What type of substance doesn't have a consistent volume: liquid, solid, or gas?
A gas does not have a consistent volume.
Matter in the solid state has a fixed volume and shape. Its particles are close together and fixed in place.
Matter in the liquid state has a fixed volume, but it has a variable shape that adapts to fit its container. Its particles are still close together but move freely.
Matter in the gaseous state has both variable volume and shape, adapting both to fit its container. Its particles are neither close together nor fixed in place.
Hope this helps.