Chemical reactions and how we spot them

Physical and chemical changes

When ice melts, it is still water. This is a physical change, and no new substance is made. When a candle burns, the wax and the oxygen of the air change into carbon dioxide and water vapour. New substances are made. This is a chemical change, or a chemical reaction.

In a reaction, the substances that we start with are the reactants. The new substances are the products.

Examples that you can try

Magnesium burning. A strip of magnesium ribbon is cleaned with sandpaper and held in a flame. It burns with a dazzling white flame, and a white powder is left. The powder is magnesium oxide. (Do not look straight at the flame. Hold the ribbon with tongs.)

Zinc and acid. A piece of zinc is put in dilute sulphuric acid. Bubbles of hydrogen gas come out, and the test tube becomes warm.

Lead nitrate. White lead nitrate is heated. It gives out brown fumes of nitrogen dioxide and leaves a yellow solid.

Two clear solutions. Sodium sulphate solution and barium chloride solution are both clear. When they are mixed, a white solid forms.

Signs of a chemical reaction

1. A gas is given out. Bubbles appear.
2. The colour changes. A blue solution turns green, or a bright metal turns black.
3. A solid forms. A solid that forms in a liquid is called a precipitate.
4. The temperature changes. The mixture becomes hot or cold.
5. The state changes. A solid becomes a gas or a liquid, in a reaction that makes new substances.

Be careful. Some of these signs also appear in a physical change. Ice melting is a change of state, and it is not a chemical reaction. A reaction is sure when a new substance with new properties has been made.

You should find that the curve always levels off at the same height for the same mass of magnesium, however strong the acid is.

Examples from daily life

• Milk left in a warm place turns sour.
• Digesting food in the body.
• Burning wood, coal or cooking gas.
• A cut apple, or a potato, turns brown in the air.
• An iron gate in the rain turns brown, because it rusts.

Practise with new numbers

Try it in Python

This program lists the signs of a reaction with one example of each.

Remember these

• Chemical reaction: new substances are made. Reactants change into products.
• Signs: a gas, a change of colour, a solid (precipitate), a change of temperature, a change of state.
• A sign is not sure proof. The sure sign is a new substance.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Is the melting of ice a chemical reaction?

2. What is the name of a solid that forms in a liquid in a reaction?

3. Name the reactants and the product when magnesium burns in air.

4. A cut apple turns brown in the air. Which sign of a chemical reaction is this?