Question

Answer the following questions with the help of the data given in the Table.

Solubility of various salts (in g per 100 g of water)  at different temperatures
Salts	Temperature (°C)
	10°C	20°C	30°C	40°C	60°C	80°C
Potassium nitrate	21	32	45	62	106	167
Sodium chloride	36	36	36.3	36.5	37	37
Potassium chloride	35	35	37.4	40	46	54
Ammonium chloride	24	37	41	41	55	66

1. What mass of potassium nitrate would be needed to prepare its saturated solution in 50 g of water at 40°C?
2. A student makes a saturated solution of potassium chloride in water at 80°C and leaves the solution to cool at room temperature (25°C). What would she observe as the solution cools? Explain.
3. What is the effect of a change in temperature on the solubility of salts? Also, compare the changes in the solubility of the four given salts with increasing temperature from 10°C to 80°C.

Answer 1

(i) Based on the Table, the solubility of potassium nitrate at 40°C is 62 g per 100 g of water.

Mass of potassium nitrate needed for 50 g of water

= `62/100 xx 50 = 31  "g"`

Thus, in order to make a saturated solution in 50 g of water at 40°C, 31 g of potassium nitrate are required.

(ii) The solubility of potassium chloride is 54 g per 100 g of water at 80°C, but it decreases to about 36 g per 100 g of water at 25°C. When the hot saturated solution is cooled, its ability to hold dissolved solute reduces. As a result, the excess potassium chloride can no longer remain dissolved and separates out of the solution as crystals.

(iii) In general, the solubility of solid salts in water increases as the temperature rises.

From 10°C to 80°C, the change in solubility is as follows:

1. Potassium nitrate: increases from 21 to 167, a rise of 146 g, which is the highest and shows a very steep increase.
2. Ammonium chloride: increases from 24 to 66, an increase of 42 g.
3. Potassium chloride: increases from 35 to 54, an increase of 19 g.
4. Sodium chloride: increases from 36 to 37, an increase of only 1 g, showing almost no change.

As a result, sodium chloride has the least change in solubility with temperature, whereas potassium nitrate exhibits the largest rise. The order of increase is Potassium nitrate > Ammonium chloride > Potassium chloride > Sodium chloride.