Gas. Molecules in gas are able to move and vibrate around each other more freely than the molecules in liquids and/or solids due to the great amount of space in between them. Hope this helps! :)
It is a Physical property because like water it changes into a ice cube but it can be melted into water or be turned back into an ice cube.
We can explain this in a molecular level. We know that the difference between a gas and a liquid of the same composition is how fast their molecules are moving. So given a gas, their molecules are farther and faster when moving, but when they are cooled their bulk kinetic energy decreases. In other words their molecules start to move closer and move slower until it behaves more like a liquid molecule. This is the time when gases condense.
