Choosing the right way to separate a mixture
Most substances we use come as mixtures, and we separate them to remove unwanted parts or to collect useful ones. The method depends on the properties of the components. Handpicking removes stones from grain, threshing separates grain from stalks, and winnowing uses wind to blow away light husk. Sieving separates particles of different sizes, such as bran from flour. Sedimentation and decantation let heavy particles settle so the liquid can be poured off.
Filtration uses filter paper to separate insoluble solids from liquids, like tea leaves from tea. Evaporation removes water to leave behind a dissolved solid, which is how salt is obtained from sea water. Distillation boils a liquid and condenses the vapour to collect a pure liquid. Chromatography separates the different coloured dyes in ink as they travel at different speeds up a paper strip. Centrifugation spins a mixture so heavier parts collect at the bottom, as when cream is separated from milk. A magnet removes iron filings from a mixture.
Why it matters
Separation methods are used every day in kitchens, farms and factories, from straining tea to making butter. Water treatment plants use sedimentation and filtration to give us clean drinking water. Laboratories use centrifuges to test blood and chromatography to check food colours. This topic leads to the study of pure substances and mixtures in Class 9.
Key terms
- Filtration
- Separating an insoluble solid from a liquid by passing the mixture through a filter.
- Evaporation
- Heating a solution to remove the liquid and leave the dissolved solid behind.
- Chromatography
- A method of separating different dissolved substances as they move through paper or another medium.
- Centrifugation
- Spinning a mixture rapidly so denser particles collect at the bottom.