Page 82 - Chemistry ICSE Class IX

70 ICSE Chemistry – 9

Effect of pressure on the solubility of a gas in a liquid
The effect of pressure on the solubility of a gas in a liquid (e.g. water) is
given by the Henry’s law. The Henry’s law is stated as follows:
At any given temperature, the mass of a gas dissolved in a certain
ſZGF XQNWOG QH C NKSWKF QT UQNXGPV KU FKTGEVN[ RTQRQTVKQPCN VQ VJG
pressure of the gas over the surface of the liquid.
The Henry’s law is applicable to gases which do not react chemically
with the solvent. Therefore, the Henry’s law will not apply to the solution
of gases like ammonia (NH ) and hydrogen chloride (HCl) in water because
3
these gases react chemically with water.
–
+
NH (g) + H O q NH OH(aq) NH (aq) + OH (aq)
q
4
2
3
4
+
–
HCl(g) + H O o H O (aq) + Cl (aq)
3
2
A typical application of the Henry’s law in industry is the preparation
of aerated water/cold drinks.
Carbon dioxide is fairly soluble in water at room temperature. Its
solubility in water can be increased by increasing the pressure of carbon
dioxide. This is the way carbonated drinks, such as soda water, are
manufactured/prepared.
Why does a chilled soda water bottle fizz out when
opened
Soda water is a solution of carbon dioxide gas [CO (g)] in water. Soda
2
water bottle is sealed when the CO gas is under high pressure. Because of
2
high pressure and low temperature (because it is chilled), a large amount
of gas gets dissolved in water. As soon as the bottle is opened under
Fig. 3.9 A soda water bottle when normal atmospheric conditions, the dissolved gas escapes out to reach a
opened ¿zz out new equilibrium state required by the lower pressure (the atmospheric
pressure). At lower pressure, solubility of gas in water decreases.
CO (in soda water) CO (in soda water) + CO (g)
q
2
2
2
under high pressure, the at lower pressure the excess CO gas
2
amount of the dissolved (atmospheric pressure), escapes out of soda
CO is large the solubility of CO in water in the form of zz
2
2
water decreases
However, if the soda water bottle is left open to air for some time,
most of the carbon dioxide escapes out and it turns ƀCV.
Effect of temperature on the solubility of a gas in a liquid
The solubility of a gas in a liquid decreases when its temperature is
dŚĞ ƐŽůƵďŝůŝƚLJ ŽĨ ƐŽůŝĚƐ ŝƐ ŶŽƚ increased. This is because at higher temperatures, the kinetic energy of the
ĂīĞĐƚĞĚ ŶŽƟĐĞĂďůLJ ǁŝƚŚ Ă ĐŚĂŶŐĞ ŝŶ
ƉƌĞƐƐƵƌĞ͘ gas molecules increases. As a result, the gas molecules are able to overcome
the binding forces and escape out of the liquid.
How does the solubility of a solid substance in a liquid
(say water) vary with temperature

In general, the solubility of a solid in any liquid (called solvent) increases
with a rise in temperature.
However, there are certain cases where this general trend is not
observed. The solubility curves of some such cases are described below:
ƒ The solubility of sodium nitrate, potassium nitrate, lead nitrate,
potassium chlorate, potassium chloride, potassium sulphate, etc. in
water increases with a rise in temperature.