Solubility of solid solutes in liquid depends on the following factors:
(i) Nature of solute. Dissolution of solid solutes in liquids can be summed up in a phrase ”like dissolves like”.
This means that, polar solutes dissolve in polar solvents and non-polar solutes dissolve in non-polar solvents. For example, ionic substances such as NaCl, KCl, KN03, etc., have larger solubilities in polar solvents like water but they have poor solubilities in non-polar solvents like CC14, CS2, etc. On the other hand, non-polar solids such as !2, S8, etc., are more soluble in non-polar solvents like CC14, CS2 but they are almost insoluble in polar solvents like water.
(ii) Temperature. Saturated solution represents equilibrium between undissolved solute and dissolved solute
If the value of ΔSo1H <. 0; i.e., the solution process Is exothermic, then by Le-Chatelier’s Principle, the solubility of such a solute will decrease with the rise in temperature. On the other hand, if B sol H > 0; i.e., solution process is endothermic, then, solubility of such a solute will increase with rise in temperature.
Measurement of solubility at different temperatures
The solubility can be measured at different temperatures by repeating Activity 28.1 using saturated solutions.
An alternative method of finding the solubility of a substance is to prepare an unsaturated solution of known concentration at higher temperature. The solution is then cooled and the temperature at which crystals begin to form in the solution is recorded. This gives the solubility of the substance at that temperature. This method is used in Activity 28.2.
The graphs obtained by plotting solubility of a solid vs temperature are referred to as solubility curves. The various ionic substances can be divided into three categories on the basis of effect of temperature on solubility.
(i) Solids whose solubility increase continuously and gradually with rise in temperature. This happens in case of solids whose Δ Sol H is > 0. Some examples are NaCI, KCl, KNO3, NaNO3 and hydrated salts.
(ii) Solids whose solubility decrease gradually with rise in temperature. This happens in case of solids whose d Δ sol < 0. Li2 SO4 and anhydrous salts like CuSO4, Na2SO4 are common examples of this category.
(iii) Solids whose solubility does not increase or decrease regularly or continuously. This happens if a substances undergoes a change from one polymeric form to another at particular temperature. For example, dissolution of N2SO4.10Hp is endothermic but dissolution of Na2SO4 is exothermic. The conversion of N2SO4.10Hp to N2SO4 occurs at 34°C. Thus, solubility of N2SO4.10Hp increases with the rise in temperature till34°C and thereafter it starts decreasing. The temperature at which this reversal of solubility occurs is often called transition temperature. The solubility curves of some substances are shown in Fig. 28.1
Fig. 28.1. Solubility curves of some substances.
INFORMATIONS CONVEYED BY SOLUBILITY CURYE
In order to understand the information that we can obtain from the solubility curve, consider the solubility for the dissolution of A as shown in Fig. 28.2. The curve shows
- Solubility of A increases with increase in temperature.
- Any point on curve (say y) represents saturated solution.
- The corresponding point a represents the solubility at temperature t1.
- Increasing temperature from t1 àt2 till point X means that the solution becomes unsaturated.
- Decreasing temperature from t1 à t3 results in the decrease of solubility to b mol dm-3, and the amount of solute equal to (a-b) moles separate out as crystals.
- Since the solubility increases with rise in temperature, it means Δ sol H > 0 i.e., solution process is endothermic.