IMAT Chemistry · States of Matter and Gas Laws
Pure Substances, Mixtures and Separation Techniques
Elements and compounds are pure substances with fixed properties; mixtures can be separated by physical methods that exploit a difference in size, solubility, boiling point or attraction.
Why this matters
No past IMAT question has been set on this page yet, but it is in the official syllabus. Expect a short fact: which technique separates a given mixture, or whether a material is an element, a compound or a mixture.
Concept 1 of 2: Elements, compounds and mixtures
A pure substance is one kind of thing all the way through, so it has a fixed composition and sharp melting and boiling points. A mixture is two or more substances mingled without bonding, so its composition can vary and it can be pulled apart without a chemical reaction.
Definition
- An element contains only one kind of atom and cannot be split into simpler substances by chemical means.
- A compound is two or more elements chemically bonded in a fixed ratio. Its properties differ from those of its elements, and only a chemical reaction can separate them.
- A mixture contains substances that are not bonded together; its composition can vary.
- Homogeneous mixtures (solutions, air, alloys) look the same throughout. Heterogeneous mixtures (sand in water, oil and water) have visibly different parts.
- A pure substance melts and boils at sharp, fixed temperatures; an impure one melts and boils over a range.
| Type | What it is | Examples | How it can be separated |
|---|---|---|---|
| Element | One kind of atom | Iron, oxygen gas, | Cannot be broken down chemically |
| Compound | Elements bonded in a fixed ratio | Water, sodium chloride, carbon dioxide | Only by a chemical reaction |
| Homogeneous mixture | Uniform throughout, one phase | Salt water, air, brass | Physical methods such as distillation |
| Heterogeneous mixture | Not uniform, visible parts | Sand in water, oil and water, granite | Physical methods such as filtration |
Practice this conceptself-check · 3 quick reps
Looking uniform does not make a substance pure
Salt water and air look the same throughout, but they are homogeneous mixtures: their composition can change and their parts can be separated physically. A pure substance has a fixed composition and sharp melting and boiling points.
Concept 2 of 2: Separation techniques: filtration, distillation, chromatography and others
Every separation method uses one property in which the parts of a mixture differ. Different particle sizes suggest filtration, different boiling points suggest distillation, different attractions to a surface suggest chromatography. Name the difference first and the method follows.
Definition
- Filtration separates an insoluble solid from a liquid: the solid stays on the filter paper as the residue, the liquid passes through as the filtrate.
- Evaporation and crystallisation recover a dissolved solid from its solution.
- Simple distillation recovers a solvent from a solution (pure water from salt water). Fractional distillation separates miscible liquids with different boiling points (ethanol and water, crude oil, liquid air).
- Chromatography separates dissolved substances, such as the dyes in an ink, by how strongly each is attracted to the stationary phase (paper) compared with the mobile phase (solvent). = distance moved by the spot / distance moved by the solvent front, always between 0 and 1.
- A separating funnel separates immiscible liquids; centrifugation separates suspended solids by density (blood cells from plasma).
| Technique | Separates | Based on | Example |
|---|---|---|---|
| Filtration | Insoluble solid from a liquid | Particle size | Sand from water |
| Evaporation or crystallisation | Dissolved solid from its solution | The solvent is volatile, the solid is not | Salt from sea water |
| Simple distillation | Solvent from a solution | Very different boiling points | Pure water from salt water |
| Fractional distillation | Miscible liquids | Boiling points closer together | Ethanol from water; fractions of crude oil |
| Chromatography | Dissolved substances in small amounts | Attraction to the stationary and mobile phases | Dyes in an ink; amino acids |
| Separating funnel | Immiscible liquids | Density; liquids do not mix | Oil from water |
| Centrifugation | Suspended solid from a liquid | Density | Blood cells from plasma |
Practice this conceptself-check · 4 quick reps
Filtration cannot remove a dissolved substance
A dissolved solid passes through filter paper with the solvent. To get salt out of salt water you must evaporate the water, or distil it off. Filtration only removes undissolved solids.
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)
Elements, compounds and mixtures4 rows
| Type | What it is | Examples | How it can be separated |
|---|---|---|---|
| Element | One kind of atom | Iron, oxygen gas, | Cannot be broken down chemically |
| Compound | Elements bonded in a fixed ratio | Water, sodium chloride, carbon dioxide | Only by a chemical reaction |
| Homogeneous mixture | Uniform throughout, one phase | Salt water, air, brass | Physical methods such as distillation |
| Heterogeneous mixture | Not uniform, visible parts | Sand in water, oil and water, granite | Physical methods such as filtration |
Separation techniques: filtration, distillation, chromatography and others7 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 | The solvent is volatile, the solid is not | Salt from sea water |
| Simple distillation | Solvent from a solution | Very different boiling points | Pure water from salt water |
| Fractional distillation | Miscible liquids | Boiling points closer together | Ethanol from water; fractions of crude oil |
| Chromatography | Dissolved substances in small amounts | Attraction to the stationary and mobile phases | Dyes in an ink; amino acids |
| Separating funnel | Immiscible liquids | Density; liquids do not mix | Oil from water |
| Centrifugation | Suspended solid from a liquid | Density | Blood cells from plasma |
Watch out for (2)
- Looking uniform does not make a substance pure→ Elements, compounds and mixtures
- Filtration cannot remove a dissolved substance→ Separation techniques: filtration, distillation, chromatography and others
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