Physical Separation of Mixtures
This topic covers the fundamental principle that mixtures are separated using physical processes, not chemical reactions. The choice of technique depends entirely on the differing physical properties of the components, such as their state, solubility, or boiling point.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Separating mixtures involves physical changes only; no new chemical substances are formed.
- The technique used to separate a mixture exploits differences in the physical properties of its components.
- Key properties used for separation include solubility, particle size, boiling point, and density.
- Insoluble solids (e.g., sand in water) are separated from liquids based on particle size using techniques like filtration.
- Dissolved solids (e.g., salt in water) are separated from their solvent based on differences in boiling point, for example by evaporation.
- Immiscible liquids (e.g., oil and water) do not mix and can be separated based on their differing densities, while miscible liquids (e.g., ethanol and water) form a uniform solution and must be separated by other means, like fractional distillation based on boiling points.
Definitions
- Mixture
- A substance containing two or more elements or compounds that are not chemically bonded together.
- Physical Process
- A process that alters the form or physical properties of a substance, but does not change its chemical composition.
- Insoluble
- A property of a substance (solute) that will not dissolve in a solvent to form a solution.
- Immiscible
- A property of two or more liquids that do not mix to form a single, uniform phase.
- Miscible
- A property of two or more liquids that can be mixed together in any proportion to form a homogeneous solution.
Worked example
A student has a mixture containing solid iron filings, solid naphthalene, and a salt solution (sodium chloride dissolved in water). Naphthalene is insoluble in water but soluble in ethanol, and iron is magnetic. Which set of physical properties is correctly matched to an effective sequence for separating all three solid components? A) Magnetism, then Volatility, then Solubility B) Solubility, then Density, then Magnetism C) Magnetism, then Solubility, then Volatility D) Particle Size, then Magnetism, then Solubility
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Step 1:
Identify the most unique physical property for the first separation.
Iron is magnetic, while naphthalene and the salt solution are not.
Using a magnet (exploiting magnetism) is a simple, non-invasive first step to remove the iron filings.
- 2
Step 2:
The remaining mixture is naphthalene solid in a salt solution.
Naphthalene is insoluble in water.
This means it can be separated from the salt solution by filtration, which exploits the difference in particle size and solubility.
- 3
Step 3:
The final mixture is a salt solution.
To recover the solid salt, the water must be removed.
This is done by evaporation or boiling, which exploits the difference in volatility (or boiling point) between the non-volatile salt and the volatile water.
- 4
Step 4:
The sequence of properties exploited is therefore:
Magnetism (for iron), then Solubility/Particle Size (for naphthalene via filtration), then Volatility (for salt from water).
Option C matches this logic most closely, considering filtration as a process based on solubility.
Answer: C
Common mistakes
- ×Confusing a physical separation with a chemical reaction. Remember, filtration or distillation does not create new substances.
- ×Mistaking a homogeneous solution (e.g., salt water) for a pure substance. Even though you can't see the different parts, it is still a mixture.
- ×Applying an incorrect technique based on properties. For instance, you cannot filter a dissolved solid because its particles are too small and will pass through the filter paper with the solvent.
No-calculator tips
- ✓This is a qualitative topic. Focus on the vocabulary: 'soluble', 'insoluble', 'miscible', 'immiscible', 'volatile'. Your reasoning will depend on understanding these terms correctly.
- ✓Always plan the order of separation. It is usually most efficient to remove large, insoluble components first before tackling dissolved substances or separating miscible liquids.
- ✓For any given mixture, quickly classify each component by its state (solid/liquid) and its interaction with the solvent (soluble/insoluble). This mental checklist will guide you to the correct separation principle.