Physical changes of matter

​​In a nutshell

A chemical change undergoes a chemical reaction and can sometimes be reversible. Physical changes undergo a change in state which means that the process can be reversed with a change in temperature. Density is the mass per unit volume and generally speaking, solids are the most dense and gases are the least dense. 

Physical changes

There are two types of change, a chemical change and a physical change. For a chemical change to happen, there needs to be a change in chemistry. This means that the substance has to undergo a chemical reaction which alters the chemical composition of the substance. This involves the creation of a new chemical element or compound.

Physical changes are when there is no change to the chemical structure of the substance, but there is a change in state. Changes in state are when a material changes from a solid, liquid or gas into another state.

Curiosity: Sublimation is a change of state where substances go from a solid straight to a gas, missing the liquid stage. When a gas goes to a solid it is called deposition.

Reversible changes

As the chemical properties of the material is not being changed during a physical change, all physical changes of matter are reversible. 

Example

If you put water in the freezer, it will turn into ice, which is still water. If you then place the ice in a glass and place it on a table, eventually it will melt back into water. There has been no chemical changes in the water and this process can be repeated indefinitely. 

Temperature

The determining factor for substances undergoing physical changes is the temperature the substance is at. 

Starting from a solid, an increase in temperature starts to break some of the intermolecular bonds between the molecules. This means the molecules in the substance can start to move a bit more freely. The substance has now melted and turned into a liquid.

If you continue to increase the temperature of a liquid, eventually the bonds holding the liquid molecules together will all break and the individual molecules can escape. The substance has then been boiled and turned into a gas.

The same is true in reverse, if you were to lower the temperature of a gas it would condense into a liquid and eventually freeze back to a solid.

Density

Recall that density is defined as the mass per unit volume. In simple terms, how much matter is in a given space. The diagram below shows the particle arrangement in solids, liquids and gases.

1 Solid 2 Liquid 3 Gas

There are a lot more particles in the same volume for the solid than there is for the liquid and gas. This means that the solid has the greatest mass, as it has the most amount of matter. 

Liquids have slightly less particles per unit volume than solids, but still a lot more than gases. Gases hardly have any particles in the given volume. This means in general that solids are the most dense, followed by liquids and gases are not dense at all. There are some exceptions to this generalisation, but can be assumed to be true.

Curiosity: Ice as a solid is less dense than water, which is why ice floats in your drinks!