Exploring Mixtures and their Separation · Lesson 4 of 13
Solubility of Substances
“Some solids love warm water; others barely notice the temperature change.”
• Define solubility at a stated temperature. • Distinguish saturated and unsaturated solutions. • Explain the usual temperature trends for solids and gases. • Read and compare solubility curves. • Calculate the mass deposited when a hot saturated solution cools.
Continue adding sugar to a glass of water and stirring. At first every spoonful disappears. Eventually some sugar remains at the bottom, no matter how long you stir. The water has reached a limit under those conditions. Warm the mixture, and more sugar may dissolve; cool it again, and some may return as solid crystals.
Solubility is the maximum amount of a solute that can dissolve in a fixed quantity of solvent, commonly 100 g or 100 mL, at a given temperature.
A saturated solution contains the maximum amount of dissolved solute possible at a given temperature and cannot dissolve more of that solute under the same conditions.
An unsaturated solution contains less dissolved solute than the maximum possible at that temperature and can dissolve more solute.
Temperature must accompany a solubility value because solubility can change when temperature changes. For many solid solutes in liquid solvents, solubility increases as temperature rises. Gases commonly show the opposite trend: their solubility in liquids generally decreases as temperature rises.
Activity: Let Us Represent Solubility Graphically
A graph of solubility against temperature is called a solubility curve. Temperature is placed on the horizontal axis, while the vertical axis shows grams of solute that can dissolve in 100 g water. Each point gives the maximum amount at one temperature.
If the curve for compound B lies above the curve for compound A at a selected temperature, more B than A can dissolve in the same mass of water at that temperature. If B rises more steeply, its solubility is more strongly affected by temperature. A nearly horizontal curve indicates comparatively little change.
Problem
A saturated solution contains 287 g compound B in 100 g water at 60 °C. At 40 °C, its solubility is 241 g per 100 g water. Find the mass deposited.
- 1.At 60 °C, 287 g is dissolved in 100 g water.
- 2.At 40 °C, only 241 g can remain dissolved in the same 100 g water.
- 3.Mass that can no longer remain dissolved = 287 g − 241 g.
- 4.Mass deposited = 46 g.
- 5.The calculation assumes that the water amount remains 100 g during cooling.
Problem
Potassium nitrate has a solubility of 62 g per 100 g water at 40 °C. How much is needed for 50 g water?
- 1.The table gives 62 g solute for 100 g water.
- 2.Fifty grams water is half of 100 g.
- 3.Required solute = 62 × 50 ÷ 100.
- 4.Required solute = 31 g.
- 5.Therefore, 31 g potassium nitrate makes a saturated solution in 50 g water at 40 °C.
| Salt | Solubility at 10 °C | Solubility at 80 °C | Change |
|---|---|---|---|
| Potassium nitrate | 21 g | 167 g | Very large increase |
| Sodium chloride | 36 g | 37 g | Very small increase |
| Potassium chloride | 35 g | 54 g | Moderate increase |
| Ammonium chloride | 24 g | 66 g | Large increase |
Problem
A saturated potassium chloride solution made at 80 °C is cooled to about 20 °C. What happens?
- 1.At 80 °C, 54 g potassium chloride can dissolve in 100 g water.
- 2.Near 20 °C, only about 35 g can remain dissolved in 100 g water.
- 3.The cooling solution temporarily contains more dissolved salt than it can retain at the lower temperature.
- 4.Excess potassium chloride separates as crystals.
- 5.The approximate deposited mass is 54 g − 35 g = 19 g per 100 g water.
Solubility is the maximum possible dissolved amount at a specified temperature. Concentration describes the actual amount present and may be below that maximum.
Quiz
Which description best matches Solubility?
Which description best matches Saturated Solution?
Which term matches this description: Solubility is the maximum amount of a solute that can dissolve in a fixed quantity of solvent, commonly 100 g or 100 mL, at a given temperature.
Which term matches this description: A saturated solution contains the maximum amount of dissolved solute possible at a given temperature and cannot dissolve more of that solute under the same conditions.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain why temperature must be stated with solubility.
- Compare the likely curve shapes of potassium nitrate and sodium chloride using the table.
- A saturated solution contains 80 g solute at a high temperature but only 52 g can remain dissolved after cooling. Find the deposited mass.
- Why do warm aerated drinks lose dissolved gas more readily than cold ones?
Key Takeaways
• Solubility is a maximum value at a stated temperature. • A saturated solution has reached that maximum. • Many solid solutes become more soluble on heating. • Dissolved gases generally become less soluble on heating. • Cooling a hot saturated solution can cause the excess solute to crystallise.