Introduction
Matter around us is rarely found in a pure form. Air is a mixture of gases, seawater contains dissolved salts, and soil is a complex mixture of many substances. A mixture is a combination of two or more substances that are not chemically combined. Understanding mixtures and how to separate them is a fundamental chemistry skill.
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Key Concepts
- 1. Pure Substances vs Mixtures
- Pure substance: has a definite composition and fixed properties (e.g., distilled water, pure gold, oxygen gas).
- Mixture: two or more substances combined physically in any proportion. Components retain their individual properties.
2. Types of Mixtures
- Homogeneous mixtures (solutions)
- Uniform composition throughout.
- Cannot distinguish components by naked eye.
- Examples: saltwater, air, steel, vinegar.
- Solute: the dissolved substance (smaller quantity).
- Solvent: the substance that dissolves (larger quantity).
- Concentration = (mass of solute / mass of solution) x 100%
- Heterogeneous mixtures
- Non-uniform composition; components are visible.
- Examples: soil + water (suspension), sand + iron filings, oil + water.
- Colloids: intermediate between solutions and suspensions. Particles (1-100 nm) scatter light (Tyndall effect). Examples: milk, fog, smoke.
- Suspensions: particles > 100 nm; settle on standing; can be filtered. Example: muddy water.
- 3. Properties Used for Separation
- Different physical properties allow components of a mixture to be separated:
- Particle size: filtration, sieving
- Density / settling: sedimentation, decantation
- Boiling point: distillation, evaporation
- Magnetic properties: magnetic separation
- Solubility: crystallisation
- Volatility / affinity for stationary phase: chromatography
4. Separation Techniques
Evaporation: Separates a dissolved solid from its liquid solvent. Heat evaporates the liquid, leaving the solid behind. Example: getting salt from seawater.
Distillation: Separates liquids with different boiling points, or a soluble solid from its solvent when the liquid is also needed. The mixture is heated; the component with a lower boiling point vaporises first, is cooled in a condenser, and collected. Example: separating alcohol from water.
Fractional Distillation: Used when two or more liquids have close boiling points. A fractionating column with glass beads or plates is used. Example: separating crude oil into petrol, kerosene, and diesel; separating liquid air into nitrogen and oxygen.
Sublimation: Separates a sublimable solid (one that converts directly from solid to gas) from a non-sublimable solid. Example: separating iodine from sand; camphor from salt.
Chromatography: Separates components based on their different speeds of movement through a stationary phase carried by a mobile phase. Example: separating dyes in ink; identifying amino acids; detecting drugs in urine.
Magnetic Separation: Separates a magnetic substance from a non-magnetic one. Example: iron filings from sulphur.
Sedimentation and Decantation: Allow heavy particles to settle, then carefully pour off the liquid. Example: separating sand from water.
Centrifugation: Rapid spinning separates denser particles faster. Example: separating cream from milk; blood components.
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Worked Examples
Classify the following as homogeneous or heterogeneous: (a) bronze (b) oil and water (c) soda water.
(a) Bronze (Cu + Sn alloy) — homogeneous (uniform solid solution).
(b) Oil and water — heterogeneous (two visible layers).
(c) Soda water (CO2 dissolved in water) — homogeneous.
A 250 g saltwater solution contains 20 g of salt. Calculate the concentration.
Concentration = (20 / 250) x 100 = 8%
How would you separate a mixture of iron filings and sulphur powder?
Use a magnet. Iron is magnetic; it is attracted to the magnet and separates from the non-magnetic sulphur.
Why is distillation preferred over evaporation to obtain pure water from saltwater?
Evaporation leaves behind the salt and the water vapour is wasted. Distillation collects and condenses the water vapour, giving you pure distilled water. Evaporation is used when the solid (salt) is the desired product.
A ink spot on chromatography paper travels 4 cm while the solvent front travels 8 cm. Calculate the Rf value.
Rf = distance moved by component / distance moved by solvent = 4 / 8 = 0.5
What is the Tyndall effect and which type of mixture shows it?
When a beam of light is passed through a colloid, the colloidal particles scatter the light, making the beam visible. This is the Tyndall effect. Solutions do not show this effect because their particles are too small to scatter light.
How would you separate a mixture of camphor and sand?
Use sublimation. Heat the mixture; camphor sublimes (solid to gas) and the vapour can be collected on a cool surface (cold plate above). Sand remains behind, being non-sublimable.
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Common mistakes
- Confusing colloids and suspensions: colloids have smaller particles (1-100 nm), do not settle, and show the Tyndall effect; suspensions have larger particles, settle, and can be filtered.
- Thinking distillation always works for any mixture — it works only when components have different boiling points.
- Calling alloys impure — alloys are homogeneous mixtures; they are mixtures, not impure substances.
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Summary
Mixtures are classified as homogeneous (solutions) or heterogeneous (suspensions and colloids). Each separation technique exploits a different physical property. Evaporation, distillation, chromatography, sublimation, magnetic separation, and centrifugation are key techniques. Understanding which method to apply requires knowing the physical properties of the components in the mixture.