What Is a Mixture

Learn what makes a mixture different from a pure substance and how its parts can be separated.

About 3 min read
  • Explain what a mixture is
  • Tell a mixture from a pure substance
  • Choose a method to separate a simple mixture
Before you start

You know that materials have properties such as size, state and whether they dissolve. We use these properties to separate mixtures.

Mixtures are all around us

Air, sea water and a bowl of cereal are mixtures. A mixture contains two or more substances that are together but not chemically joined. Each substance keeps its own properties. Because no new substance forms, we can usually separate the parts again using physical methods.

Definition: mixture

Two or more substances mixed together but not chemically joined.

Definition: pure substance

A material that contains only one type of substance.

Separating mixtures

The best method depends on the properties of the parts. Sieving separates solids with different particle sizes. Filtering separates an undissolved solid from a liquid. Evaporation leaves a dissolved solid behind when the liquid turns to gas.

Sand and salt water

A beaker contains sand, salt and water. How can you collect the sand and salt?

Filter the mixture. The sand stays in the filter paper, while the salty water passes through.

Heat the salty water. The water evaporates and salt remains.

Answer: Filter first, then evaporate.

Pure can mean two things

In everyday life, pure can mean natural or clean. In science, it means containing only one substance.

Exam focus
Say it like this“Filter the mixture because sand does not dissolve.”
Not like this“Use a filter.” — Name the method and link it to a property.
Common mistakeTrying to filter out a dissolved solid. Dissolved particles pass through filter paper.

Practice

Is sea water a mixture or a pure substance?
A mixture, because it contains water, salts and other substances.
How would you separate pebbles from sand?
Use a sieve. The small sand grains pass through and the pebbles stay behind.
How would you get solid salt from salt water?
Evaporate the water by heating it, leaving the salt behind.
Going further
A student has iron filings, sand and salt. Plan how to separate all three.
Use a magnet to remove the iron. Add water to dissolve the salt. Filter to remove the sand. Evaporate the water to collect the salt.

Quick check

Mixtures quick check

Which of these is a mixture?

It contains nitrogen, oxygen and other gases that are not chemically joined.

Which method separates sand from water?

Water passes through the paper while the larger undissolved grains stay behind.

What stays behind when salt water evaporates?

Only the water becomes a gas, so the dissolved solid remains in the dish.
Key takeaways
  • A mixture contains substances that are not chemically joined.
  • Each part keeps its own properties.
  • Sieving, filtering and evaporating separate mixtures.
  • Choose a method by comparing properties.
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