Explain why pure water has maximum water potential.

Pure water has maximum water potential because it represents the highest possible free energy of water molecules under standard conditions. Water potential is a measure of the potential energy of water per unit volume compared to pure water at atmospheric pressure and temperature.

Here’s why pure water has maximum water potential:

No Solute Potential Contribution: Pure water has no dissolved solutes (like salts or sugars), which means it has the maximum possible solute potential (Ψ_s = 0). Solute potential represents the effect of dissolved substances on lowering the water potential compared to pure water.

Pressure Potential at Atmospheric Pressure: At standard atmospheric pressure (1 atm or 101.3 kPa), pure water also has no pressure potential (Ψ_p = 0). Pressure potential depends on the hydrostatic pressure exerted on water due to surrounding conditions.

Combined Water Potential: The water potential (Ψ) of pure water is therefore the sum of its solute potential (Ψ_s = 0) and pressure potential (Ψ_p = 0), which gives a value of zero:

Ψ
pure water
=
Ψ
𝑠
+
Ψ
𝑝
=
0
+
0
=
0
Ψ
pure water
​
=Ψ
s
​
+Ψ
p
​
=0+0=0

Thus, pure water, under standard conditions where it is not under any external pressure and has no dissolved solutes, has the maximum possible water potential of zero. This means water will tend to move from areas of higher water potential (such as pure water) to areas of lower water potential (where solutes are dissolved or where there is positive pressure) in processes like osmosis and water transport in plants.