Solutions: Chapter Introduction
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Soda fizzes when opened and the sea does not freeze at 0 °C: both are solution behaviour. The chapter covers concentration, Henry's and Raoult's laws and colligative properties.
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Key points
- A solution is a homogeneous mixture; the component present in the largest amount is the solvent, the dissolved one the solute.
- Molarity = moles of solute per litre of solution; also molality, mole fraction, ppm.
- Henry's law: gas solubility rises with pressure (p = K_H x); opening soda lowers the pressure, so CO₂ bubbles out.
- Raoult's law: a non-volatile solute lowers the solvent's vapour pressure; colligative properties (boiling point ↑, freezing point ↓, osmotic pressure) depend only on the number of solute particles.
Solution, solvent, solute
Salt water is a solution: water (the larger component) is the solvent, salt is the solute. The mixture is uniform throughout (homogeneous).
Concentration
| Term | Definition |
|---|---|
| Molarity (M) | moles of solute / litres of solution |
| Molality (m) | moles of solute / kg of solvent |
| Mole fraction (x) | moles of a component / total moles |
| ppm | parts of solute per million parts of solution |
0.5 mol NaCl in 2 L solution → M = 0.5 / 2 = 0.25 M
Henry's and Raoult's laws
Henry: p = K_H · x Raoult (non-volatile solute): (p° − p) / p° = x_solute
Soda is bottled under high CO₂ pressure. When it is opened the pressure falls, the gas's solubility falls, and the extra CO₂ escapes as bubbles. A non-volatile solute takes up part of the surface, so fewer solvent molecules escape and the vapour pressure is lower.
Colligative properties
- Relative lowering of vapour pressure.
- Elevation of boiling point.
- Depression of freezing point (sea water freezes near −1.9 °C).
- Osmotic pressure.
All four depend on the number of solute particles, not on what they are.
Quick quiz
4 questions. Choose an answer for each, then submit to see your score and the explanations.