Physical and Chemical Changes

Physical changes alter the form of a substance without creating new materials, while chemical changes produce entirely new substances.

What Is a Physical Change?

A physical change is a change in the appearance or form of a substance, but no new substance is created. The material is still made of the same atoms and molecules; they have just been rearranged or the state has changed.

Examples of physical changes include: cutting paper, melting ice, dissolving sugar in water, crushing a can, and bending a metal wire. In all these cases, the material can often be returned to its original form. Melted ice can be refrozen. Dissolved sugar can be recovered by evaporating the water. The chemical identity of the substance has not changed.

Note

Dissolving is a physical change, not a chemical one! When salt dissolves in water, the salt and water are still the same substances — they have just mixed together. You can get the salt back by evaporating the water.

What Is a Chemical Change?

A chemical change (also called a chemical reaction) creates one or more new substances with different properties from the starting materials. In a chemical change, atoms are rearranged and new chemical bonds are formed or broken. The new substances are called products, and the original substances are called reactants.

Examples of chemical changes include: burning wood (which produces ash, carbon dioxide, and water), rusting iron, cooking an egg, baking a cake, and the reaction of vinegar with bicarbonate of soda. These changes are usually difficult or impossible to reverse.

How to Recognise a Chemical Change

Several signs can tell you that a chemical change has taken place:

Colour change (iron turning rusty orange)
Gas is produced (bubbles when vinegar meets baking soda)
Light or heat is produced (fire)
Precipitate forms (a new solid appearing in a liquid)
Irreversible change (you cannot unburn something)

However, not every colour change is a chemical reaction (food colouring in water is physical), and not every reaction produces visible signs. Chemistry requires careful observation and testing.

Tip

A simple way to remember: if you can undo it easily, it's probably physical. If something new is made and you can't easily get back to the original, it's probably chemical. Tearing paper — physical. Burning paper — chemical.

Conservation of Mass

In any chemical change, the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass, stated by Antoine Lavoisier in the 18th century. Atoms are never created or destroyed in a chemical reaction — they are only rearranged.

For example, when magnesium burns in air, it combines with oxygen. The ash (magnesium oxide) weighs more than the original piece of magnesium because it now includes the oxygen atoms from the air. No matter is lost — it just changes form.

Warning

A common exam mistake is to say that burning a piece of wood "destroys" matter. It does not! The wood's atoms are still there — they have just turned into carbon dioxide gas and water vapour in the air, plus ash. The total mass is conserved.

Real world: cooking — physical and chemical changes together

Making a cake involves both types of change. Mixing flour and butter is a physical change (blending without new substances). Baking transforms the batter: proteins in eggs denature (chemical change), raising agents release CO₂ gas making it rise (chemical change), and the Maillard reaction browns the surface (chemical change). Understanding the distinction helps explain why a baked cake cannot be "unbaked."

Real world: recycling metals

Melting aluminium cans in a recycling furnace is a physical change — aluminium melts (solid → liquid) and is recast into new shapes, but remains aluminium throughout. This makes metal recycling highly efficient: no new chemical bonds need to be formed or broken, so it requires far less energy than extracting aluminium from bauxite ore. Knowing that melting is physical (reversible in principle) justifies the entire economics of metal recycling.

Real world: combustion engines

Burning petrol in a car engine is a chemical change. Octane (C₈H₁₈) reacts with oxygen to produce carbon dioxide, water vapour, and energy: $\text{C}_8\text{H}_{18} + \text{O}_2 \to \text{CO}_2 + \text{H}_2\text{O} + \text{energy}$ . New substances form, energy is released as heat and work, and the reaction is irreversible. The exhaust from a car is direct evidence of this chemical change.

Tip Signs of a chemical change: colour change, gas produced, heat/light released, precipitate forms, difficult to reverse. Signs of a physical change: change of state or shape, easily reversible, same substance throughout.

Practice Problems

Is melting chocolate a physical or chemical change? Explain.

Show answer

Melting chocolate is a physical change. The chocolate changes state from solid to liquid, but the same molecules are present. When it cools, it solidifies back into chocolate — no new substance is created.

Give three signs that a chemical change has occurred.

Show answer

Any three from: a new colour appears, gas (bubbles) is produced, light or heat is given off, a new solid (precipitate) forms, or the change is very difficult to reverse.