• Those changes which can be reversed are called reversible changes.
• Reversible changes are temporary.
• Melted chocolate can be changed back into solid chocolate by cooling.
• Molten ice cream can be changed back to its solid form.
• Thus, melting is a reversible change.
• Melting of butter and chocolate are also reversible.
• Freezing is a reversible change.
• We can freeze orange juice to make ice lollies.
• The ice lollies can be changed back into orange juice when kept at a room temperature.
Boiling, evaporation and condensation:
• Boiling, evaporation and condensation are the examples of reversible changes.
• If we could capture all the steam that is made when it is boiled in a closed container, we could turn it back into water by cooling it.
• When this water is heated for some time, water starts to boil (liquid starts to evaporate) and steam escapes from the container.
• Now, if we hold a lid over the container, the steam will liquefy or condense into small droplets of water on coming in contact with the cold lid.
• This water can be cooled down and then kept in the freezer to form ice again.
• Thus, the three physical states of water are reversible and can be changed by heating or cooling.
• Dissolving is an example of a reversible change.
• When salt is mixed with water, it disappears because it dissolves in the water and becomes salty water.
• Substances that dissolve in water are called soluble substances.
• Substances that do not dissolve in water are called insoluble substances.