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(A) If both assertion and reason are true, and reason is the true explanation of the assertion.

When hydrogen is burnt in oxygen the reaction is highly exothermic, it produces very high temperature nearly 4000°C which is used for cutting and welding purposes.

H₂O—Covalent 

B₂H₆- Covalent 

NaH- Ionic

(i) Statements A and R both are correct and R is the correct explanation of A.

(i) Dihydrogen reacts with chlorine give hydrogen chloride. Dihydrogen reduces chlorine into CI⁻ ion and itself gets oxidized to H⁺ ion by chlorine to form hydrogen chloride.

H₂ (g) + Cl₂(g) → 2NCI(g)

(ii) Dihydrogen reacts with sodium to give sodium hydride. Dihydrogen is reduced by sodium to form NaH.

H₂ (g) + 2Na(g) → 2NaH(s)

Dihydrogen reduces copper (II) oxide to copper in zero oxidation state and itself gets oxidised to H₂O, which is a covalent molecule.

H₂(g) + CuO(s) → Cu(s) + H₂O(l)

H₂O(Covalent, electron rich) , B₂H₆ (Covalent Hydride, electron deficient),NaH( ionic hydride).

Now lllg CaCl₂ = 100g of CaCO₃

1 mg of CaCl₂ = \(\frac{100}{111}\) × 1 mg of CaCO = 0.9 mg of CaCO₃

(ii) Mol. Mass of MgCk = 95

Now 95g of MgCl₂ = 100g of CaCO₃

1 mg of MgCl₂ = \(\frac{100}{95}\) × 1 mg of CaCO₃ = 1.05 mg of CaCO₃

Thus, 1 litre of hard water contain = 0.90 + 1.05 = 1.95 mg of CaCO₃.

Correct Answer - C Option
T i H 1.73 is a Metallic hydride

Since 1M – H₂O₂ solution contains 17g H₂O₂

∴ 2 M – H₂O₂ solution contains 34g of H₂O₂

But 68g of H₂O₂ contains = 22400x 34/68

= 11200ml of O₂ at NTP

Thus 1000ml of H₂O₂ soln. gives off O₂ = 11200ml at NTP

Hence 1 ml of H₂O₂ soln gives off

= 11200/1000 = 11.2 ml

Thus volume strength of H₂O₂ = 11.2

In the presence of metal surfaces or traces of alkali (present in glass containers), the above reaction is catalyzed. It is, therefore, stored in wax-lined glass or plastic vessels in dark. Urea can be added as a stabilizer. It is kept away from dust because dust can induce explosive decomposition of the compound.

Fluorine being more electronegative than oxygen can oxidize water to oxygen. In this, water acts a reducing agent and hence itself gets oxidized to O₂. 

2F₂(g) + 2H₂O(l) → 4H⁺(aq) + 4F_–m(aq) + O₂(g)

Atomic hydrogen is highly unstable and hence very reactive. Therefore, it combines with almost all the elements, However, molecular hydrogen has large bond dissociation enthalpy (458.5 kJ mol,m^-1) and therefore, less reactive.

Concentrated sulphuric acid is not used for the preparation of hydrogen because it reacts with zinc to form SO₂ gas instead of hydrogen. 

Zn + 2H₂SO₄ → ZnSO₄ + 2H₂O + SO₂↑

Since the ionic hydride of alkali metal has significant covalent character, therefore, it is LiH. Since LiH is very stable, therefore, it is almost unreactive towards O₂ and Cl₂. It reacts with Al₂Cl₂ form lithium aluminium hydride.

8LiH + Al₂Cl₆ → 2LiAlH₄ + 6LiCl