Horrible Science: Witch Mix

What is a reversible change?

A witch in a grey hat is looking at a large sieve.

When a material, such as salt, can return to its original state, we call it a reversible change. For example, salt dissolved in water can be reversed by evaporating the water, which leaves behind the salt.

An irreversible change is when a material cannot return to its original state. For example, a fried egg cannot be turned back into an uncooked egg.

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Explore how reversible changes can return a material to its original state.

How can we change materials?

Image of a witch in a hat holding a salt shaker. She has grey hair and there is smoke coming from the cauldron.

Materials can be either solids, liquids or gases. These are called states of matter. A material’s state can change under certain conditions. For example, ice is a solid. If we melt ice, it becomes water, which is a liquid. If we boil water, it becomes water vapour or steam, which is a gas.

Changing between states of matter is a reversible change because we can undo these changes. For example, water vapour or steam is a gas. If we condense water vapour or steam, it turns back into water (a liquid). If we freeze water, it turns back into ice (a solid).

Dissolving is another example of a reversible change. Dissolving occurs when a solid such as salt is mixed with a liquid such as water or magic potion to form a solution. We can reverse dissolving through evaporation.

Another example of a reversible change is mixing. Mixing occurs when a solid, such as sand, is mixed with a liquid, such as water to form a suspension. The sand is not dissolved in the water, and it floats around. We can reverse mixing by sieving, if the solid is large enough, or filtering, if the solid is smaller).