Water potential is the potential energy of the water in a device relative to pure water, as both temperature and pressure are preserved in the same way. This can also be defined as a measure of how free molecules of water may move in a given environment or device. It is expressed in kPa (kilopascals) and is expressed by Ψ (Greek letter). ;Water potential is never positive but has a maximum value of zero, that is, at atmospheric pressure, it is pure water. The more soluble it is, the more negative it is when it comes to impure water or water that has solutes in it, as the solute molecules attract water molecules and limit their freedom to travel.