(i) At room temperature, HCI is a colourless gas and the solution of HCI in water is called hydrochloric acid and it is in liquid state.
(ii) Sodium hydride NaH is an ionic compound and it is made of sodium cations (Na⁺) and hydride (H^-) anions. It has the octahedral crystal structure. It is an alkali metal hydride.

(i) 2 H_2(g) + O_(g) ⟶ 2 H₂O(1) - Water
(ii) H_2(g) + CI_2(g) ⟶ 2 HCI_(g) - Hydrogen Chloride
(iii) 2 Na_(s) + H_2(g) ⟶ 2NaH_(s) - Sodium hydride

It occurs naturally as a result of nuclear reactions induced by cosmic rays in the upper atmosphere.
\(_{ 7 }^{ 14 }{ N }+_{ 0 }^{ 1 }{ n }\rightarrow _{ 6 }^{ 12 }{ C }+_{ 1 }^{ 3 }{ H }\)
\(_{ 1 }^{ 2 }{ H }+_{ 1 }^{ 2 }{ H }\rightarrow _{ 1 }^{ 3 }{ H }+_{ 1 }^{ 1 }{ H }\)
\(_{ 3 }^{ 6 }{ Li }+_{ 0 }^{ 1 }{ n }\rightarrow _{ 2 }^{ 4 }{ He }+_{ 1 }^{ 3 }{ H }\) 

The largest use of plaster of paris is in the building industry as well as plasters. It is used for immobilising the affected part of organ where there is a bone fracture or sprain. It is also employed in dentistry, in ornamental work and for making casts of statues and busts.

The lattice energy of LiF is higher due to the smaller size of Li⁺ and F^-. So LiF has lower solubility.

6Li + N₂ ⟶ 2Li₃ N

(i) The order : NH₃ < H₂O < HF
(ii) Strength of H bond depends upon the atomic size and electronegativity of the other atom to which H atom is covalently bonded. 
Since electronegativity of F > O > N, the strength of H-bond is in the order H-F......H > H - O......H > H-N......H.
H-bonds are much weaker than covalent bonds.

Metal Hydride (Hydrogen Sponge):
The best studied binary hydrides are the palladium - hydrogen system. Hydrogen interacts with palladium in a unique way, and forms a limiting monohydride, PdH. Upon heating, H atoms diffuse through the metal to the surface and recombine to form molecular hydrogen. Since no other gas behaves this way with palladium, this process has been used to separate hydrogen gas from other gases.

\(2 \mathrm{Pd}(\mathrm{s})+\mathrm{H}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{PdH}(\mathrm{s})\)
The hydrogen molecule readily adsorbs on the palladium surface, where it dissociates into atomic hydrogen. The dissociated atoms dissolve into the interstices or voids (octahedral/tetrahedral) of the crystal lattice.
Technically, the formation of metal hydride is by chemical reaction but it behaves like a physical storage method, i.e., it is absorbed and released like a water sponge. Such a reversible uptake of hydrogen in metals and alloys is also attractive for hydrogen storage and for rechargeable metal hydride battery applications.

Interstitial hydrides have metallic bonding. These are non - stoichiometric; their composition varies with temperature and pressure. The crystal lattice expands due to the inclusion of dihydrogen and there is distortion of crystal lattice. So they have lower density than the parent metal. Recent studies have shown that except the hydrides of Ni, Pd, Ce & Ac, other hydride are different from the parent metals.

(i) WO₃ + 3H₂ ➝ W + 3H₂O
(ii) H₂ + Cl₂ ➝ 2HCl.

a) HCl is a gas. It is an electron rich covalent hydride. It is a covalent molecule with strong covalent bond. However the forces between the molecules are not as strong.
b) NaH is a solid. It is an ionic hydride of group I sodium metal. It is an insoluble hydride

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.

Heavy water is toxic when taken in large quantities. Heavy water is not radioactive. The deuterium in it is stable; it does not decay. Nobody will be in danger at all from radiation. It is heavier than plain water. D₂O performs little different from H₂O in chemical reactions. One has to drink a lot of D₂O to kill him.

Efflorescence is the spontaneous loss of water by a hydrated salt, which occurs when the aqueous vapor pressure of the hydrate is greater than the partial pressure of the water vapour in the air. This is the property of salts
Ex: Glauber's salt- Na₂SO₄·10H₂O
Epsom salt - MgSO₄·7H₂O

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.

(i) By treatment with catalyst like Pt or Fe.
(ii) By passing an electric discharge
(iii) By heating to 800°C or more.
(iv) By mixing with paramagnetic molecules like O₂,NO,NO₂·
(v) By mixing with nascent hydrogen or atomic hydrogen.

(i) Due to small size of Nitrogen atom.
(ii) Due to polar nature of N-H bond.
(iii) Due to inter molecular H- bonding which are stronger than London forces present in other hydrides. Other hydrides lack H - bonding