State the main characteristics of chemical reactions. Give at least one example in each case. [4 marks]

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Solution

Main characteristics of a chemical reaction are:

(a) Evolution of gas:
In many chemical reactions, one of the product is a gas.

$Z n$ + $H_{2} S O_{4}$ $\to$ $Z n S O_{4}$ + $H_{2}$
[1 mark]

(b) Change of colour:
Some chemical reactions are characterised by a change in the colour of the reactants.

$F e$ + $C u S O_{4 (a q)}$ $\to$ $F e S O_{4}$ + Cu
[1 mark]

(c) Formation of precipitates:
Certain chemical reactions are characterised by the formation of an insoluble substance called precipitate.

$A g N O_{3 (a q)}$ + $N a C l$ $\to$ $A g C l_{p p t}$ + $N a N O_{3 (a q)}$
[1 mark]

(d) Change of state:
In many chemical reactions, change of state is observed. The reaction might start with gaseous or liquid reactants and end with a solid product and vice-versa.

$N H_{3 (g)}$ + $H C l_{(g)}$ $\to$ $N H_{4} C l$
[1 mark]

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