Separating and purification are some methods to collect pure substance from mixture based on different physical properties of components in mixture by physical means.
1. Filtration
Filtration is a separating method which can be removed insoluble solid from liquid, such as sand in water.
There are several other methods can be used to achieve this purpose. For example, decanting (倾倒法) can be used when the size of insoluble particles is large. Centrifugation (离心法) can be used to separate extremely small particles in liquid, such as different components in blood.
The principle can be particle size or solubility. When two solids are mixture together, and one of them is soluble and another is insoluble, we can use solvent (always water) to dissolve one soluble solid and then use filtration to separate them.
The liquid collected in the beaker is called filtrate (滤液), and the solid left on the filter paper is called residue (滤渣).
2. Evaporation and crystallisation
Evaporation and crystallisation are two processes to collect solid when soluble solids dissolved in water.
Hydrated crystal (含水的晶体), such as copper(II) sulfate pentahydrate (CuSO4•5H2O), cannot be produced by evaporation. Because evaporation will remove all water in solution, including water in CuSO4•5H2O. Finally, a white powder CuSO4 will be collected.
The principle of crystallisation is large difference in solubility under different temperature. As shown in the figure above, solubility of copper(II) sulfate increased significantly with increasing temperature. We can prepare a saturated CuSO4 solution under a higher temperature. When the solution is cooling down, some CuSO4•5H2O will be produced due to difference in solubility under different temperature. This process is called crystallisation. For sodium chloride, the change in solubility under different temperature, it cannot be produced by crystallisation.
3. (Simple) distillation
Simple distillation can be used to separate two substances which have large difference in boiling points, such as seawater (salt and water). When seawater was heated, water will be boiled to produce water vapor, and water vapor will pass through condenser and condensed into water again. Finally, distilled water can be collected in conical flask.
Notice: cooling water should be entered from bottom of condenser for full-filling the condenser.
4. Fractional distillation
Fractional distillation can be used to separate miscible liquids which have small difference in boiling points, such as alcohol, which contains water (100 ℃) and ethanol (78 ℃). When alcohol was heated to 80 ℃, ethanol vapor will be produced and pass through the condenser. Finally, ethanol could be collected in conical flask. Meanwhile, water can be evaporated to produce water vapor as well. The purpose of fractionating column is to condense water vapor, and prevent water vapor pass through the condenser.
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