Year 9 · Science · Chemical sciences
Physical and chemical change
Students identify evidence of chemical change, such as gas production, colour change and temperature change, and compare it with physical change.
Awaiting educator review. This material was drafted by our content team with AI assistance and is waiting for review by a qualified educator. Please check it suits your child before using it. Created 10 October 2026.
These lessons were written around Australian Curriculum strands and matched to New Zealand year levels by age (NZ Year 1 is the first year of school). They are not yet mapped to The New Zealand Curriculum, so check them against your own learning programme.
Learning goals
Learners will
- Distinguish between physical and chemical changes.
- Identify evidence that a chemical reaction has occurred.
- Write word equations for simple reactions.
Success looks like
- I can classify a change as physical or chemical and justify my answer.
- I can list at least four signs of a chemical reaction.
- I can identify reactants and products in a word equation.
The big idea
In a physical change, no new substance is formed. Melting, freezing, dissolving, cutting and crushing are physical changes. Ice melting is still water.
In a chemical change (a chemical reaction), atoms rearrange to form new substances with different properties. Burning, rusting, cooking and digesting food are chemical changes.
Evidence that a chemical change may have occurred includes: a gas forming (bubbles not caused by boiling), a colour change, a temperature change, light being given off, a new smell, or a solid (precipitate) forming when two liquids are mixed.
None of these signs proves a chemical change on its own. Bubbles also appear when water boils, which is physical. Scientists look for several pieces of evidence and ask whether a new substance has been made.
We write reactions as word equations. Reactants go on the left and products on the right: sodium bicarbonate + acetic acid → sodium acetate + water + carbon dioxide.
In a chemical reaction, atoms are not created or destroyed. They are just rearranged. That is why the total mass stays the same in a sealed container.
Worked examples
A reaction in a bag
What evidence shows that bicarbonate of soda and vinegar react chemically?
- Place half a teaspoon of bicarbonate of soda in one corner of a zip-lock bag; wrap it in a twist of paper towel.
- Pour about 50 mL of vinegar into the bag, squeeze out air and seal it, then let the vinegar reach the powder. Keep the bag pointed away from faces.
- Observe: fizzing, the bag puffs up with gas and feels cooler.
- Evidence: a gas is produced and the temperature drops, so new substances have formed.
Answer: Gas production and a temperature decrease show a chemical change.
Classifying changes
Classify: (a) chocolate melting, (b) a nail rusting, (c) sugar dissolving in tea.
- (a) Melting is a change of state; the chocolate is still chocolate, so it is physical.
- (b) Rust (iron oxide) is a new substance formed from iron, oxygen and water, so it is chemical.
- (c) Sugar is still sugar, just spread through the tea, so it is physical.
Answer: (a) physical, (b) chemical, (c) physical.
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
Which is a chemical change?
Toast burning — Burning toast forms new substances such as carbon and gases.
In the equation iron + oxygen → iron oxide, what is iron oxide?
A product — Substances on the right of the arrow are products.
Which observation does NOT by itself prove a chemical change?
Bubbles form in boiling water — Boiling water bubbles because liquid water turns to water vapour, a physical change.
Which gas is produced when bicarbonate of soda reacts with vinegar?
Carbon dioxide — The reaction produces carbon dioxide gas.
In a sealed container, what happens to the total mass during a chemical reaction?
It stays the same — Atoms are rearranged, not created or destroyed, so mass is conserved.
Is a puddle drying up in the sun a physical or chemical change?
physical — Evaporation is a change of state; water vapour is still water.
What name is given to an insoluble solid formed when two solutions react?
precipitate — This solid is called a precipitate.
Substances present at the start of a reaction are called r________.
reactants — Starting substances are reactants.
Watch out for
- Students often think dissolving is a chemical change because the solid 'disappears'. No new substance forms.
- Some believe gas produced in a reaction has no mass, so mass is 'lost'. In an open container the gas escapes; in a sealed one the mass stays the same.
- Some think all bubbles mean a chemical reaction. Boiling and fizzy drinks releasing dissolved gas are physical changes.
Tips for parents
- Do the zip-lock bag reaction outside or over a sink, with small amounts only.
- Ask your child to back up every classification with evidence: 'I think it is chemical because…'.
- Safety: wear eye protection, never seal reactions in rigid bottles or jars, and do not taste anything. Do not mix household cleaners: some combinations, such as bleach and acids, release toxic gases.
Go further
- Add a tablespoon of vinegar to a cup of warm milk (adult warms it). Observe curds forming: the acid changes milk proteins and they clump into a solid.
- Put a soap-free steel wool pad in a damp jar for a week and observe rusting.
- Research how cement hardening in a building is a chemical change.