Solubility Ch 14

Key definitions The solubility of a substance refers to the maximum amount of that substance that can be dissolved in a given quantity of solvent at a given temperature. A solution in which no more solute can be dissolved at that temperature is said to be saturated. The standard measure of solubility, or concentration, of a substance at a given temperature is how many grams will dissolve in 100 g of water to form a saturated solution.

An unsaturated solution is one where less than the maximum amount of solute has been dissolved at a given temperature. More crystals could be dissolved at that temperature. If a saturated solution is cooled, some crystals would come out of solution. If a saturated solution is cooled carefully, a supersaturated solution may be formed. The crystals do not precipitate - more solute is dissolved than is theoretically possible. This is an unstable solution, and by adding a seed crystal, the excess crystals can come out of solution.

Solubility Curves A solubility curve can be obtained by plotting a graph showing the relationship between solubility and temperature. Any point on the curve represents a saturated solution , while points below the curve indicate an unsaturated solution – ie more solute could be dissolved at that particular temperature before a saturated solution is formed. Any points above the curve indicate an unstable, supersaturated solution.

1. What is the solubility of sucrose at 20oC? 2. Which salt, potassium nitrate or sodium nitrate, is more soluble at a) 25oC? b) 75oC?   a) What mass of CuSO4.5H2O will just dissolve in 100g of water at 75oC? b) What mass of CuSO4.5H2O will be deposited if this solution is cooled to 25oC?   4. What mass of sodium chloride will just dissolve in 200g of water at 80oC?    5. What mass of water will be required to just dissolve 40g of silver nitrate at 10oC?

Solubility of gases Polar gases dissolve in water easily Non polar gases do not dissolve easily Temperature – as temperature ↑, solubility ↓ Pressure – as pressure ↑, solubility ↑

The solubility of gases tends to decrease as the temperature of water increases – how can this be explained?

Crystallisation Crystals can be formed by 1. cooling a solution so the excess crystals that can no longer be dissolved are crystallised out 2. evaporate the solvent so the excess solute will crystallise out The size of the crystals formed depends on The rate of cooling – quick cooling produces small crystals, slow cooling produces large crystals Rate of evaporation - quick evaporation produces small crystals, slow evaporation produces large crystals Nucleation – or seeding The nature of the compound