Physical and chemical changes

Learning objectives

By the end of this topic you should be able to:

• describe physical changes and explain why they are usually reversible
• identify chemical changes using their signs
• explain what iron needs to rust and how to prevent it
• explain burning and what fire needs
• explain conservation of mass in open and closed containers
• discuss preventing rust and the effects of firecrackers

Before you start

You will use these skills. If any feel shaky, review them first.

• solids, liquids and gases (see States of matter)
• mixtures and dissolving (see Pure substances, mixtures and separation)
• reading tables of results

Key vocabulary

Physical change

A change in which no new substance is made, such as melting or dissolving.

Chemical change

A change in which new substances are made (a chemical reaction).

Reversible

Able to be changed back.

Rusting

A chemical change in which iron reacts with oxygen and water.

Combustion

Burning: a reaction with oxygen that gives out heat and light.

Conservation of mass

The total mass stays the same in any change, if nothing escapes.

Understanding the ideas

1. What is it?

   In a physical change, such as melting, freezing or dissolving, the substance keeps its identity; no new substance is made, and the change can usually be reversed.

2. Why does it happen?

   In a chemical change, new substances are made. Signs include gas bubbles, colour changes, temperature changes, light, and a solid forming. Burning, rusting, cooking and souring milk are all chemical changes.

3. How do we know?

   Experiments like the rusting tubes show exactly what a reaction needs. Weighing shows that mass is never created or destroyed: if a gas escapes from an open container the mass falls, but in a sealed container it stays the same.

4. Why does it matter?

   Understanding changes helps us prevent rust, put out fires safely, cook food and reduce pollution from burning.

5. What does it connect to?

   This topic links to states of matter and mixtures in chemistry, to digestion and respiration in biology, and later to acids, fuels and chemical equations.

Physical or chemical?

• Physical: melting, freezing, boiling, dissolving, cutting, bending. No new substance; usually reversible.
• Chemical: burning, rusting, cooking, rotting, baking soda with vinegar. New substances; usually hard to reverse.
• Signs of a chemical change: gas bubbles, colour change, temperature change, light or sound, a solid (precipitate) forming.
• Fire needs fuel, heat and oxygen (the fire triangle).
• Rusting needs iron, oxygen and water.

Changes in the real world

Steel bridges and ships are painted or galvanised to stop rust. Fire blankets and lids put out cooking fires by cutting off oxygen. Many cities limit firecrackers during festivals because of smoke and injuries.

Worked examples

Assessment tips

Questions on changes often use observations and simple mass data. Expect to:

• Classify changes as physical or chemical, with reasons.
• Use experiment results to explain rusting and burning.
• Explain changes in mass using conservation of mass.
• Plan fair tests.

Common mistakes: saying dissolved substances disappear; thinking mass is lost in reactions; saying rusting needs only water; and calling every hard-to-reverse change chemical.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.