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Question
State the main characteristics of chemical reactions. Give at least one example in each case.
Solution
The main characteristics of chemical reactions are as follows:
- Evolution of gas: In certain chemical reactions, one of the products is a gas. For example., When zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with effervescence.
\[\ce{Zn + H2SO4 -> ZnSO4 + H2}\] - Change of colour: Certain chemical reactions are characterized by a change in the colour of the reactants. For example., When a few pieces of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green due to the formation of ferrous sulphate.
\[\ce{\underset{[iron]}{Fe} + \underset{[blue solution]}{CuSO4_{(aq)}} ->\underset{[green solution]}{FeSO4} + \underset{(brown/red deposit)}{\underset{[copper]}{Cu}}}\] - Formation of precipitate: Certain chemical reactions are characterized by the formation of insoluble solid substances called precipitates. For example., When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green precipitate of ferrous hydroxide is formed.
\[\ce{\underset{[ferrous sulphate]}{FeSO4_{(aq)}} + \underset{[sodium hydroxide]}{2NaOH_{(aq)}} -> \underset{(dirty green ppt)}{\underset{[ferrous hydroxide]}{Fe(OH)2\downarrow}} + \underset{[sodium sulphate]}{Na2SO4_{(aq)}}}\] - Change of state: In many chemical reactions, a change of state is observed. The reaction might start with gaseous or liquid reactants and end up with solid products, and vice versa. For example., Ammonia gas reacts with hydrogen chloride gas to produce solid ammonium chloride.
\[\ce{NH3_{(g)} + HCl_{(g)} <=> NH4Cl_{(s)}}\]
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