In water molecule, O is sp³ hybridized. Due to stronger lone pair-lone pair repulsion than bond pair-bond pair repulsions, the HOH bond angle decreases from 109.5° to 104.5°. Thus water is bent molecule.

(a) Dihydrogen is prepared from water by the action of alkali metals like Na and K which is a strong reducing agent.
2Na + 2H₂O \(\longrightarrow\) 2NaOH + H₂
2K + 2H₂O \(\longrightarrow\) 2KOH + H₂
(b) For welding purposes.
\({ H }_{ 2 }\left( g \right) +\frac { 1 }{ 2 } { O }_{ 2 }\left( g \right) \longrightarrow { H }_{ 2 }O\left( g \right) +heat\)

The aqueous solution of H₂O₂ is weakly acidic in nature.
H₂O₂ + H₂O \(\rightleftharpoons \) H₃O⁺ + \(HO^-_2\)
It gives two types of salts with alkalies, peroxides and hydroperoxides.
2NaOH + H₂O₂ \(\longrightarrow\) NaO₂ + 2H₂O
\(NaOH+{ H }_{ 2 }{ O }_{ 2 }\longrightarrow \underset { Sodium\\ hydroperoxide }{ { NaOH }_{ 2 } } +{ H }_{ 2 }O\)
MnO₂ and finely divided metals like Pt and Fe catalyse the decomposition of H₂O₂.

As oxidising agent.
2I^- + H₂O₂ + 2H⁺ \(\longrightarrow\) I₂ + 2H₂O
As reducing agent.
H₂O₂ + Ag₂O \(\longrightarrow\) 2Ag + H₂O + O₂

Water is amphoteric in nature because it acts as an acid
\(\underset { Base \ 1 }{ { H }_{ 2 }O(l) } +\underset { Acid\ 2 }{ { H }_{ 2 }S(aq) } \longrightarrow \underset { Acid \ 1 }{ { H }_{ 3 }{ O }^{ + }(aq) } +\underset { Base \ 2 }{ { HS }^{ - }(aq) } \)
\(\underset { Acid \ 1 }{ { H }_{ 2 }O(l) } +\underset { Base \ 2 }{ { NH }_{ 3 }(aq) } \longrightarrow \underset { Acid \ 2 }{ { NH }_{ 4 }^{ + }(aq) } +\underset { Base \ 1 }{ { OH }^{ - }(aq) } \)

Cation exchange resins have large organic molecule with SO₃H group which are insoluble in water. Ion exchange resin (RSO₃H) is changed to RNa on treatment with NaCl. The resin exchange Na⁺ ions with Ca²⁺ and Mg²⁺ ions present in hard water and make it soft.
\(2RNa(s)+{ M }^{ 2+ }(aq)\longrightarrow { R }_{ 2 }M(s)+2{ Na }^{ + }(aq)\)
where, M = Mg, Ca.
The resins can be regenerated by adding aqueous NaCl solution.

Ice crystallizes in the normal hexagonal form. However, at very low temperatures it condenses in cubic form. In the normal hexagonal ice each oxygen atom is tetrahedrally surrounded by four other hydrogen atoms.

(i) Electron deficient hydrides: Compounds in which central atom has incomplete octet, are called electron deficient hydrides. For example, BeH₂, BH₃ are electron deficient hydrides.
(ii) Electron precise hydrides: Those compounds in which exact number of electrons are present in central atom or the central atom contains complete octet are called precise hydrides e.g., CH₄, SiH₄, GeH₄ etc. are precise hydrides.
(iii) Electron rich hydrides: Those compounds in which central atom has one or more lone pair of excess electrons are called electron rich hydrides" e.g., NH₃, H₂O.

Due to this property hydrogen is used in atomic hydrogen welding/cutting torch.

\(CO(g)+2{ H }_{ 2 }(g)\overset { 700k,200atm }{ \underset { ZnO,Cr{ O }_{ 3 } }{ \rightarrow } } \underset { Methanol \ or \ methyl \ alcohol }{ { CH }_{ 3 }OH(l) } \)

Anhydrous \(BaO_{ 2 }\) is not used because the \(BaSO_{ 4 }\)formed during the reaction forms a protective layer around unreacted \(BaO_{ 2 }\) and the reaction stops after sometime.

Hydrogen as a fuel has the following advantages.
(i) It has calorific value.
(ii) During combustion, it does not produce smoke or any unpleasant fumes.
(iii) It leaves no ash after burning. The only product of combustion is water.
(iv) It can be used in a fuel cell to generate electricity.
(v) It can be used in the internal combustion engines with slight modifications.
(vi) It does not pollute the air because no pollutant is produced during its combustion.

100 mL of \({ H }_{ 2 }O_{ 2 }\)solution contains \({ H }_{ 2 }O_{ 2 }\) = 3g
\(\therefore \)1000 mL of \({ H }_{ 2 }O_{ 2 }\)solution will contains
\({ H }_{ 2 }O_{ 2 }=\frac { 3 }{ 100 } \times 1000=30g\)
Consider the chemical equation,
\(\underset { 2\times34 \ = \ 68g }{ 2H_{ 2 }{ O }_{ 2 } } \rightarrow \underset { 22.7 \ L \ at \ NTP }{ { 2H }_{ 2 }{ O }+{ O }_{ 2 } } \)
Now 68 g of \({ H }_{ 2 }{ O }_{ 2 }\) gives \({ O }_{ 2 }\)at NTP = 22.7 L
\(\therefore \)30 g of \({ H }_{ 2 }O_{ 2 }\)will give \(O_{ 2 }\)at NTP = \(\frac { 22.7 }{ 68 } \times30\approx 10.014\)
But 30g of \({ H }_{ 2 }O_{ 2 }\) are present in 1000 mL of \({ H }_{ 2 }O_{ 2 }\)
Hence, 1000mL of \({ H }_{ 2 }O_{ 2 }\) solution gives \(O_{ 2 }\)at NTP
\(=\frac { 10014 }{ 1000 } =10.01mL\)
Hence, the volume strength of 3%\({ H }_{ 2 }{ O }_{ 2 }\) solution = 10.01

\(\underset { Peroxydisulphuric \ acid }{ HO_{ 3 }SOOSO_{ 3 }H } (aq)\overset { { H }_{ 2 }O }{ \rightarrow } 2HSO_{ 4 }^{ - }(aq)+2{ H }^{ + }(aq)+\underset { Hydrogen \ peroxide }{ { H }_{ 2 }{ O }_{ 2 }(aq) } \)

Given CaCl₂ present in 10³ g = 12 mg
CaCl₂ Present in 10⁶g = 12g
∴ MgCl₂ present in 10⁶g = 12g
Now, 1 mol (111g) CaCl₂ = 1mol (100g) CaCo₃
∴ 12g CaCl₂ = \(\frac { 100\times 12 }{ 111 } =10.81g\)
Similarly 1 mol (95g) MgCl₂= 1 mol (100g)CaCo₃
∴ 12gMgCl₂\(\frac { 100\times 12 }{ 95 } =12.63g\)
Total hardness in terms of CaCO₃ = (10.81+12.63)
= 23.44 ppm