Page 123 - ICSE Chemistry 8

Procedure
• Take some zinc granules in a conical fl ask.
• Add dilute sulphuric acid over zinc granules. conical fl ask

Observa on: You will observe that bubbles of hydrogen gas will be
formed around the zinc granules. dilute
sulphuric hydrogen gas
Conclusion: Hydrogen gas is evolved when zinc granules are added acid bubbles
to dilute sulphuric acid. zinc granules

2. Change of colour: Some reac ons are characterised by change in colour. The reactants are of a
diff erent colour and the products formed are diff erent in colour.

Examples:
• When solid lead nitrate is heated, reddish-brown nitrogen gas is evolved with oxygen. Also, a
yellow-coloured lead monoxide is formed.

Heat
2[Pb(NO ) ](s) 2PbO(s) + 4NO (g)  + O (g) 
2
2
3 2
Lead nitrate Lead oxide Nitrogen dioxide Oxygen
(yellow residue) (reddish-brown)
• When an iron nail is dropped in blue-coloured copper sulphate solu on, the blue colour of
the solu on fades and eventually turns into light green.
Fe(s) + CuSO (aq) FeSO (aq) + Cu(s)
4
4
Iron Copper sulphate Iron sulphate Copper
(blue) (green solu on)

• When green-coloured copper(II) carbonate is heated strongly, it decomposes to form black-
coloured copper(II) oxide and carbon dioxide gas is evolved.
heat
CuCO (s) CuO(s) + CO (g) 
2
3
Copper(II) carbonate Copper(II) oxide Carbon dioxide

Activity 6.2

Aim: To observe colour change during a chemical reac on

Materials required: A beaker, copper sulphate, an iron nail, some water
Procedure

• Take some water in a beaker. Add some copper sulphate in it. You will get a blue coloured solu on.
• Drop an iron nail into it. Leave it for a few hours.
Observa on: The blue colour changes to green colour.

Conclusion: This shows that a new substance has formed. This confi rms that chemical reac ons are
accompanied by change in colour.

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