Explain why pure water has the …

Water molecules possess kinetic energy. In liquid and gaseous form they are in random motion that is both rapid and constant.

The greater the concentration of water in a system, the greater is its kinetic energy or 'water potential'.

Hence, it is obvious that pure water will have the greatest water potential. If two systems containing water are in contact, random movement of water molecules will result in net movement of water molecules from the system with higher energy to the one with lower energy.

Thus water will move from the system containing water at higher water potential to the one having low water potential. This process of movement of substances down a gradient of free energy is called diffusion.

Water potential is denoted by the Greek symbol Psi or y and is expressed in pressure units such as pascals (Pa), By convention, the water potential of pure water at standard temperatures, which is not under any pressure, is taken to be zero.