Solubility and the factors affecting it
Solubility is the mass of solute that saturates 100 g of solvent at a given temperature. It depends on the nature of the substances, temperature and pressure.
What solubility means
Water cannot dissolve an endless amount of salt. At 25 °C, 100 g of water stops taking salt after about 36 g: the solution is saturated. So the solubility of common salt at 25 °C is 36 g per 100 g of water. Solubility always refers to 100 g of solvent, not of solution, and to one stated temperature.
Factors affecting solubility
| Factor | Effect | Example |
|---|---|---|
| Nature of solute and solvent | Like dissolves like. Ionic and polar solutes (with charged ends, like water molecules) dissolve in polar solvents; non-polar solutes dissolve in non-polar solvents. | Salt dissolves in water; grease and wax dissolve in kerosene but not in water |
| Temperature: solids in liquids (endothermic) | If dissolving takes in heat, solubility rises as temperature rises. | Potassium nitrate (KNO₃), sugar |
| Temperature: solids in liquids (exothermic) | If dissolving gives out heat, solubility falls as temperature rises. | Calcium hydroxide (slaked lime), Ca(OH)₂ |
| Temperature: gases in liquids | Solubility of a gas falls as temperature rises. | Warm water holds less dissolved oxygen |
| Pressure | Affects gases only: more pressure, more gas dissolves. | CO₂ is forced into soda water under pressure |
Worked example: finding solubility
50 g of a saturated solution of sodium chloride at 30 °C contains 13.2 g of salt. Find the solubility of NaCl at 30 °C.
- Mass of solvent (water) = mass of solution − mass of solute = 50 g − 13.2 g = 36.8 g
- Formula: solubility = (mass of solute ÷ mass of solvent) × 100
- Solubility = (13.2 ÷ 36.8) × 100
- = 0.3587 × 100 = 35.87 g (rounded to two decimal places)
Solubility of NaCl at 30 °C = 35.87 g per 100 g of water.
Worked example: using solubility
The solubility of NaCl at 25 °C is 36 g. What mass of NaCl will dissolve in 250 g of water at 25 °C?
- 100 g of water dissolves 36 g of NaCl
- 1 g of water dissolves 36 ÷ 100 = 0.36 g
- 250 g of water dissolves 0.36 × 250 = 90 g
90 g of NaCl dissolves in 250 g of water at 25 °C.
Gases, heat and pressure in daily life
- Opening a soda bottle lowers the pressure, so dissolved CO₂ escapes as bubbles.
- A soda left in the sun goes flat faster, because gases are less soluble in warm liquids.
- On hot afternoons fish come up to the surface: warm water holds less oxygen.
A soda bottle fizzes when opened because the pressure drops and CO₂ escapes. Hot tea can hold more sugar than cold water, which is why sugar is stirred in while the tea is hot.
2 marks: “What is solubility? Write the formula used to calculate it.” The number of grams of solute that dissolve in 100 g of solvent to form a saturated solution at a given temperature. Solubility = (mass of solute ÷ mass of solvent) × 100.
2 marks: “How does pressure affect the solubility of a gas in a liquid? Give an example.” More pressure makes more gas dissolve. Soda water is made by forcing CO₂ into water under pressure; opening the bottle lowers the pressure and the gas bubbles out.
4 marks: “How do the nature of the substances, temperature and pressure change solubility? Give one example for each.” Nature of solute and solvent (like dissolves like); temperature (endothermic solids rise, exothermic solids fall, gases fall); pressure (gases only: more pressure, more gas dissolves). One example each.
Like dissolves like. Heat helps most solids but chases gases away. Pressure pushes gases in.