Permanent hardness is due to the presence of soluble salts of magnesium and calcium in form of sulphates and chlorides. Permanent hardness is not removed by boiling.

(a) D₂O retards the growth of living organisms like plants and animals.
(b) Pure heavy water kills small fishes, tadpoles and mice when fed upon it.

The carbon monoxide of the water gas can be converted to carbon dioxide by mixing the gas mixture with more steam at 400^oC and passed over a shift converter containing iron/copper catalyst. This reaction is called as water-gas shift reaction.
CO + H₂O ➝ CO₂ + H₂

a) Oxidation is performed in acidic medium:
Eg : H₂O₂ Oxidises FeSO₄ to Fe (SO₄)₃
H₂O₂ + 2H⁺+ 2e^- ➝ 2H₂O (E⁰ = +1.77 V)
2FeSO₄ + H₂SO₄ + H₂O₂ ➝ Fe₂(SO₄)₃ + 2H₂O
b) Reduction is performed in basic medium:
Eg : H₂O₂ reduces KMnO₄ to MnO₂
HO₂^- + OH^- ➝ H₂O + 2e^- (E⁰ = +0.08 V)
\(2 \mathrm{KMnO}_{4}+3 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow 2 \mathrm{MnO}_{2}+2 \mathrm{KOH}+2 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{O}_{2}\)

Hydrogen has electronic configuration 1s¹. So it resembles ns¹ general valence shell configuration of alkali metals.
Its similarity with alkali metals are:
1. Form uni positive (H⁺) ions like Na⁺, K⁺, Cs⁺
2. Form halides, (HX), oxides (H₂O) peroxides (H₂O₂) and sulphides (H₂S) like (Nax, Na2O, Na₂O₂, Na₂S)
3. Acts as reducing agent. 
4. Has +1 oxidation state.
5. If it is placed in the halogen family it should be the most electro negative element which is not true.

(i) Used in emergency exit signs.
(ii) Used in illumination of wrist watches in place of radium.

Tritium is the radio active isotope of Hydrogen with a half-life of 12.3 years. It's nucleus consists of one proton and two neutrons.

₁H₂ or ₁D₂ . It occurs naturally in very small traces. It's nucleus consists of a proton, a neutron and one electron revolving around the nucleus. Its chemical properties are similar to those of protium but their reaction rates are different.

NOTE:
At any condition ice cube does not sink in water.
At 0⁰C, ice and liquid water will be in equilibrium and will coexist. Hence no freezing or melting will occur.

When conc. H₂SO₄ absorbs H₂O from moist H₂ it produces so much of heat since its highly exothermic and so it may catch fire.

(i) Resemblance with alkali metals :
Its electronic configuration is 1s¹ which is similar to alkali metals. 
It acts as a good reducing agent. 
Like alkali metal, it loses one electron to form H⁺, 
It is liberated at cathode when HCI is electrolysed.
It is electropositive in nature.
(ii) Resemblance with halogens :  
It is a non-metal. 
It is a gas like F₂ and Cl₂ 
It is slightly electronegative in nature 
Like halogens it is ready to accept one electron and attain the stable inert gas configuration.

The element which occupies group number (16) and period number (2) is oxygen. (B) is D₂O which is used as a moderator in nuclear reactions.
So (A) must be deuterium, which is an isotope of hydrogen
    \(2\underset { (A) }{ { D }_{ 2 } } +{ O }_{ 2 }\rightarrow 2\underset { (B) }{ { D }_{ 2 }O } \)
So (B) is D₂O
(A) adds to (C) as follows :
\(3 \mathrm{D}_{2}+\mathrm{C}_{3} \mathrm{H}_{6} \rightarrow \mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{2}\)
So (D) is 1,2 - dideutero propane.