When you mix some substances you cannot get them back again. This is known as an irreversible change. For example, if you mix water, sand and cement you get concrete. This sets hard and you can't get the original substances back again.
Separation
Mixtures can be separated by methods like sieving, filtering and evaporating.
Sieving
A mixture made of solid particles of different sizes, for example sand and gravel, can be separated by sieving.
Filtering
You can separate a mixture of sand and water by passing it through a piece of filter paper. The water is able to pass through the tiny gaps in the paper but the sand particles are too big and are left on the surface of the filter paper.
Evaporating
By dissolving salt in water you make a solution. You can separate the salt from the water again by boiling the solution. The water will evaporate until it is all gone. The salt will be left behind.
When you mix some substances you cannot get them back again. This is known as an irreversible change. For example, if you mix water, sand and cement you get concrete. This sets hard and you can't get the original substances back again.
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