Many things around us are mixtures of several substances. We will learn methods for extracting a particular substance from a mixture.
Basic Knowledge
Substances are divided into pure substances (one kind of substance) and mixtures (two or more substances). Removing a desired substance from a mixture is called separation, and increasing its purity is called purification.
Main separation methods:
① Filtration: separates a liquid from an insoluble solid (the solid remains on the filter paper).
② Evaporation: evaporates water to obtain the solute (heating salt water).
③ Distillation: uses differences in boiling points (water + ethanol → ethanol comes out first).
④ Recrystallization: uses differences in solubility caused by temperature (such as potassium nitrate).
⑤ Chromatography: uses differences in the speeds at which substances move (separation of dyes).
Pure substance（consists of one kind of substance. Water, oxygen, table salt, and so on）
Mixture（a combination of multiple substances. Air, seawater, petroleum）
Filtration（separating a liquid and a solid. Guide the liquid with a glass rod）
Distillation（separation using differences in boiling points. A distilling flask and a condenser）
Recrystallization（dissolving at high temperature, cooling, and obtaining crystals）
Boiling chips（must be added during distillation to prevent bumping）
Further Exploration (Background and Significance)
Separation technology is the foundation of industry. Crude oil purification is carried out by fractional distillation, producing gasoline, kerosene, diesel fuel, heavy oil, and other products in order of increasing boiling point. Salt production is an ancient technology that evaporates seawater, and the salt fields of the Seto Inland Sea are famous.
Recrystallization is a powerful method for removing impurities and is used to purify medicines and produce high-purity semiconductor materials such as silicon. Chromatography is an essential technique in modern chemistry and biochemistry, with applications ranging from DNA analysis and drug testing to new drug development.
- Pure substance = one kind; mixture = two or more kinds
- Filtration = separates a liquid and a solid
- Evaporation = removes water and leaves the solute
- Distillation = separates substances using differences in boiling points
- Recrystallization = increases purity using differences in solubility
- Chromatography = separates substances using differences in movement speed
- Boiling chips are essential for distillation (to prevent bumping)
Points to Note (Easily Confused)
① Do not confuse evaporation (removing the solvent) with distillation (separating substances by differences in boiling points). ② During filtration, place the tip of the funnel against the wall of the beaker and guide the liquid with a glass rod. ③ A mixture can be distinguished from a pure substance because its melting point and boiling point are not constant. ④ In recrystallization, substances with a greater difference in solubility between temperatures are more effectively purified (potassium nitrate is a typical example).
Practice
- Is air a pure substance or a mixture?
- What method is used to obtain table salt from salt water?
- Explain why boiling chips are added during distillation.