3.3 Mixtures can be separated according to their properties

3.3

Learning intentions

By the end of this topic, you will be able to: • describe the processes of decanting, sedimentation, flotation and magnetic separation • define sediment, magnetism and flocculant • explain how the properties of a substance can be used to separate them from a mixture.

Key ideas

• Properties are how a substance looks (size, mass, texture, shape, volume) and how it behaves around other substances (magnetic, soluble). • A magnet can be used to separate particular metals. • Decanting can be used to separate sediment from a liquid.

Figure 1 First Nations Australians use different techniques to separate mixtures.

magnetic separation the process of using magnets to separate magnetic materials from non-magnetic materials

decanting a technique used to separate sediment from the liquid it is in by carefully pouring the liquid away

Mixtures can be separated according to their properties

Simple separation

Some mixtures are quite simple to separate. Sometimes we can simply pick out the bits we need to separate. The Alyawarre, Anmatyerre, Warlpiri and Pitjantjatjara peoples in the desert parts of the Northern Territory use their fingers to individually separate the good desert raisins (Solanum centrale) from the stalks and rotten fruit (Figure 1). This leaves the ripe fruit that is good for eating. This works well if there is a small number of raisins that are big enough to see. But what if the raisins were too small to see? You may need another way of Figure 2 Magnets can be used to separate tin cans separating out your favourite fruit. (left), which are magnetic, from aluminium cans Magnetic separation (right), which are not. Do you separate recyclables from your rubbish? Have you ever wondered how the different recyclable materials are separated once they are out of your house? Magnetic separation uses magnets to attract and separate particular objects. Some metals are magnetic. Magnetic substances are attracted to a magnet. They are made of iron or a mixture containing iron.

Magnetism will only separate substances containing iron, so magnetic materials such as iron nails can be separated from other nonmagnetic materials such as glass, aluminium and paper.

Tin cans are magnetic, whereas aluminium cans are not. Sometimes large magnets are used to separate tin cans in the rubbish from aluminium cans (Figure 2). This means both types of cans can be recycled Figure 3 Decanting wine separates the in different ways. undrinkable sediment.

Decanting Have you ever had a piece of food in the bottom of your drink? Did you use a spoon to remove it? Or maybe you carefully poured your drink into another glass, leaving the food behind? The careful pouring of DRAFT liquid, or decanting, is often done to remove high density sediment from wine (Figure 3). The objects or liquids that sink are denser than the liquid on the top. The particles in dense objects are packed together more tightly than those in less dense objects. Oil floats on top of water because the particles in the oil are packed very loosely. The water particles pack together more tightly, so they sink to the bottom below the oil.

The particles in a grain of sand are packed together very tightly. The sand is more dense than water. Therefore, the sand settles to the bottom of a glass of water. The sand forms a sediment in the glass.

Sedimentation and flotation

Sedimentation and fl otation are used in sewage treatment to separate the mixture of substances (Figure 4). Sewage is left in settling ponds to allow the sediment to settle to the bottom. Fats and oils that fl oat to the top of the ponds can be scooped off for digestion by bacteria. Oil spills can be cleaned up using the fact that oil fl oats on the surface of water (Figure 5). Cork and other substances can be sprinkled on top of the oil to soak it up, and these substances are then scooped off and squeezed out. In certain situations, sedimentation is more diffi cult. Chemicals called fl occulants can be added to a mixture to make suspended particles clump together. This makes them heavy enough to settle to the bottom. Flocculation is regularly used to separate substances from water. First Nations peoples separate mixtures Some mixtures can be separated by their ability to be blown or fl oated away. The Alyawarre people of the Sandover River in the Northern Territory separate mixtures using these properties. In one technique, a variety of local seeds are collected before being beaten with sticks to remove the seeds from their pods. Blowing air gently under the mixture blows away the light pods, leaving the heavier seeds to be used for food.

The Yindjibarndi people in Western Australia separate sand and dirt from seeds in a process called yandying. This technique involves placing the mixture in a shallow wooden dish, called a yandy or a coolamon in other places in Australia, which is gently shaken back and forth. The dense sand sinks to the bottom and the less dense dirt and larger seeds fl oat on the surface, allowing them to be collected for food or growing. This process is very similar to gold panning. Figure 4 Sewage treatment involves sedimentation and flotation. Figure 5 Oil floats on the surface of water. DRAFT

sedimentation the process of a substance settling to the bottom in a mixture

fl otation the action of fl oating in a liquid or gas

fl occulant a chemical added to a mixture to make suspended particles clump together

3.3 Check your learning

Retrieve

1 Defi ne the following terms. a sediment b fl occulation c decant d density

Comprehend

2 Describe the property that allows the separation of tin cans from aluminium cans. 3 Explain why fl otation allows oil spills to be cleaned up more easily. 4 Use an example to describe what can be done to encourage sedimentation if a suspension does not separate. 5 Use an example to describe a situation where people need to separate a mixture by hand. Apply

6 Discuss a situation where magnetism cannot be used to separate a mixture. Evaluate how the properties of the materials cause this problem.

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