When matter changes state, sometimes the change can be undone but sometimes it’s changed forever. Let’s have a look at reversible and irreversible changes:

Reversible changes are changes that can be undone. When these changes happen, sometimes the materials look and feel different, but new material isn’t created. 

Examples of reversible changes are: 

Dissolving - This is when substances are mixed in a liquid, like sugar in hot tea. The sugar looks like it has disappeared but it’s really just mixed very well. The mixture is called a solution. Salt and sugar can dissolve because they are soluble substances, but have you ever tried dissolving sand in water? It’s impossible! That’s because sand is an insoluble substance. Even when salt dissolves, it can still go back to the way it was before. Salt that has dissolved in water can be returned back to salt by evaporating the water. 

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Evaporating - In a bathroom, hot water from the shower can change state into gas. This is called evaporation and it can be reversed. If the gas (vapour) touches something cold, like a mirror, it turns back into water.

Melting and freezing - Ice can turn into water by melting and water can turn back into ice by freezing. They are reversible.

Irreversible changes are changes that cannot be undone. The material that changed cannot go back to how it was before, instead, new material is formed.

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Examples of irreversible changes are:

Heating - If you cook a pizza or fry an egg you can’t uncook them. Cooking or heating the food changes them forever.

Burning - If you burn a log of wood, the wood turns into ashes and smoke. The ashes and smoke cannot turn back into wood.

Mixing - When you make a cake by mixing different ingredients together, the cake cannot change back into its ingredients. When you mix cement you cannot unmix the cement afterwards. These are all irreversible changes.