical Changes
Cutting paper	Burning paper
Melting butter	Rusting iron
Freezing water	Baking bread
Crushing chalk	Fireworks exploding
Dissolving sugar	Cooking an egg

Common Mistakes

• Thinking all visible changes are chemical changes.
• Confusing dissolving with reacting.
• Assuming bubbling always means boiling.
• Forgetting to identify new substances.
• Looking directly at burning magnesium.

Teacher Notes

• Emphasise that the formation of a new substance is the most important test.
• Reinforce the idea that physical changes do not alter chemical identity.
• Learners commonly confuse dissolving with chemical change.
• Use magnesium burning to demonstrate a clear chemical reaction.
• Encourage learners to justify their classifications scientifically.

Teacher Tip

Ask learners after every investigation: Did the substance stay the same, or did a new substance form?

Expected Results

• Melting ice: physical change.
• Dissolving salt: physical change.
• Vinegar and baking soda: chemical change.
• Burning magnesium: chemical change.

Extension Activity

Classify the following as physical or chemical changes:

• Tearing paper
• Rusting iron
• Cooking food
• Boiling water
• Freezing juice
• Burning candles
• Breaking glass
• Digesting food

Real-World Applications

Physical and chemical changes are important in cooking, manufacturing, recycling, water purification, medicine, metal production, fire safety and environmental science.

Understanding these changes helps scientists and engineers work safely with materials and reactions.