Mixtures and Solutions

A mixture is a combination of two or more substances that are not chemically joined together. Unlike compounds (where atoms bond chemically and produce a new substance), mixtures keep their original substances intact and can usually be separated again. The world is full of mixtures: air is a mixture of gases, sea water is a mixture of water and salts, milk is a mixture of fats, proteins and water, smoke is a mixture of tiny particles in air. There are three main kinds of mixture: solutions, suspensions and colloids.

• Mixtures vs compoundsNot chemically joinedCan be separated
• Three main typesSolutions, suspensions, colloids
• Solution exampleSalt waterSalt fully dissolved
• Suspension exampleSand in waterParticles settle out
• Colloid exampleMilkTiny particles that do not settle
• Separation methodsFiltering, distillation, evaporation

Mixtures vs compounds

The difference matters.

• Compound: atoms chemically bonded. Fixed ratio of ingredients. New properties. Can only be separated by chemical reactions. Examples: water (H2O), salt (NaCl), sugar.
• Mixture: substances physically combined but still separate. Ratio can vary. Original properties preserved. Can be separated by physical means. Examples: salt water, air, salad, smoke.

The three main types

• Solutions: one substance is completely dissolved in another. The particles are too small to see even with a microscope. The mixture looks uniform throughout. Sugar in tea, salt in sea water, alcohol in wine, oxygen in air.
• Suspensions: bigger particles mixed in a liquid or gas. The particles eventually settle out when left alone. Sand in water, flour in water, dust in air.
• Colloids: in-between mixtures with particles big enough to scatter light but small enough that they never settle out. Milk, fog, mayonnaise, paint, blood.

How to separate mixtures

Since the substances in a mixture are not chemically joined, you can usually separate them with simple physical methods.

• Filtering: separates a solid from a liquid (or two solids of different sizes). Used in coffee makers, kidneys, water filters.
• Evaporation: leaves dissolved solids behind when the liquid evaporates. How sea salt is harvested.
• Distillation: boils a liquid and collects the steam separately. Used to purify water, make whisky, separate crude oil into petrol and diesel.
• Decanting: pouring off a clear liquid from a settled solid.
• Magnetism: separates magnetic materials (iron) from non-magnetic ones.
• Chromatography: a special technique that separates substances based on how fast they travel through a material like paper. Used to separate ink colours or analyse blood samples.

Fact The air you breathe is a mixture, not a compound. It is approximately 78% nitrogen, 21% oxygen, and 1% other gases (argon, carbon dioxide, water vapour and traces of many others). Because it is a mixture, the ratios can vary slightly from place to place. Cities have more car exhaust gases. Forests have more oxygen. The air above the ocean has more water vapour. Mountain air has less oxygen.

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