Calcium oxide

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution. 

Ans. Pb(CH₃COO)₂ + HCl → PbCl₂ + CH₃COOH

This is a Double Displacement reaction 

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas. 

Ans. 2Na + 2C₂H₅ONa + H₂

(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas. 

Ans. 2Fe₂O₃ + 3CO → 2Fe + 3CO₂

This is decomposition reaction.

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid Sulphur and liquid water.
 Ans. 
 2H₂O + O₂ -> 2S + 2H₂O 
 
 This is a Redox reaction.

Silver chloride on exposure to sunlight may decompose as per the following reaction.

2 AgCl → 2Ag + Cl₂ 

Therefore, it is stored in dark coloured bottles.

(a) Write the chemical formulae of X and Y

Ans. X is MgO Y is  Mg₃N₂

(b) Write a balanced chemical equation, when X is dissolved in water. 

Ans. MgO + H₂O → Mg(OH)₂ 

Zinc is above hydrogen whereas copper is below hydrogen in the activity series of metals. That is why zinc displaces hydrogen from dilute hydrochloric acid, while copper does not. 

Zn + HCl → ZnCl₂ +H₂ 

Cu + HCl → No reaction

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.
Ans. Metals such as silver when attacked by substances around it such as moisture, acids, gases etc, are said to corrode and this phenomenon is called corrosion.
(b) Name the black substance formed and give its chemical formula.
Ans. The black substance is formed because silver (Ag) reacts with H₂S  present in air. It forms thin black coating of silver sulphide (Ag₂S)

(a) Write a balanced chemical equation of the reaction.
Ans. Balanced chemical equation
2Cu(NO₃)₂(s)  → 2CuO +O₂ + 4NO₂(g)

(b) Identity the brown gas X evolved.
 Ans. The brown gas X evolved is nitrogen dioxide (NO₂).
 
(c) Identity the type of reaction.
 Ans. This is a decomposition reaction
 
(d) What could be the pH range of aqueous solution of the gas X?
Ans. Nitrogen dioxide dissolves in water to form acidic solution because it is an oxide of non-metal. Therefore, pH of this solution is less than 7.