Trap seawater in wide salt pans and let the sun dry it, and the water disappears into the air, leaving only white salt behind.
Something made of two or more substances mixed together is called a mixture, and if the substances have different properties, those differences can be used to separate them again.
If the grains are different sizes you filter them; if one substance is dissolved in water you evaporate the water; and if only one sticks to a magnet you pick it out with a magnet.
Use the buttons to see filtering, evaporation, and magnets in turn, and compare which difference in properties each one uses.
How can we separate a mixture?
To separate a mixture, the first step is not picking a tool but asking how the two substances differ. A mixture like sand and water, where the grains differ in size, a mixture like salt water, where one part has dissolved and looks clear, and a mixture like iron filings and sand, where only one part sticks to a magnet, each need a different tool. If you only memorise the names of the methods, you end up using filter paper on every mixture, and if you filter salt water, the dissolved salt slips straight through the paper. Once you find what is different, it becomes obvious whether to filter, evaporate, or use a magnet.
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Three ways to separate a mixture: filtering, evaporation, and magnets
Choosing one of the three buttons switches the whole picture to that method. In filtering, sand grains are caught on the filter paper in the funnel, and the water that passed through collects in the beaker below. In evaporation, water in the dish over the flame turns into steam and rises away, and white grains of salt are left on the bottom of the dish. With the magnet, grey iron filings hang from the underside of a magnet with N and S poles, while the brown sand stays behind in the pile below.
🔬 Filtering
①Uses the difference in grain size
②Example: sand + water → filter through filter paper
③It uses a difference in properties between the mixed substances
④If one method does not separate everything, use several in turn
Why can mixtures be separated?
It is easy to think filter paper can separate any mixture. But salt that has dissolved in water is in pieces too tiny to catch, so it slips through the holes in the paper; salt water cannot be filtered and the water has to be evaporated instead. Iron filings and sand have grains of similar size, so even a sieve does not separate them well, but bring a magnet close and only the iron filings come away. Evaporation also is not finished just by boiling the water off: you have to look at what is left in the dish to know you have got the salt. Before picking up a tool, work out whether the difference is size, dissolving, or sticking to a magnet.
💡 Differences in properties are the key
①If the grains differ in size → filter with a sieve or filter paper
②If one part is dissolved in water → evaporate the water to get the dissolved substance
③If only one part sticks to a magnet → pick it out with a magnet
④If sand, salt, and iron filings are all mixed, take out the iron with a magnet, dissolve the rest in water and filter it, then evaporate the filtered water
⑤Things very different in size, like beans and rice, can be separated with a sieve
Summary
How separating works
Difference in properties → method of separating
It uses differences in grain size, dissolving in water, and sticking to a magnet
Three typical methods
Filtering (size) / Evaporation (leaves the dissolved substance) / Magnet (sticks to a magnet)
First find which property is different
🎯 Key points
①A mixture is two or more substances mixed together
②Filtering uses the difference in grain size (sand and water)
③Evaporation drives off only the water and leaves the dissolved substance (salt water)
④A magnet uses the difference in sticking to a magnet (iron filings and sand)
⑤In daily life we separate mixtures too, like salt from salt pans or the filter in a water purifier
Examples and Unit Test
Example 1
How can you pick the sand out of a mixture of sand and water?
1
Sand grains are bigger than the holes in filter paper, but water can pass through them.
2
Filter it through filter paper, and the water runs through while the sand stays caught on the paper.
▸ Filter it through filter paper (using the difference in grain size)
Even if the filtered water looks clear, anything dissolved in it passes straight through the paper.
Example 2
Iron filings and salt are mixed together. What tool can you use to pick out only the iron filings?
1
Iron filings stick to a magnet, but salt does not.
2
Bring a magnet close and only the iron filings come away on it, separating them from the salt.
▸ A magnet (using the difference in sticking to a magnet)
If you put the magnet inside a plastic bag, it is easy to pull the filings off afterwards.
Unit-test style
Which statement about separating mixtures is correct?
①Filtering uses the difference in boiling temperature
②A magnet can pick out substances that stick to it, such as iron
③The salt in salt water is caught by filter paper
④A mixture can be separated by only one method
⑤Evaporation uses the difference in grain size
▸ ② A magnet can pick out substances that stick to it, such as iron
1
Iron sticks to a magnet, so bringing a magnet close picks out substances like iron.
2
Filtering uses the difference in grain size (①), evaporation drives off the water and leaves the dissolved substance (⑤), dissolved salt passes through filter paper (③), and a mixture can be separated by using several methods in turn (④).