As the temperature of a liquid increases, the solubilities of gases in that liquid decrease. We can use the Second Law of Thermodynamics to explain why. Heating a solution of a gas enables the particles of gas to move more freely between the solution and the gas phase. The Second Law predicts that they will shift to the more disordered, more highly dispersed, and therefore, more probably gas state.

In a similar way, we can explain why the substances that are solids at room temperature and pressure tend to become more soluble when the temperature rises. Heating a solution of a solid makes it easier for the particles of solid to move between the solution and the solid phase. The Second Law predicts that they will shift to the more disordered, more highly dispersed, and therefore, more probably solution state.

The table below shows the relationship between temperature and solubility for several substances. Notice that substances that are gases at room temperature and pressure become less soluble with increased temperature, and the substances that are solids at room temperature and pressure tend to become more soluble when the temperature rises.

 

Temperature and Water Solubilities in Grams of Solute per 100 mL of Water

 

Temperature ºC	Solid citric acid	Solid potassium phosphate	Gaseous nitrogen	Gaseous oxygen
0	49	44	0.0030	0.0070
20	59	50	0.0020	0.0050
30	64	*	*	*
50	71	62	*	0.0031
70	76	*	*	*
100	84	*	*	0.0029

*Not available

 

EXAMPLE  - Predicting the Effect of Temperature on Solubility:

Would you expect the water solubility of the following substances to be higher or lower at 100 °C than at 20 °C?

a. acetaminophen (the pain reliever in Tylenol) – melting point = 169-170 °C

b. dimethyl ether (used as a refrigerant) – melting point = -138.5 °C, boiling point = -25 °C

Solution:

a. Higher – Because acetaminophen is a solid at room temperature, we expect its solubility to increase with increasing temperature.

b. Lower – Because dimethyl ether is a gas at room temperature, we expect its solubility to decrease with increasing temperature.