Types of Solutions
Before you can understand colligative properties or solubility rules, you need the vocabulary. A solution is a homogeneous mixture of two or more substances - meaning the composition is uniform throughout. Unlike a salad (heterogeneous), a solution looks the same no matter which part you sample.
Solute and Solvent
Every solution has two components:
- Solvent - the substance present in the greater amount (the “dissolver”). It determines the phase of the solution.
- Solute - the substance present in the lesser amount (the “dissolved stuff”).
When water is the solvent, the solution is called an aqueous solution - by far the most common type on the MCAT. But solutions are not limited to liquids dissolving solids.
Types of Solutions
Solutions can exist in any combination of phases:
| Solute Phase | Solvent Phase | Example |
|---|---|---|
| Gas | Gas | Air (O₂ and other gases dissolved in N₂) |
| Gas | Liquid | Carbonated water (CO₂ dissolved in H₂O) |
| Liquid | Liquid | Vodka (ethanol dissolved in water) |
| Solid | Liquid | Saltwater (NaCl dissolved in H₂O) |
| Solid | Solid | Brass (zinc dissolved in copper) |
Solid-in-solid solutions are called alloys. Gas-in-liquid solutions are critical for biology - dissolved O₂ and CO₂ in blood are the reason you can breathe.
Concentration - How Much is Dissolved?
Concentration describes the amount of solute per amount of solution (or solvent). You already learned the main concentration units in Chapter 4 (molarity, molality, mole fraction), but here is a quick review since they show up constantly in solutions problems:
| Unit | Formula | Temperature Dependent? |
|---|---|---|
| Molarity (M) | mol solute / L solution | Yes (volume changes with T) |
| Molality (m) | mol solute / kg solvent | No (mass does not change) |
| Mole fraction (χ) | mol solute / total mol | No |
| Mass percent (%) | (mass solute / mass solution) × 100 | No |
Saturation
When you dissolve sugar in water, at some point the water cannot hold any more. That limit is defined by three levels:
- Unsaturated - less solute is dissolved than the maximum. More can dissolve.
- Saturated - the maximum amount of solute is dissolved at that temperature. If you add more, it will not dissolve - it just sits at the bottom as undissolved solid (precipitate). A dynamic equilibrium exists between dissolving and precipitating.
- Supersaturated - MORE solute is dissolved than the maximum. This is an unstable, metastable state. It is created by dissolving solute at a high temperature (where solubility is greater), then slowly cooling without disturbing. The tiniest disturbance - a scratch, a seed crystal, even a vibration - causes the excess solute to crash out of solution dramatically.
Electrolytes vs. Nonelectrolytes
This distinction matters enormously for colligative properties:
- Strong electrolytes dissociate completely into ions in water. They conduct electricity well. Examples: NaCl, HCl, KOH, CaCl₂.
- Weak electrolytes dissociate partially. They conduct electricity weakly. Examples: CH₃COOH (acetic acid), NH₃, HF.
- Nonelectrolytes do not dissociate at all. They dissolve as intact molecules. No ions, no conductivity. Examples: glucose (C₆H₁₂O₆), sucrose, urea.
Why does this matter? Because NaCl dissolves into 2 particles (Na⁺ + Cl⁻), but glucose dissolves into 1 particle. Two particles have twice the effect on colligative properties. This is exactly what the van ‘t Hoff factor (i) captures - but we will get to that in Section 11.