Solubility and Solutions

1. Types of Solutions

A solution is a homogeneous mixture of two or more substances. It consists of a solute (the substance being dissolved) and a solvent (the medium doing the dissolving).

• Unsaturated Solution: A solution that can still dissolve more solute at a given temperature.
• Saturated Solution: A solution that contains the maximum amount of solute that can be dissolved in a given volume of solvent at a specific temperature in the presence of undissolved solute.
• Supersaturated Solution: A solution that contains more solute than it should normally hold at a specific temperature. These are unstable and often formed by cooling a saturated solution carefully.

2. Solubility and Calculations

Solubility is defined as the maximum amount of solute in moles or grams that will dissolve in 1 dm³ (1 liter) of solvent at a specific temperature. The unit is typically mol/dm³ or g/dm³.

Formula: Solubility (mol/dm³) = (Mass of solute / Molar mass) / Volume of solvent in dm³.

3. Solubility Curves

A solubility curve is a graph showing the variation of solubility with temperature. Usually, the solubility of solids increases with temperature, while the solubility of gases decreases as temperature increases.

• Deductions: You can determine the amount of solute that will precipitate (fall out of solution) when a hot saturated solution is cooled by subtracting the solubility at the lower temperature from the solubility at the higher temperature.

4. Solvents for Fats, Oils, and Paints

The principle 'like dissolves like' applies here. Polar solutes dissolve in polar solvents (like water), and non-polar solutes dissolve in non-polar solvents.

• Turpentine: Used as a solvent for paints.
• Kerosene/Petrol: Used to remove grease, fats, and oil stains.
• Ethanol: Used for perfumes and some resins.
• Carbon Tetrachloride (CCl4): A powerful solvent for fats and oils (though less used now due to toxicity).

5. False Solutions: Suspensions and Colloids

A Suspension is a heterogeneous mixture in which the solute particles are large enough to be seen with the naked eye and settle upon standing (e.g., muddy water, Harmattan haze, water paints).

A Colloid is a mixture where the particle size is intermediate between a true solution and a suspension. They do not settle and exhibit the Tyndall Effect (scattering of light).

• Examples: Fog (liquid in gas), Milk (emulsion), Aerosol sprays, Rubber solution.