(a) Photochemical decomposition reaction: Those reactions in which a compound breaks down into simple substances in presence of light are called photochemical decomposition reaction. The colour of salt will change from white to grey.
(b)  \( 2 \mathrm{AgCl}(\mathrm{s}) \stackrel{\text { Sunlight }}{\longrightarrow} 2 \mathrm{Ag}(\mathrm{s})+\mathrm{Cl}_{2}(\mathrm{~g}) \)
(c) Silver chloride is used in photography.

'X' is sodium hydrogen carbonate (baking soda)
Chemical formula  \(\longrightarrow \) NaHCO₃
During cooking   \(\longrightarrow \) It decomposes to sodium carbonate, carbon dioxide and water
2NaHCO₃(s)\(\overset { heat }{ \longrightarrow } \) Na₂CO₃ + CO₂ + H₂O

(a) (i) Ca(OH)₂ + CO₂\(\longrightarrow \) CaCO₃ + H₂O
(ii) Ca(OH)₂(aq) + CO₂(g)\(\longrightarrow \) CaC0₃(s) +H₂O(I)
CaCO₃(s) + H₂O(I) + CO₂(g) \(\longrightarrow \) Ca(HCO₃)₂ (aq)
(b) Aragonite and calcite

(i) Before heating, it is pale green.
After heating, it is brown or reddish brown.
Two observations are:
(a) Change in state and colour
(b) Evolution of gas
(ii) Decomposition reaction
2FeSO₄(s) \(\overset { Heat }{ \rightarrow } \) Fe₂O₃(s) + S0₂(g) + SO₃(g) 

Chemical formula - CaOCI₂
Chemical equation - Ca(OH)₂ + CI₂ ⟶ CaOCI₂ + H₂O
Uses:
(i) For bleaching cotton and linen in textile industry
(ii) As an oxidising agent in a chemical industry. Y,
(iii) For disinfecting water.

(i) Five molecules of water.
(ii) Formula - CuSO₄.5H₂O
[iii) When heated, its colour changes from blue to white.

(i) Alkali. e.g., NaOH (Sodium hydroxide).
(ii) Tooth decay starts when the pH of the mouth is lower than 5·5.It can be prevented by using tooth pastes which are generally basic.
(iii) Bee-sting has acid that causes pain and irritation. Baking soda is alkaline, neutralises the acid and gives relief.

pH in increasing order of H⁺ion concentration is C < E < D < A < B

The food taken by the living beings is ultimately broken down to glucose by the digestive system. The glucose so formed is slowly oxidised to carbon dioxide and water with the release of heat energy. Thus, respiration is an exothermic reaction.
C₆H₁₂O₆(aq) + 6O₂(g) \(\rightarrow\) 6CO₂(g) + 6H₂O(l) + energy
   (Glucose)

(i) H₂ + Cl₂ \(\rightarrow\) 2HCI
(ii) 3BaCl₂ + Al₂(SO₄)₃ \(\rightarrow\) 3BaSO₄ \(\downarrow\)+ 2AlCl₃
(iii) 2Na + 2H₂O \(\rightarrow\) 2NaOH + H₂