How to tell pure substances from mixtures and separate them
In chemistry, a pure substance contains only one kind of particle and has fixed properties, such as a definite melting point. Elements and compounds are pure substances. Mixtures contain two or more substances. Homogeneous mixtures, like salt water or air, have a uniform composition throughout, while heterogeneous mixtures, like sand in water, do not.
Mixtures are classified by particle size. In a true solution, such as sugar in water, particles are smaller than 1 nm, do not settle and do not scatter light. In a suspension, like chalk in water, particles are larger than 100 nm and settle down. A colloid, such as milk, fog or jelly, has particles in between; they do not settle but scatter a beam of light, which is called the Tyndall effect. The concentration of a solution can be expressed as mass percent. Mixtures are separated by evaporation, centrifugation, separating funnel, sublimation, chromatography, distillation and fractional distillation, chosen according to the properties of the components.
Why it matters
Separation techniques are used to get salt from sea water, cream from milk, gases from air and petrol from crude oil. The Tyndall effect is what makes sunbeams visible in a dusty room or forest. Doctors and forensic scientists use chromatography to test blood and identify substances. This topic links to chemical reactions and atoms and molecules.
Key terms
- Pure substance
- A substance made of only one type of particle, with fixed properties.
- Colloid
- A mixture with particles between 1 and 100 nm that do not settle and scatter light.
- Tyndall effect
- The scattering of a light beam by particles in a colloid, making its path visible.
- Fractional distillation
- A method to separate miscible liquids with close boiling points.