3.1
Mixtures are a combination of two or more substances In this topic, you will learn that:
emulsion a type of colloid in which two or more liquids are mixed together, with one suspended in the other as tiny droplets mixture a substance made up of two or more pure substances mixed together solution a mixture of a solute dissolved in a solvent
In the previous chapter, you learnt about the particle model of matter. The particles in a pure sample of ice, water and water vapour are all identical (H 2O). This is different from water found in the sea, which is a mixture of salt, water and many other types of particles. Sea water is a mixture of different types of particles.
Properties of mixtures
There are many different types of mixtures, each with different characteristics. For this reason, scientists group mixtures according to their properties: what they are made of and how they behave. Knowing the type of mixture and the properties of each particle help us to work out ways to separate the different particles into pure substances.
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suspension a cloudy liquid containing insoluble particles
• the different properties of substances can determine the type of mixture.
pure substance something that contains only one type of substance (e.g. a single element or a single compound) sediment substance or matter that settles to the bottom in a mixture colloid a type of mixture that always looks cloudy, because clumps of insoluble particles remain suspended throughout it rather than settling as sediment
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Solutions When you mix salt into water, it seems to disappear. But we know the salt is still there because we can taste it. The particles of salt become so small that they spread evenly throughout the water. This clear mixture of salt and water will not separate by itself. It is a solution. A solution is a mixture of one substance dissolved evenly throughout another. Solutions are usually transparent (see-through).
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Interactive 3.1B Is it pure?
• the different substances in a mixture can have different properties
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Interactive 3.1A Elements, compounds and mixtures
• a pure substance is one where all the particles are identical
Suspensions
Dirty water is an example of a suspension. A suspension is a mixture of two substances, in which a solid is mixed through (dispersed), undissolved, in a liquid. The result is a cloudy liquid where the dirt is suspended or ‘hanging around’ in the middle of the water. Sand in water is also a suspension. If you shake a container of sand and water, the sand spreads through the water, forming a cloudy liquid. The sand will then settle to the bottom of the container as a sediment. Suspensions often need to be shaken or stirred before use to spread the sediment through the liquid.
Colloids When two types of particles are mixed, they do not always separate out with time. Suspensions that do not separate easily are referred to as colloids. These can be formed when a solid is suspended in a liquid, such as hot chocolate in milk. Occasionally different particles are suspended in a gas. Fog is an example of this: small drops of water suspended in the air.
Figure 1 Most of the things we use every day are mixtures. What mixtures can you see in this photograph? Can you see any pure substances?
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