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The water which contains dissolved salts of bicarbonates, sulphates and chlorides of calcium and magnesium is called hard water. Hard water does not produce lather with soap solution. 

Soft water is free from bicarbonates, sulphates and chlorides of calcium and magnesium. It produces lather with soap solution easily. e.g., distilled water, rain water.. 

10 volume solution of H₂O₂ means that 1L of this H₂O₂ will give 10 L of oxygen at STP 

2H₂ O₂ ( l) → O₂( g) +2 H₂O( l) 

2×34 g       22.4 L at STP 

22.4 L of O₂ at STP is produced from H₂O₂ = 68 g 

20 L of O₂ at STP is produced from H₂O₂ =68X20/22.4 = 60.72 g

Therefore, strength of H₂O₂ in 20 volume H₂O₂ = 60.72 g/L

a) because calcium and magnesium bicarbonates decomposes on heating or boiling 

b) It precipitates out calcium carbonate and magnesium hydroxide which can be filtered off. 

c) Permutit/Zeolite is said to be exhausted when all the sodium in it is used up. It is regenerated for further use by treating with an aqueous sodium chloride solution. 

Salt-like hydrides are also called ionic hydrides. They are formed by group 1 and 2 elements with H₂. They are basic in nature. They dissolve in water forming alkali and give H₂ gas, .e.g., NaH, CaH₂, KH etc. Covalent hydrides are formed between less electropositive or electronegative elements. 

Ex : B₂H₆, AlH₃, BeH₂, They are insoluble in water.

Its position in periodic table is not justified because it resembles both alkali metals and halogens.

(i) Its electronic configuration is similar to alkali metals. 

(ii) It is non-metal like halogens. 

(iii) Its ionization energy is high like halogens. 

(iv) It can lose electrons to form H⁺ like alkali metals.

Hard water on boiling forms precipitates of MgCO₃, CaCO₃ and CaSO₄ which form scales in the boilers. As a result of these scales in the boilers. The boiler gets deteriorated due to over heating. Moreover, these scales are non-conducting and therefore, more fuel is consumed. Therefore, in order to prevent the formation of scales, hard water softened before using in boilers.

D₂O₂ can be prepared by the reaction of D₂SO₄ dissolved in water over BaO₂.
BaO₂ + D₂SO₄ → BaSO₄ + D₂O₂  

D₂O₂ can be prepared by the reaction of D₂SO₄ dissolved in water over BaO₂.

BaO₂ + D₂SO₄ → BaSO₄ + D₂O₂

Heavy hydrogen or deuterium was separated from liquid hydrogen by fractional evaporation by H.C. Urey. For this great contribution, he was awarded the Noble Prize in Chemistry in 1934.

Degree of hardness is defined as the number of parts of calcium carbonate or equivalent to various calcium and magnesium salts present in a million parts of water by mass. It is expressed in ppm.

The region of the periodic table from groups 7-9 which do not form hydrides is called the hydride gap.

Mixtures of CO and H₂ are called syngas. It is so named because it is used for the synthesis of methanol and a number of hydrocarbons.

It is because Ca²⁺ and Mg²⁺ ions present in hard water react with soap to form Ca²⁺ and Mg²⁺ salts of fatty acids which are insoluble in water. 

Advantages of soap over detergent; soap are biodegradable where as detergents are non-biodegradable.

(a) Carbon monoxide and dihydrogen

(b) C_nH_2n+2 + nH₂O \(\overset{1270\,K}{\underset{Ni}{\longrightarrow}}\) nCO + (2n + 1)H₂

(c) Synthetic gas or syngas or water gas.

(d) CO(g) + H₂O(l) \(\overset{673\,K}{\underset{FeCrO_4}{\longrightarrow}}\) CO₂(g) + H₂(g)

Water-gas shift reaction is the special name of this reaction.

Atomic number 17 = Chlorine, and Atomic number 20 = Calcium. 

(b) H₂ + Cl₂ → 2HCl 

Cl₂ is oxidizing agent, H₂ is reducing agent. 

Ca + H₂ → CaH₂

(a) Coal gas, water gas and liquid hydrogen are some significant fuel.

(b) Since hydrogen is lighter than air. it is possible to collect the gas by downward displacement of air. But it is not safe to do so since a mixture of hydrogen and air can lead to an explosion.

(c) Impure hydrogen gas bums in air with a pop sound. This is because of the presence of impurities in it.

(d) If there is a small leakage of hydrogen in a balloon, it forms a mixture with air that can explode. So helium has replaced hydrogen.

(e) Hydrogen cannot be prepared by the action of nitric acid on metals because it also releases nitrous oxide and nitric oxide and oxides the hydrogen to form water.