IMAT Chemistry · Solutions and Concentration
Solutions, Mixtures and How to Separate Them
A solution is a mixture that looks the same all the way through; each kind of mixture is separated by the property its parts do not share.
Why this matters
Mixtures were asked in 2017 and 2022: whether a mixture is homogeneous or heterogeneous, which pairs of liquids mix, and which technique separates a named component.
Concept 1 of 2: Solutions and mixtures: the key terms
A mixture keeps the separate substances in it, in any proportion. If the particles are spread so finely that every part of the sample is the same, the mixture is homogeneous and is called a solution. If you can see, or spin out, separate parts, it is heterogeneous.
Definition
- A solution is a homogeneous mixture of a solute (the substance dissolved, usually the smaller amount) in a solvent (the substance it dissolves in, usually the larger amount). Water is the commonest solvent; the solution is then aqueous.
- Homogeneous: one phase, the same composition everywhere. Solutions can be liquid (salt water), gas (air) or solid (alloys such as brass).
- Heterogeneous: two or more visible phases or particles.
- Two liquids that mix in any proportion are miscible; liquids that form layers are immiscible.
| Term | Meaning | Example |
|---|---|---|
| Solution | Homogeneous mixture; particles too small to see or settle | Sugar dissolved in water |
| Suspension | Heterogeneous; particles large enough to settle or be filtered | Muddy water |
| Colloid | Particles about 1 to 1000 nm across; they scatter light and do not settle | Milk, fog |
| Miscible liquids | Mix completely to give one layer | Water and ethanol |
| Immiscible liquids | Separate into two layers | Hexane and water |
| Blood | Heterogeneous: cells suspended in plasma, which is itself a solution | Spinning blood separates the cells |
Practice this conceptself-check · 4 quick reps
Homogeneous does not mean pure, and blood is not homogeneous
Salt water is homogeneous but still a mixture of two substances. Blood looks uniform but is heterogeneous: its cells can be separated by spinning. Options calling blood homogeneous, or calling a solution a pure substance, are wrong.
Concept 2 of 2: Separation techniques and when to use each
To separate a mixture, find a property its parts do not share: particle size, boiling point, density, or how strongly each sticks to a surface. Each technique exploits one of these, so name the difference first and the technique follows.
Definition
- Filtration removes an insoluble solid from a liquid. It cannot remove a dissolved solute.
- Evaporation and crystallisation recover a dissolved solid from its solution.
- Simple distillation recovers the solvent from a solution of a solid.
- Fractional distillation separates miscible liquids whose boiling points are close.
- A separating funnel separates immiscible liquids.
- Chromatography separates dissolved substances that move at different speeds through a stationary phase.
- Centrifugation spins suspended particles to the bottom of a tube.
| Technique | Separates | Based on | Example |
|---|---|---|---|
| Filtration | Insoluble solid from a liquid | Particle size | Sand from water |
| Evaporation or crystallisation | Dissolved solid from its solution | Solvent evaporates, solid stays | Salt from sea water |
| Simple distillation | Solvent from a solution | Large difference in boiling point | Pure water from sea water |
| Fractional distillation | Miscible liquids | Close boiling points | Ethanol from water; crude oil fractions |
| Separating funnel | Immiscible liquids | Density; liquids do not mix | Oil from water |
| Chromatography | Dissolved substances in a mixture | Different attraction to the paper and the solvent | Plant pigments, ink dyes |
| Centrifugation | Suspended particles from a liquid | Density | Blood cells from plasma |
Practice this conceptself-check · 4 quick reps
Filtration cannot remove a dissolved substance
Dissolved particles pass through filter paper with the solvent. Salt cannot be filtered out of salt water; it needs evaporation, or distillation to recover the water. Two miscible liquids need fractional distillation, not filtration or a separating funnel.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Solutions and mixtures: the key terms6 rows
| Term | Meaning | Example |
|---|---|---|
| Solution | Homogeneous mixture; particles too small to see or settle | Sugar dissolved in water |
| Suspension | Heterogeneous; particles large enough to settle or be filtered | Muddy water |
| Colloid | Particles about 1 to 1000 nm across; they scatter light and do not settle | Milk, fog |
| Miscible liquids | Mix completely to give one layer | Water and ethanol |
| Immiscible liquids | Separate into two layers | Hexane and water |
| Blood | Heterogeneous: cells suspended in plasma, which is itself a solution | Spinning blood separates the cells |
Separation techniques and when to use each7 rows
| Technique | Separates | Based on | Example |
|---|---|---|---|
| Filtration | Insoluble solid from a liquid | Particle size | Sand from water |
| Evaporation or crystallisation | Dissolved solid from its solution | Solvent evaporates, solid stays | Salt from sea water |
| Simple distillation | Solvent from a solution | Large difference in boiling point | Pure water from sea water |
| Fractional distillation | Miscible liquids | Close boiling points | Ethanol from water; crude oil fractions |
| Separating funnel | Immiscible liquids | Density; liquids do not mix | Oil from water |
| Chromatography | Dissolved substances in a mixture | Different attraction to the paper and the solvent | Plant pigments, ink dyes |
| Centrifugation | Suspended particles from a liquid | Density | Blood cells from plasma |
Watch out for (2)
- Homogeneous does not mean pure, and blood is not homogeneous→ Solutions and mixtures: the key terms
- Filtration cannot remove a dissolved substance→ Separation techniques and when to use each
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