Solubility:
Solubility of a solute in a solvent is the maximum amount of a solute in moles or in grams that can dissolve/ saturate 1dm3 of solution at a particular temperature.
For example, sugar dissolving in water is a common illustration of solubility.
Solubility of a solute depends on several factors:
i. Nature of solute and solvent: “Like dissolves like” — polar substances dissolve in polar solvents, and non-polar in non-polar.
ii. Temperature: For most solid solutes, solubility increases with increase in temperature, while for gases solubility generally decreases with increase in temperature
iii. Pressure: Mainly affects gases; higher pressure increases the solubility of gases in liquids.
A solution can be:
i. Unsaturated : one that can still dissolve more solute at a particular temperature.
ii. Saturated : one that contains the maximum amount of solute it can hold at a particular temperature in the presence of undissolved solute particles
iii. Supersaturated : one that contains more solute than it can normally hold at a particular temperature (and is unstable).
Determination of Solubility
Solubility is commonly determined experimentally by preparing a saturated solution at a known temperature and then
Weigh the remaining solute.
Solubility is then calculated using:
Solubility= Mass of solute x 100
Mass of solvent
It is often expressed as grams of solute per 100 g of solvent.
A solubility curve is a graph that shows how the solubility of a substance changes with temperature.
To generate a solubility curve for salts like NaCl, Na₂SO₄, KNO₃, Na₂CO₃, and Ca(OH)₂, we usually rely on experimental data plotted as a graph.
Sample Solubility Data (g per 100 g of water)
| Temperature (°C) | NaCl | KNO₃ | Na₂SO₄ | Na₂CO₃ | Ca(OH)₂ |
|---|---|---|---|---|---|
| 0. | 36 | 13 | 5 | 7 | 0.19 |
| 20 | 36 | 32 | 20 | 22 | 0.17 |
| 40 | 37 | 64 | 45 | 48 | 0.14 |
| 60 | 37 | 110 | 30 | 50 | 0.12 |
| 80 | 38 | 170 | 10 | 46 | 0.10 |
| 100 | 39 | 245 | 5 | 45 | 0.08 |
How the Solubility Curves Look
If you plot this data (Temperature on x-axis, Solubility on y-axis), you’ll observe:
-
NaCl (Sodium chloride)
Almost a flat line → solubility changes very little with temperature. -
KNO₃ (Potassium nitrate)
A steep upward curve → solubility increases rapidly with temperature. -
Na₂SO₄ (Sodium sulfate)
Shows a peak (unusual behavior) → increases up to about 40°C, then decreases (due to change in crystal form). -
Na₂CO₃ (Sodium carbonate)
Moderate increase, then slightly levels off at higher temperatures. -
Ca(OH)₂ (Calcium hydroxide)
Downward slope → solubility decreases as temperature increases (rare for solids).
A combined solubility curve for different salts highlights how substances respond differently to temperature changes.
While most solids like KNO₃ become more soluble at higher temperatures, some like Ca(OH)₂ show the opposite trend.
Others, like NaCl, remain nearly constant. These variations are important in industrial processes, crystallization, and chemical separation techniques.
Objective Questions
- Solubility is defined as the:
A. Rate of dissolving a solute
B. Maximum amount of solute dissolved in a solvent at a given temperature
C. Amount of solvent in a solution
D. Volume of solution formed
- A solution that can dissolve more solute is said to be:
A. Saturated
B. Supersaturated
C. Unsaturated
D. Concentrated
- Which of the following factors does NOT affect solubility?
A. Temperature
B. Pressure
C. Nature of solute
D. Colour of solute
- The solubility of most solid substances in water generally:
A. Decreases with temperature
B. Increases with temperature
C. Remains constant
D. Becomes zero
- The solubility of gases in liquids:
A. Increases with temperature
B. Decreases with temperature
C. Is unaffected by temperature
D. Becomes constant
- Increasing pressure increases the solubility of:
A. Solids in liquids
B. Liquids in liquids
C. Gases in liquids
D. Solids in gases
- A supersaturated solution is:
A. Stable
B. Contains less solute than required
C. Contains more solute than it can normally hold
D. Cannot exist
- “Like dissolves like” means:
A. All substances dissolve in water
B. Polar dissolves polar, non-polar dissolves non-polar
C. Solids dissolve only in solids
D. Liquids dissolve only in gases
- Which of the following is an example of a saturated solution?
A. Contains no solute
B. Contains maximum solute at a given temperature
C. Contains excess solvent
D. Contains only gas
- The unit of solubility is commonly expressed as:
A. mol/dm³
B. g/dm³
C. g per 100 g of solvent
D. kg/m³
- A solubility curve shows the relationship between:
A. Pressure and volume
B. Temperature and solubility
C. Mass and density
D. Volume and pressure
- On a solubility curve, a point below the curve represents:
A. Saturated solution
B. Supersaturated solution
C. Unsaturated solution
D. Boiling solution
- Which substance shows little change in solubility with temperature?
A. KNO₃
B. NaCl
C. NH₃
D. CO₂
- Which of the following has decreasing solubility with increase in temperature?
A. NaCl
B. KNO₃
C. Ca(OH)₂
D. Na₂CO₃
- A point above the solubility curve represents:
A. Unsaturated solution
B. Saturated solution
C. Supersaturated solution
D. Dilute solution
- Solubility depends on:
A. Colour only
B. Temperature and nature of substances
C. Shape of container
D. Time of day
- When a saturated solution is cooled, crystals may form because:
A. Solubility increases
B. Solubility decreases
C. Pressure increases
D. Volume increases
- Which of the following best describes a solution?
A. A mixture of two solids
B. A homogeneous mixture
C. A heterogeneous mixture
D. A suspension
- The process of obtaining solid crystals from a solution is called:
A. Filtration
B. Evaporation
C. Crystallization
D. Distillation
- Which of the following is least soluble in water at room temperature?
A. NaCl
B. KNO₃
C. Ca(OH)₂
D. Na₂CO₃
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