Solubility Curves

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The solubility of a given solute in water, as done above, can be determined at many different temperatures, say 0^oC, 10^oC, 20^oC,…..100^oC. The result so obtained can be plotted on a graph by taking temperatures along the X-axis, and solubility along the Y-axis. By joining the points so obtained, you get a curve, called the solubility curve. The solubility curves of six solutes in water are given in the graph.

Observe the graph carefully and notice the following, regarding the solubility of these six solutes in water.

a) Sodium nitrate has the highest solubility at 0^oC.

b) Potassium chlorate has the lowest solubility at 0^oC.

c) Potassium nitrate has the highest solubility at 100^oC.

d) The solubility of potassium nitrate is low at 0^oC but increases drastically with increase in temperature.

e) The solubility of sodium chloride is very slightly affected by the increase in temperature; the increase in solubility is only 4.1 grams from 0^oC to 100^oC.

Advantages of Solubility Curves

a) It gives a general idea about the ability of a solute to dissolve in water, at different temperatures.

b) The solubility of the solute at any temperature, between the given lowest and highest temperatures, can be more or less accurately read.

c) The mass of crystals deposited, when a saturated solution so cooled from a higher to a lower temperature, can be calculated.

For e.g.,

As per the given graph, the solubility of potassium nitrate at 60^oC= 110 g and at 50^oC = 91 g.

Hence, by cooling a saturated solution of potassium nitrate containing 100 g of water from 60^oC to 50^oC, 19 g of the solute (110 g - 91 g) is deposited in the form of crystals.