(POS) Solubility Curves Lesson

Solubility Curves

A common way to express the solubility of a given solute-solvent pair is to state the maximum amount of solute that can be dissolved by 100 grams of the solvent. The temperature dependence of the solubilities for various compounds in water is shown in the graph below.

The aqueous solubility of a given substance is determined experimentally by dissolving increasing amounts into a known mass of water at a specific temperature until no more solid dissolves. A solution that cannot hold any more of a given solute is said to be saturated. For example, in the graph above we see that the solubility of sodium nitrate (NaNO₃) is approximately 90 g per 100 g of H₂O at 20°C. At this temperature, a 100 gram sample of H₂O in which 90 grams of sodium nitrate is dissolved would be saturated, and a solution in which less sodium nitrate is dissolved would be unsaturated. Note that this ratio also holds for samples in which the solvent is present in different amounts; 50 grams of water would hold 45 grams of sodium nitrate at 20°C, and 300 grams of water would hold 270 g of NaNO₃. Notice that even relative solubilities of various compounds are temperature dependent. For example, at 20°C, KCl has a higher solubility than NaCl, but at 50°C, this relationship is reversed.

We can quantify supersaturation by looking at solubility curves. If the ratio of solute to solvent is above the saturation curve at the given temperature, the solution is supersaturated. If it is on the curve, the solution is saturated, and if it is below the curve, the solution is unsaturated.

Practice Problems

Can you use the graph above to answer the following questions?  You will be required to read and interpret solubility graphs.

• Describe conditions under which a solution of ammonium chloride (NH₄Cl) would be unsaturated, saturated, and supersaturated at 50°C.

At 50°C, adding 30 grams of NH₄Cl would be unsaturated. Any amount of solute added under the saturation line will be unsaturated, meaning that more solute can be dissolved into the solvent (water).

At 50°C, adding 50 grams of NH₄Cl would be saturated. This is the point following on the line indicating the maximum amount of solute that will dissolve into the solvent (water).

At 50°C, adding 70 grams of NH₄Cl would be supersaturated. Any amount of solute added above the saturation line will be supersaturated.

Look at the picture below to see how to find these answers on the graph:

Activity

It is now time to practice reading and interpreting a solubility curve. Test your understanding of solubility curves by answering the questions below.

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