Page 73 - Chemistry ICSE Class IX
P. 73
Water 61
Water — A Chemical Compound
Why is water considered a compound
Water is a compound containing hydrogen and oxygen in the
ratio of 2 : 1 by volume and 1 : 8 by mass. The molecular formula
of water is H O.
2
Water is a compound because
Water is obtained by the chemical combination of hydrogen
and oxygen in a xed ratio, 2 : 1 by volume and 1 : 8 by
mass. Fig. 3.1 Water cycle in nature
During the formation of water from hydrogen and oxygen,
energy is given out. i.e., formation of water from hydrogen and oxygen
is an exothermic reaction.
H (g) + 1 2 O (g) o H O(l) + 286 kJ
2
2
2
The properties of water are entirely different from that of its constituent
elements, hydrogen and oxygen.
Hydrogen is combustible and oxygen is a supporter of combustion.
Water is neither combustible nor a supporter of combustion.
Water cannot be separated into its constituent elements, hydrogen
and oxygen, by any physical method. Water can only be separated
into its constituent elements by electrolytic decomposition (i.e., by
electrolysis).
How will you show that water is a polar compound
Suspend a burette on a burette-stand. Fill it with distilled water and place
a beaker (250 mL) under the jet of the burette. Allow a thin stream of water
to run out of the burette. The stream runs out of the jet almost vertically
downwards.
Rub a piece of thermocol against a sheet of annel and bring the
rubbed edge of the thermocol near the running stream of water. The stream
of water gets attracted towards the thermocol piece. This can be explained
as follows.
Explanation: When thermocol is rubbed against annel, it develops a
negative charge on it. When this negatively charged thermocol is brought
nearer to the stream of water, the water stream gets attracted towards it
(Fig. 3.2). This shows that water molecules have some positive charge on
them.
Fig. 3.2 Experiment to show that water is a polar compound
