The substances in a mixture are separated by the
differences in their physical properties, such as their particle size. The more
different the properties are, the easier it is to separate the substances. Tea
leaves do not dissolve in water, so you can use a strainer to FILTER them. The particles in other mixtures can be far smaller;
in CHROMATOGRAPHY, microscopic substances are separated by how
easily each sticks to another substance.
To search for tiny particles of gold in rivers, a mixture of sand,
mud, and gravel is scooped up in a pan and swirled around. Gold particles are
heavier than the other particles, so they settle to the bottom of the pan. The
lighter particles stay suspended in the water, and are decanted (poured off).
This technique of panning for gold is called decanting. Cream is also separated
from milk by decanting – the cream is less dense than the milk.
In distillation, a mixture of liquids is heated in a flask. The
liquid with the lower boiling point evaporates (changes to a vapour) first, and
is condensed (changes back to a liquid) and collected. The liquid with the
higher boiling point and any solid particles are left behind in the flask.
Fractional distillation separates liquids one by one as they boil. The oil
industry separates crude oil using this technique.
When the substances in a mixture have different particle
sizes, they are separated by filtration. The mixture is poured through a sieve
or filter. The smaller particles slip through the holes, but the larger
particles do not. Filtration is the first stage in water recycling. Chemists
use filters called zeolites, which have holes so tiny that they can remove
microscopic particles from water.
Scientists separate many liquid mixtures using
chromatography. The mixture is dissolved in a liquid or a gas to make a
solution. The solution is put on a solid material and the substances that
dissolved most easily travel farthest up the solid material. The separated
substances form bands of colour called chromatograms. Food scientists study
chromatograms to discover which colourings a food contains.
Food scientists separate food colouring for analysis using paper
chromatography. A drop of colouring is put onto filter paper. The edge of the
filter paper is dipped in water. As the water flows up through the paper, it
carries the colours with it. Some colours travel faster than others, so the
substances split into different coloured bands.
THIN LAYER CHROMATOGRAPHY
Genetic scientists use thin layer chromatography (TLC) to study
the substances that make up our genes. In TLC, the solid material is a plate of
glass or plastic coated with a chemical, usually aluminium oxide or silicon
oxide. When the liquid mixture travels up the plate, some of the substances
move farther up the plate than others. The substances appear as spots on the
plate. Scientists study genes to learn about inherited characteristics.
