Physical and Chemical Changes: Unit Review

Introduction

This unit has explored how matter changes: melting and dissolving, fizzing and colour changes, rusting nails and roaring flames. These ideas all rest on a few big principles. This review page draws them together so that you can classify any change, explain it with particles, write equations for it and use the evidence confidently — in exams and in the world around you.

Core explanation

1. Physical versus chemical change.

Feature Physical change Chemical change --- --- --- New substance? No Yes Particles Same particles, rearranged or moved apart Atoms rearranged into new particles Reversibility Usually easy (melting, dissolving) Usually hard; some reactions are reversible Energy change Usually small Often large (exothermic or endothermic) Examples Melting, boiling, dissolving, freezing Burning, rusting, cooking, neutralisation

2. Signs of chemical change. Colour change, gas production (effervescence), formation of a precipitate, heat or light given out or taken in, and a new smell. No single sign proves a chemical change; weigh the evidence and ask whether a new substance has formed.

3. Conservation of mass. Atoms are neither created nor destroyed. In a closed system, total mass stays the same. In an open container, mass seems to fall when a gas escapes (burning wood, fizzing reactions) or rise when a gas from the air is added (burning magnesium, rusting iron).

4. Reversibility. Physical changes are usually reversible. Most chemical changes are not, but some are: heating blue hydrated copper(II) sulfate gives white anhydrous copper(II) sulfate and water, and adding water reverses it. The symbol ⇌ shows a reversible reaction. Thermal decomposition, such as calcium carbonate → calcium oxide + carbon dioxide, breaks a compound down using heat.

5. Equations. Word equations name reactants and products; balanced symbol equations show the same number of each atom on both sides, reflecting conservation of mass.

6. Oxidation, rusting and burning. Both rusting and burning are oxidation reactions — gaining oxygen.

- Rusting is the slow corrosion of iron. It needs both water and oxygen ; salt and acidic conditions speed it up. Rust is hydrated iron(III) oxide. It is prevented by barriers (paint, oil, grease, plastic) or by sacrificial protection with a more reactive metal such as zinc (galvanising). - Burning (combustion) is a rapid, exothermic reaction with oxygen. It needs fuel, oxygen and heat — the fire triangle . Complete combustion of hydrocarbons gives carbon dioxide and water; incomplete combustion gives toxic carbon monoxide and soot. Combustion products affect the environment through climate change, acid rain and air pollution.

Step-by-step reasoning

A reliable routine for any change:

1. Describe what is observed (colour, gas, solid, temperature, smell). 2. Decide whether a new substance forms — physical or chemical. 3. Explain using particles: same particles or rearranged atoms. 4. Write a word equation (and symbol equation if possible) and check mass is conserved. 5. Comment on reversibility and energy change.

Visual explanation

Picture a concept map with "Change in matter" at the centre. Two branches lead to "Physical" (melting, dissolving, boiling) and "Chemical" (signs of change, equations). From "Chemical", a branch labelled "Oxidation" splits into "Rusting (slow)" and "Burning (fast)", each with its own sub-branches for conditions and prevention or products.

Real-world analogy

The unit's ideas work like a detective's toolkit. The signs of change are clues, conservation of mass is the rule that nothing vanishes, and equations are the written case report. A good detective uses several clues together before reaching a verdict.

Real-world example

A car shows the whole unit at once. Petrol evaporates (physical) and burns in the engine (chemical, exothermic), producing carbon dioxide, water and pollutants that a catalytic converter treats. Meanwhile the steel body is protected from rusting by galvanising and paint, and the air-conditioning cycles a refrigerant through evaporation and condensation.

Why?

Why do we group rusting and burning together although they look so different? Both involve a substance combining with oxygen to form oxides and releasing energy. The main difference is rate: burning is fast, so heat and light are obvious, while rusting is slow, so the heat escapes unnoticed.

Common misconception

"Chemical changes can never be reversed and physical changes always can." Some chemical changes are reversible, such as hydrating copper(II) sulfate, and some physical changes are hard to reverse in practice. The real test is whether a new substance forms.

Worked example

Question: Iron wool is weighed, heated strongly in air until it glows, and weighed again. The mass increases from 5.0 g to 5.8 g. Explain.

Reasoning: Glowing and a new black solid show a chemical change. Iron combines with oxygen from the air; mass is conserved, so the gain is the oxygen added.

Answer: The iron has been oxidised (burned) to iron oxide; 0.8 g of oxygen from the air combined with it.

Quick check

1. What two substances must be present for iron to rust? Answer: Water and oxygen.

Exam focus

Review questions often combine topics: classify a change, give evidence, write an equation and explain a mass change. Use precise language — "new substance formed", "plentiful or limited oxygen", "more reactive metal corrodes instead" — and always link observations to explanations.

Advanced insight

Later chemistry refines these ideas. Oxidation is redefined as the loss of electrons, which includes reactions without oxygen at all, and reversible reactions reach a dynamic equilibrium in a closed system. Energy changes are explained by bond breaking and bond making. The simple models in this unit remain the foundation for all of them.

Summary

Physical changes form no new substance and are usually reversible; chemical changes form new substances, shown by signs such as colour change, gas, precipitate, energy change and smell. Mass is always conserved. Some chemical changes are reversible. Rusting and burning are both oxidation: rusting needs water and oxygen and is prevented by barriers or sacrificial protection; burning needs fuel, oxygen and heat and can be complete or incomplete.

Practice questions

1. Give one difference between a physical and a chemical change in terms of particles. Answer: In a physical change the same particles are simply rearranged or moved apart; in a chemical change atoms are rearranged to form new particles. 2. Why does the mass of an open beaker fall when marble chips react with acid? Answer: Carbon dioxide gas is produced and escapes into the air; the total mass of all substances is still conserved. 3. Explain how galvanising protects iron from rusting. Answer: The zinc coating acts as a barrier and, being more reactive than iron, corrodes in preference to it even if scratched (sacrificial protection). 4. State the products of the complete and incomplete combustion of methane. Answer: Complete: carbon dioxide and water. Incomplete: carbon monoxide and/or carbon (soot) and water. 5. Give an example of a reversible chemical change. Answer: Heating hydrated copper(II) sulfate to form anhydrous copper(II) sulfate and water, which is reversed by adding water.