Qualitative Effect of temperature on solubility
We know that the solubility of any salt is directly proportional to the temperature of a solution but this effect is not uniform in the case of various salts. This is due to different nature of constituent ions of salt with solvent water. When salt dissolves in water then partially positive part of water attracts negative part of the salt and the partially negatively charged oxygen side attracts the positive part of the salt. These forces play an important role to separate ions of the salt. Each salt has unique inter–ionic forces. Therefore, the solubility of each salt is different in similar solvents, like water at different temperatures.
We
can understand this behaviour of different salt with the help of solubility
curves of few salts.
Solubility of KNO3 in water
On the basis of solubility curve in Figure, we can say that potassium nitrate is less soluble in cold water but it is highly soluble in hot or boiling water. In the graph, y-axis shows the solubility in grams (gms) per 100 mL (gm/100 mL) of water and x-axis shows the temperature in Celsius. From the graph, we can find that at 0°C the solubility of potassium nitrate is 13 gm/100 mL which gradually increases with rising in temperature and reaches 240 gm/ 100 mL at 100°C.
Solubility of NaCl in water
On
the basis of the solubility curve in Figure, we can say that sodium chloride is
soluble in cold water but its solubility does not change highly with the
increase in temperature like potassium nitrate. This can be seen in the
following graph where y-axis shows the solubility in grams (gms) per 100 mL (gm/100
mL) of water and x-axis shows the temperature in Celsius. From the graph, we
can find that at 0°C the solubility of sodium chloride is 35.65 gm/100 mL which
gradually increases with rising in temperature and reaches 38.99 gm/ 100 mL at
100°C.
Solubility of CaSO4 in water
On the basis of the solubility curve in Figure, we can say that the solubility of calcium
sulphate shows entirely different behaviour with respect to other salts. On the
basis of the graph, we can observe initially the solubility increase with
temperature but above 40°C it decreases gradually. From the graph, we can find
that at 0°C the solubility of calcium sulphate is 0.223 gm/100 mL which
gradually increases with rise in temperature and reaches 0.265 gm/ 100 mL at
40°C and after this it starts decreasing and at 100°C it is 0.205 gm/100 mL.
This observation can be explained on the basis of the interaction of hydrated
calcium sulphate with water which is an exothermic
process. Exothermic change is opposed by high temperature.
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