Solubility Curves

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Reading a Solubility Curve

Background Information: Solubility refers to the maximum amount of solute that will dissolve in a given amount of solvent at a speciﬁed temperature and pressure. There are three different types of solutions that will result depending on how much of a particular solute is dissolved in the solvent. • Saturated Solution • Contains the maximum amount of dissolved solute for a given amount of solvent at a speciﬁc temperature and pressure • Unsaturated Solution • Contains less dissolved solute for a given temperature and pressure than a saturated solution • Supersaturated Solution • Contains more dissolved solute than a saturate solution at the same temperature • To form a saturated solution, dissolve the solute in a solution at a high temperature and cool slowly • Under normal temperature and pressure, you would have some solute left in solution that wouldn’t dissolve, but by increasing the temperature, the solute will dissolve in the solution All of this information will help you read a solubility curve, which plots different amount of solutes that dissolve in 100g (or 100 mL) of water at particular temperatures. When ready the curve of a particular solute, it is important to see at which temperature the solute creates a saturated solution (on the line), supersaturated solution (above the solute’s solubility curve), or an unsaturated solution (below the solute’s solubility curve). See the illustration below for an example:

Let’s look at the NaNO3 (sodium nitrate) solubility curve. • At 55°C, a saturated solution would contain 120 g of solute, NaNO3. (Notice, this point is exactly on the solute’s curve). • If, for example we mixed in 140 g of NaNO3 instead of 110 g of solute at 55°C, we would create a SUPER saturated solution, because we would be at a point ABOVE the solute’s solubility curve. • Another example using the same curve, is not putting in enough of the solute to create a saturated solution. Say we only mix in 110 g of NaNO3, we would be at a point BELOW the solute’s solubility curve, thus a UNsaturated solution. Understanding how to read a solubility curve allows for many different types of questions that could be asked of you. You could, for example ﬁnd at what temperature you could a create a saturated solution for a particular solute. Since a solubility curve only shows you the amount of solute that dissolves in 100g (or 100 mL) of water, there is a handy calculation on how to ﬁnd the amount needed when dealing with a situation that requires more or less of a solution. Follow the calculation below: Solubility Start with known volume × =Amount of Solute needed to saturate 100mL of H2O Let’s look at an example of this calculation using the curve on pg. 1: • At 10oC, 80 g of NaNO3 will dissolve in 100 mL (a saturated solution) • Say we only needed 60 mL of a NaNO3 solution.

60mL H2O 80g NaNO3 × = 48g of NaNO3 100 mL H2O

• If the chart is in units of 100 g of H2O use the density of water conversion: !1 mL H2O = 1 g H2O

60mL H2O 1 g H2O 80g NaNO3 × × = 48g of NaNO3 1 mL H2O 100 g H2O

Other Useful Information: Solutes whose curves move upward with increased temperature are typically solids because the solubility of solids increases with increased temperature.

Solutes whose curves move downward with increased temperature are typically gases because the solubility of gases decreases with increased temperature.

The steeper the incline of a solute, the more soluble the solute is because it doesn’t take as much of a temperature increase to dissolve the substance. This also means that more of the solute can be dissolved versus another substance at the same temperature. Therefore, by the same rule, the ﬂatter the line, the less soluble the solute it, because it takes a larger temperature in order for a solute to dissolve compared to a steeper inclined substance.