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Importance of Water (H2O) - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main

Quick Facts

Water: Structure of Water and Ice, Heavy Water is considered one the most difficult concept.
14 Questions around this concept.

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Which one of the following statements about water is FALSE?

Water is oxidized to oxygen during photosynthesis.

Water can act both as an acid and as a base

There is extensive intramolecular hydrogen bonding in the condensed phase

Ice formed by heavy water sinks in normal water.

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Water: Structure of Water and Ice, Heavy Water

Structure of Water
Oxygen atom in water molecule is sp³ hybridized, four hybrid orbitals directed towards the corners of a tetrahedral are formed. Two of the hybrid orbitals having one electron each overlap with two hydrogen atoms and form two sigma bonds. Thus, the molecule has a bent structure with a bond angle of 104.5^o. The bond angle is less than the expected angle in tetrahedron due to the presence of two lone pairs of electrons on two uncombined hybrid orbitals which repel each other and the bonded pairs cause them to come closer and thereby reducing the bond angle from 109^o28^' to 104.5^o.

Because of high electronegativity of oxygen, the O-H bonds are polar, i.e, oxygen is partially negatively charged and each hydrogen is partially positively charged The molecule is polar and possesses dipole moment. Molecules are linked with each other by hydrogen bonding.

Structure of Ice
Ice has a highly ordered three-dimensional hydrogen-bonded structure as shown in the figure given below. Examination of ice crystals with X-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm. Hydrogen bonding gives ice a rather open type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.

HEAVY WATER, D₂O

It is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries. It is used for the preparation of other deuterium compounds, for example

$\begin{aligned} & \mathrm{CaC}_2+2 \mathrm{D}_2 \mathrm{O} \rightarrow \mathrm{C}_2 \mathrm{D}_2+\mathrm{Ca}(\mathrm{OD})_2 \\ & \mathrm{SO}_3+\mathrm{D}_2 \mathrm{O} \rightarrow \mathrm{D}_2 \mathrm{SO}_4 \\ & \mathrm{Al}_4 \mathrm{C}_3+12 \mathrm{D}_2 \mathrm{O} \rightarrow 3 \mathrm{CD}_4+4 \mathrm{Al}(\mathrm{OD})_3 \end{aligned}$

Chemical Properties of Water

These are the various chemical properties of water:

Water is neutral in nature. pH of the pure water is 7. It is a weak electrolyte and ionises into H⁺ and OH^- ions.
$\mathrm{H_{2}O\: \rightleftharpoons \: H^{+}\: +\: OH^{-}}$
It reacts with active metals and evolves hydrogen The reaction is exothermic in the case of alkali and alkaline earth metals.
$\mathrm{2Na\: +\: 2H_{2}O\: \rightarrow \: 2NaOH\: +\: H_{2}}$
Reaction with non-metals: Water reacts with non-metals like fluorine and chlorine as follows:
$\mathrm{2F_{2}\: +\: 2H_{2}O\: \rightarrow \: 2H_{2}F_{2}\: +\: O_{2}}$
Action on non-metallic oxides: Acidic oxides combine with water to form acids viz:
$\\\mathrm{CO_{2}\: +\: H_{2}O\: \rightleftharpoons \: H_{2}CO_{3}}\\\mathrm{SO_{2}\: +\: H_{2}O\: \rightarrow \: H_{2}SO_{3}}$
Action on metallic oxides: Basic oxides combine with water to form alkalies.
$\mathrm{Na_{2}O\: +\: H_{2}O\: \rightarrow \: 2NaOH}$
Action on hydrides, carbides, nitrides, phosphides: Water decomposes these compounds with the evolution of hydrogen, acetylene, ammonia, phosphine respectively.
$\mathrm{CaH_{2}\: +\: 2H_{2}O\: \rightarrow \: Ca(OH)_{2}\: +\: 2H_{2}}$

$\mathrm{CaC_2 +2 H_2O \rightarrow C_2H_2 + Ca(OH)_2}$

$\mathrm{Ca_3N_2 +6 H_2O \rightarrow 2 NH_3 + 3 Ca(OH)_2}$

$\mathrm{Ca_3P_2 +6 H_2O \rightarrow 2 PH_3 + 3 Ca(OH)_2}$

Hydrolysis: Many salts undergo hydrolysis with water.
$\mathrm{CH_{3}COONa\: +\: \: H_{2}O\: \rightleftharpoons \: CH_{3}COOH\: +\: NaOH}$
Decomposition: Water containing either alkali or acid when electrolysed gets decomposed into H₂ and O₂.
$\mathrm{2H_{2}O\: \rightarrow \: 2H_{2}\: +\: O_{2}}$
Water of crystallisation: It combines with many salts during crystallisation to form hydrates. For example, CuSO₄.5H₂O, FeSO₄.7H₂O, etc.
Water as a catalyst: Water acts as a catalyst in many reactions. Perfectly dry gases generally do not react but the presence of moisture brings the chemical change. Ammonia and hydrochloric acid gas combine only in presence of moisture.