ICSE Class 10 Answered
Expert Answer
Below are some generic examples of observations for a chemical reaction has taken place:
| 1. Evolution of Gas
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In a chemical reaction, one of the products is a gas. Example: When zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with effervescence.
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| 2. Change of Colour
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Some chemical reactions are characterized by a change in the colour of the reactants. Example: A few pieces of iron are added into a blue coloured copper sulphate solution. The blue colour of copper sulphate fades and eventually turns into light green due to the formation of ferrous sulphate.
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| 3. Formation of Precipitate
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Some chemical reactions are characterised by the formation of a precipitate. The precipitate is an insoluble solid substance. Example: When a solution of silver nitrate is added to a solution of sodium chloride, a white precipitate of silver chloride is formed.
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| 4. Change of State
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In some reactions, a change of state is observed. The reaction starts with solid or liquid reactants and ends up with gaseous products and vice versa. Example: Ammonia gas reacts with hydrogen chloride gas to form ammonium chloride, which is in a solid state.
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