Recognising a chemical reaction
A chemical reaction makes new substances. You can spot one by a change of state, colour or temperature, or by a gas given out.
What is a chemical reaction?
In a chemical reaction, the reactants (starting substances) change into products (new substances). The atoms are rearranged, so the products have new properties. Milk setting into curd, food being digested and iron rusting are all chemical reactions. A physical change, like ice melting, makes no new substance.
Signs that a reaction has happened
| Sign | Example you can see | What forms |
|---|---|---|
| Change of state | Two clear solutions, lead nitrate and potassium iodide, are mixed and a solid separates | Lead iodide, a yellow precipitate (an insoluble solid) |
| Change of colour | An iron nail is dipped in blue copper sulphate solution; the blue fades to pale green | Ferrous sulphate, and a brown coat of copper |
| Gas given out | Zinc granules in dilute sulphuric acid fizz | Hydrogen gas |
| Change of temperature | Water is added to quicklime and the beaker becomes hot | Slaked lime (calcium hydroxide) |
Activity: burning a magnesium ribbon
- 1Clean a 3 to 4 cm long magnesium ribbon with sandpaper.This removes the layer of magnesium oxide on its surface
- 2Hold it with tongs and burn it in a flame. Keep a watch-glass below it.Keep the ribbon as far as possible from your eyes
- 3Observe: it burns with a dazzling white flame and leaves a white powder.
- 4★2Mg(s) + O₂(g) → 2MgO(s)The white powder is magnesium oxide
★ key step
Two more activities from your textbook
- Zinc and dilute acid: zinc granules in dilute sulphuric acid give bubbles of hydrogen, and the flask feels warm. Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
- Lead nitrate and potassium iodide: mixing the two solutions gives a yellow precipitate. Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
- In each activity, write down what you see, then the balanced equation.
Chemical reactions happen all around you: milk setting into curd, dosa batter rising overnight, food being digested, and a silver anklet turning black. In each one, new substances form.
1 marks: “An old magnesium ribbon looks dull grey and does not catch fire easily. What should you do before burning it, and why?” Rub it with sandpaper. This removes the layer of magnesium oxide on its surface, so the magnesium burns easily.
2 marks: “List four observations that show a chemical reaction has taken place. Give an example of any one.” Change of state, change of colour, gas given out, change of temperature. Example: zinc in dilute sulphuric acid gives bubbles of hydrogen and the flask warms.
3 marks: “What do you see when lead nitrate solution is mixed with potassium iodide solution? Write the balanced equation and name the type of reaction.” A yellow precipitate of lead iodide forms. Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). It is a double displacement (precipitation) reaction.
Spot a reaction with S-C-G-T: State, Colour, Gas, Temperature ("Some Chemicals Get Transformed").