A gas will undergo a change in volume more easily than either a liquid or a solid.
Gases adapt most easily to changes in volume.
Because gas takes the shape of it's container, it can change volume quite easily. No matter what container you put it in, a gas takes that shape and volume.
The matter that can change shape and volume is gas.
A state of matter that does not have a definite shape or definite volume would be a Gas. Gases are easily compressible compared to liquids, and change to any shape they occupy.
Gas
Assuming that pressure and the amount of matter are constant (meaning they do not change), volume will increase as temperature increases.
A solid has a definite shape and volume.
shape and volume
Matter changes both its shape and volume when it is in a gaseous state. Imagine oxygen: if you have a syringe full of oxygen, the oxygen is in the shape of the syringe, but if you let it out, it would readily lose this shape as it escaped into the environment. If you do not let it out, and instead push the syringe, you are compressing the oxygen; in other words, you are making the volume of the oxygen smaller by making it more dense. Oxygen, as with any other gas, does not have a fixed shape or volume.
A liquid.
gas
gas