What are reversible and irreversible changes?
Reversible changes refer to any changes made that can be reversed.
Changing states of matter
Changing states of matter are always reversible. For example, chocolate melts when it is warm, and it sets hard when it is cold. Boiling water evaporates as steam and then it condenses back as water. These are both examples of reversible changes. So even when the state of matter is changed, it can still be changed back to its original.
When different materials are mixed together, they can usually be separated again using separating processes.
There are four processes for separating mixtures:
Solid + liquid
When a solid is mixed with a liquid, the process can be reversed by filtering. This process consists of moving a mixture through a filter. This can usually be a paper filter containing small holes. When the mixture is poured through the filter, the liquid passes through the filter, and the solid does not pass the filter; therefore, we can separate the solid and the liquid.
Solid + solid
When solids are mixed together, the process can be reversed by sieving. A sieve is a tool with a net attached that has holes. Sieves can come in different sized. When a mixture is placed in the sieve, the smaller solids pass through the sieve, helping to separate out larger solids from smaller solids.
Soluble solid + liquid
When a soluble solid is mixed with a liquid it will dissolve in the liquid to form a solution. For example, when salt is dissolved in water, it forms a salt solution. The process of dissolving can be reversed by evaporation and condensation. Evaporation consists of heating the salt solution, this leaves behind the solid salt, and the water turns into water vapour. Condensation consists of cooling the water as it evaporates.
Irreversible changes refer to any changes that can’t be undone as a new material is formed.
Some examples of irreversible changes are:
- Rusting metal
- Heating food
- Mixing ingredients
Burning is an irreversible change. When wood is burned, it becomes ashes and cannot be changed back into wood.
Rusting metal is an irreversible change. When a metal is exposed to oxygen and water for long enough, a reaction called oxidisation occurs. This causes the metal to rust.
Heating food is an irreversible change. An example of this is cooking an egg in a pan. The cooked egg can’t be reversed back to a raw egg.
Mixing ingredients is an irreversible change. An example of this is when making bread you have to mix all the ingredients to make dough. Once the ingredients are all mixed together, they can’t be separated as now the ingredients have permanently changed.