The operation of removing a desired pure substance from a mixture is called separation, while removing impurities to make it purer is called purification. Let us organize the representative separation methods.
Basic Knowledge
Main separation operations:
① Filtration: Separates a solid and a liquid by passing the mixture through filter paper. Example: separating sand and water.
② Distillation: Uses differences in boiling points by turning a liquid into a gas and then cooling it back into a liquid. Example: producing pure water from seawater.
③ Fractional distillation: Distills a liquid mixture by component using differences in boiling points. Example: fractional distillation of petroleum (gasoline, kerosene, and light oil).
④ Recrystallization: Uses differences in solubility caused by temperature. Example: separating potassium nitrate and sodium chloride.
⑤ Extraction: Uses differences in the ability to dissolve in a particular solvent. Example: extracting tea with hot water.
⑥ Sublimation: Separates substances that readily sublime (iodine, naphthalene, and dry ice) by changing them directly from a solid into a gas.
⑦ Chromatography: Separates components using differences in adsorption and distribution. Used for separating pigments and so on.
Filtration (solid–liquid separation using filter paper; sand and water)
Distillation (separation by vaporizing and condensing liquids using differences in boiling points)
Fractional distillation (continuous distillation using differences in boiling points; petroleum refining)
Recrystallization (forming crystals using differences in solubility caused by temperature to increase purity)
Extraction (removing a desired substance using differences in solubility in a particular solvent)
Sublimation (separating an easily sublimed substance by turning it into a gas; iodine and naphthalene)
Further Study (Background and Significance)
The correct use of distillation equipment is a frequently tested point. The bulb of the thermometer should be placed at the base of the side arm (to measure the temperature of the vapor leaving the flask), a boiling stone should be added (to prevent bumping), the Erlenmeyer flask (receiver) should not be sealed (to prevent it from bursting due to pressure), and water should flow through the Liebig condenser from bottom to top (to improve efficiency by preventing backflow). You should be able to explain the reason for each point.
A fractionating column separates crude oil according to differences in boiling points, from the bottom upward in order of decreasing boiling point: residual oil, lubricating oil, light oil, kerosene, naphtha, and petroleum gas. In recrystallization, remember that “the greater the change in solubility with temperature, the more efficiently pure crystals can be obtained.”
- Filtration = solid and liquid (sand and water)
- Distillation = difference in boiling points; thermometer at the base of the side arm
- Fractional distillation = important in petroleum refining; substances with higher boiling points remain lower down
- Recrystallization = uses differences in solubility caused by temperature
- Extraction = differences in solubility (tea and coffee)
- Sublimation = iodine, naphthalene, and dry ice
- Chromatography = also applied to pigment analysis and blood tests
Points to Note (Easily Confused)
① Distillation (one stage) and fractional distillation (continuous, multistage distillation) are similar but are different operations. ② Filtration is solid–liquid separation, whereas distillation is liquid–liquid separation using differences in boiling points. ③ Extraction can be used to remove a substance from either a solid or a liquid (coffee beans are also extracted). ④ Water in the condenser flows from bottom to top. The handling of laboratory equipment is a standard topic in tests.
Practice
- What operation should be performed first to separate sand and saltwater?
- Name the method used to separate kerosene and gasoline from crude oil.
- What separation method is suitable for obtaining iodine or naphthalene as a pure substance?