Page 72 - Chemistry ICSE Class IX

3 Water

Water
CONTENTS
Molecular formula H O Structure of
2
ͻ tĂƚĞƌ Ͷ Ă ĐŚĞŵŝĐĂů ĐŽŵƉŽƵŶĚ Molar mass 18 g/mol water molecule
ͻ tĂƚĞƌ ĂƐ Ă ƐŽůǀĞŶƚ
ͻ WŚLJƐŝĐĂů ƉƌŽƉĞƌƟĞƐ ŽĨ ǁĂƚĞƌ
ͻ dŚĞ ĂŶŽŵĂůŽƵƐ ďĞŚĂǀŝŽƵƌ ŽĨ ǁĂƚĞƌ Water is the most abundant and inexhaustible natural resource. It is the
ͻ ^ŽůƵƟŽŶƐ only chemical substance which is present on the earth simultaneously as a
ͻ ŝƌ ĚŝƐƐŽůǀĞĚ ŝŶ ǁĂƚĞƌ ĂŶĚ ŝƚƐ gas (water vapour), a liquid (water) and as a solid (ice, snow).
ďŝŽůŽŐŝĐĂů ŝŵƉŽƌƚĂŶĐĞ Water plays a vital role in various physical and chemical processes in
ͻ ^ŽůƵďŝůŝƚLJ biological systems. In fact/water is essential for the sustenance of plant
ͻ īĞĐƚ ŽĨ ƚĞŵƉĞƌĂƚƵƌĞ ĂŶĚ ƉƌĞƐƐƵƌĞ and animal life.
ŽŶ ƚŚĞ ƐŽůƵďŝůŝƚLJ
ͻ ,LJĚƌĂƚĞĚ ƐƵďƐƚĂŶĐĞƐ How does water occur in nature
ͻ ŶŚLJĚƌŽƵƐ ƐƵďƐƚĂŶĐĞƐ Water is widely distributed in nature, both in the free as well as in the
ͻ ŋŽƌĞƐĐĞŶĐĞ combined forms. Free water occurs as a solid (ice and snow), liquid (water)
ͻ ,LJŐƌŽƐĐŽƉŝĐ ƐƵďƐƚĂŶĐĞƐ and gas (water vapour). Distribution of water in nature is described below:
ͻ ĞůŝƋƵĞƐĐĞŶƚ ƐƵďƐƚĂŶĐĞƐ
ͻ ^ŽŌ ǁĂƚĞƌ ĂŶĚ ŚĂƌĚ ǁĂƚĞƌ 1. In solid state
ͻ ŚĞŵŝĐĂů ƉƌŽƉĞƌƟĞƐ ŽĨ ǁĂƚĞƌ Water in solid state exists as ice and snow on the peaks of high mountains
ͻ ĐƟǀŝƚLJ ƐĞƌŝĞƐ ŽĨ ŵĞƚĂůƐ ĨƌŽŵ ƚŚĞ
ĂĐƟŽŶ ŽĨ ǁĂƚĞƌ ŽŶ ƐŽŵĞ ŵĞƚĂůƐ and in the polar regions. In Antarctica, water exists mostly as ice and snow.
ͻ /ŵƉŽƌƚĂŶĐĞ ĂŶĚ ƵƐĞƐ ŽĨ ǁĂƚĞƌ
2. In liquid state
In liquid form, water occurs in oceans, seas, rivers, lakes, ponds, springs,
wells, etc. Liquid water covers more than two-thirds of the earth’s surface.
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The total ocean water is about 3 × 10 cubic metres. Animal and plant
tissues also contain water. About 75% of the body mass of animals is due
to the presence of water in their tissues.
3. In gaseous state
In gaseous state, water exists as water vapour or steam. The earth’s
atmosphere contains a large amount of water vapour. This makes the
atmosphere humid.
4. In the form of water of crystallisation
Water also exists in the combined form as water of crystallisation in a large
number of crystalline salts and minerals. Some typical salts which contain
water of crystallisation are:
ƒ Copper sulphate pentahydrate or Copper(II) sulphate pentahydrate
(CuSO .5H O)
4
2
ƒ Iron sulphate heptahydrate or Iron(II) sulphate heptahydrate
(FeSO .7H O)
2
4
ƒ Borax or Sodium borate decahydrate (Na B O .10H O)
7
2 4
2
In our country, both the surface and the ground water are available.
This water comes from the rainfall and snow as a part of the water cycle
or hydrological cycle.