Question

(a) What is meant by chemical reaction? Explain with the help of an example.
(b) Give one example each of a chemical reaction characterised by
(i) evolution of a gas
(ii) change in colour
(iii) formation of a precipitate
(iv) change in temperature
(v) change in state.

Answer 1

Answer:

chemical reaction is the reaction which can not change in it's old form after reacting

Answer 2

Explanation:

(a) Chemical reaction:

• A chemical process of substances under certain condition to form particular product is called as chemical reaction.
• For example: Photosynthesis a basic chemical reaction taking place in all the green plants.
• The reaction takes place between carbon dioxide and water; in presence of sun light. The product of the photosynthesis is oxygen and starch.

$6 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O} \stackrel{\text { sun light energy }}{\text { . }} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{O}_{2}$

(b) Examples of chemical reaction:

(i) Evolution of a gas:

$H_{2} S O_{3(a q)} \rightarrow H_{2} O_{(l)}+S O_{2(g)}$

• The sulfurous acid decomposes to form water and evolves sulphur dioxide. The reactant is thermodynamically unstable and undergoes decomposition.

(ii) Change in colour:

$C u_{(s)}+O_{2(g)}+H_{2} O \rightarrow C u(O H)_{2(s)}+O_{2(g)}$

• The copper (reddish brown colour) reacts with oxygen and water to form hydrated copper carbonate (green colour).
• This reaction happened in Statue of liberty, as the original colour of it is reddish brown and it changed to green colour due to formation of hydrated copper carbonate.

(iii) Formation of a precipitate:

$A g N O_{3(a q)}+K C l_{(a q)} \rightarrow A g C l_{(s)} \downarrow+K N O_{3(a q)}$

• The combination of aqueous silver nitrate and aqueous potassium chloride forms silver chloride which is a precipitate with potassium nitrate in the solution.

(iv) Change in temperature:

$[\mathrm{Ne}] 3 s^{2} 3 p^{5}$

• The above reaction of formation of ammonia by combination of nitrogen and hydrogen with heat release is an exothermic reaction. The temperature increase occurs as a result of this reaction.
• If the heat is absorbed then the reaction is termed as endothermic reaction.

(v) Change in state:

$N H_{3(g)}+H C l_{(g)} \rightarrow N H_{4} C l_{(s)}$

• The ammonia and hydrochloride gas reacts to form ammonia chloride which is a solid. There is a change of state in the above chemical reaction.