Four clues

The idea

Whenever a chemical change occurs, a chemical reaction has taken place; it can be recognised by a change in state, a change in colour, evolution of a gas or a change in temperature.

Something new is made

Milk turns sour in summer. An iron nail turns brown in damp air. Grapes ferment. In each one, the thing you started with has changed into something else.

How can you tell that a new substance has been made?

• By looking for clues: a new colour, bubbles, a hot or cold feel, a new solid
• Only if there is a flame
• You cannot tell without a microscope

Catch the clue

Do each experiment. Watch the picture. Then tap the sign that tells you a reaction happened.

Four signs

Whenever a chemical change occurs, a chemical reaction has taken place. We can recognise it by a change in state, a change in colour, evolution of a gas, or a change in temperature.

Careful: ice melting also changes state, but no new substance is made. A clue tells you to look closer.

• Experiment | What you saw | Sign
• Zinc + acid | Bubbles | Gas evolved
• Quicklime + water | Beaker gets hot | Temperature changes
• Iron nail + copper sulphate | Blue → green | Colour changes
• Two clear solutions mixed | White solid appears | State changes

Notes

Whenever a chemical change occurs, a chemical reaction has taken place; it can be recognised by a change in state, a change in colour, evolution of a gas or a change in temperature.

Check yourself

Which one shows a chemical reaction?

A magnesium ribbon burns with a dazzling white flame and leaves a white powder. Which two signs do you see?

Barium chloride solution is added to a clear sodium sulphate solution. A white solid settles at the bottom. Which sign is this?

• The blue copper sulphate solution turns pale green when an iron nail is dipped in — correct. Yes! A new colour means new substances (iron sulphate and copper) have formed.
• Water boils and bubbles rise. These bubbles are steam: water turned into water vapour. It is still water, so no reaction.
• Sugar dissolves in tea. Sugar is still sugar in the tea. It spread out, but nothing new was made.
• Gas evolved and state changes. Look again: no bubbles. The ribbon does not melt or turn into a gas.
• Colour changes and temperature rises — correct. Yes! Shiny grey ribbon → white powder (colour), and the flame gives out heat and light.
• Only a change in state. A white powder from a grey ribbon is a new colour too, and a flame means heat.
• Evolution of a gas. No bubbles rose. A solid settled down.
• Change in temperature. Nothing here is hot or cold. A solid appeared.
• Change in state — correct. Yes! A liquid mixture has given a solid.