chemical reactions 20 marks

Calcium is a very reactive metal. There is a layer of calcium oxide coated on the surface of calcium granule, because of the reaction between calcium and oxygen in air.
2Ca(s) + O2(g) → 2CaO(s)

When calcium granules are added into water, the oxide layer would firstly react with water to form soluble calcium hydroxide. In the reaction, there is no gas formed.
CaO(s) + H2O(l) → Ca(OH)2(aq)

When all the calcium oxide has been reacted, the uncoated calcium metal would then react with water to give hydrogen gas.
Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)

Evolution of the colourless (hydrogen) gas was at first slow but became faster after some time. This is because hydrogen gas would be formed only when the calcium oxide layer has been reacted.