Sometimes we need pure substances as opposed to impure ones. You can imagine a mixture of sand and table salt and imagine how much use you would have for it. or imagine the manufacture of drugs and medicines. Purity is highly essential here as it is crucial to our well-being and health.
A range of physical techniques are available to make necessary separations.
All separations depend one way or other on the physical properties of substances in the mixture.
The method chosen depends on.
(i) The type of mixture
(ii) Substance in the mixture which we are interested in.
Types of Mixtures and Separation
Purity based on melting/Boiling Points
- Substances can be identified using their boiling and melting points
- Pure substances change state at a constant temperature whilst impure substances change over a range of temperature.
- If a solid is not pure, its melting point will be low and its boiling point will be high.
- Impurities widen the range over which the substance is liquid.
- The surrounding pressure can increase and decrease boiling and melting points.
If the surrounding pressure falls, the boiling point falls. An increase in pressure raises the boiling point.
- No two substances have the same boiling point and same melting point
Separation Techniques
- Separating Funnel
- Used to separate liquids that are immiscible (form layers on one another).
- When a mixture of oil and water is poured into the funnel, the oil floats on top. When tap opened, water runs out and closed when layer of water finished.
- Filtration
- Used to mainly separate suspensions, e.g. chalk and water or sand and water.
- Filter paper is aligned around the inner surface of a filter funnel and the solution is passed through.
- The solute (suspension) particles are trapped by the filter paper as residue
- The solvent passes through the filter paper and collects as filtrate
- Evaporation
- Used to separate solutions usually with a solute that consists of particles, which have been spread throughout the solvent(dissolved) and are too small to be obtained in filtration.
- Solution heated and solvent evaporates leaving solid behind.
- Salt obtained from solution by this method
- Crystallization
- Solids dissolved in solution can be separated out by letting them form crystals (E.g. copper (II) sulphate). The crystals contain some water of crystalisation.
- When a solution of copper (II) sulphate is cooled, then crystals of the salt form because it is less soluble at a lower temperature.
- In crystallization, a solution of copper (II) sulphate is heated so that some of the water evaporates leaving a more concentrated solution of the salt.
- The solution can be checked to see if it is ready by placing one drop on a microscopic slide, and crystals should form on the cool glass.
- The solution is then left to cool and crystallize. The crystals are removed by filtering, rinsed with water and dried with filter paper.
- Fractional Distillation
- Used to separate two miscible liquids that have different boiling points e.g. a mixture of ethanol and water.
- Mixture is heated; at 78°C ethanol begins to boil. Some water evaporates too.
This mixture of ethanol vapour and water vapour condenses in the cool glass beads in the column, making them hot.
- When the glass beads reach 78°C, ethanol vapour no longer condenses on them, only water vapour does. The water vapour then drips back into the flask, whilst the ethanol rises through into the condenser.
- The cool condenser forces ethanol to condense (liquid ethanol) and drip into the beaker.
- When the thermometer rises above 78°C, it is a sign that ethanol has been separated so heating can stop.
- Distillation
- Method used to obtain pure solvent from a solution, e.g. obtaining pure water from salt water
- Solution heated in a round bottomed flask A. It boils and steam rises into condenser, leaving salt behind.
- A thermometer is placed above the mixture so as to control the temperature at which distillation occurs.
- Condenser is cold and steam condenses into water, which drips into beaker completely pure (distilled water).
- Paper Chromatography
- Used when chemists want to analyse a mixture (find out what substances are present in it), e.g. finding out what dyes are present in ink.
- A pencil line is normally drawn at the point where the ink drop is put to mark the starting point. Ink must never be used as it blots and messes up the final result.
- When a drop of solution applied to paper, the paper absorbs solute/binds it to surface. As the solvent rises, some solute stays put and others dissolve in the solvent and travels up the paper.
- The most soluble dye travels through the paper faster than one which is slightly soluble.
- When the solvent reaches the top of the paper, the process stops and different spots are left on the paper. The point where the solvent stops is known as the solvent front.
- Each spot represents another solute, this way they are separated.
- Many solvents used, ethanol or propanone.
- With a solvent other than water, a closed container should be used so that the vapour of the solvent surrounds the Chromatography paper.
- Can be used to separate a mixture of coloured substances (e.g. in black ink)
- On a circular filter paper, substances more soluble will form larger circles with least soluble forming smaller circles.
- Chromatography can also be used to separate substances that are not coloured and cannot be seen with our naked eyes. For this to be possible, the paper is treated with a locating agent after the chromatography run. The agent reacts with samples to produce coloured spots.