D2.3.8 (HL)—Water potential
Water potential measures the potential energy of water per unit volume, with pure water defined as zero kilopascals in standard conditions.
- Syllabus
- First assessment 2025
- Objective
- D2.3.8
- Level
- HL
Water Potential Is Relative Potential Energy per Volume
HL onlyWater potential (ψw) is the potential energy of water per unit volume and is usually measured in kilopascals (kPa).
Absolute potential energy cannot be measured, so water potentials are expressed relative to pure water at atmospheric pressure and 20°C, which is assigned ψw = 0 kPa.
Adding solute makes water potential negative relative to pure water. Applying positive pressure can raise water potential. The numerical value predicts which region's water can release more potential energy by moving.
A solution at −300 kPa has lower water potential than pure water at 0 kPa; water has a tendency to move from the pure water toward the solution if a pathway exists.
Water potential is not solute concentration alone and is not an absolute store of energy. State the reference conditions and units.
HL Water Potential
HL onlyWater potential is potential energy of water per unit volume, measured in kPa; pure water at standard conditions has water potential of 0 kPa. Water moves from higher water potential to lower water potential; solutes lower water potential by restricting water molecule movement. Water potential equals solute potential plus pressure potential; solute potential is zero or negative and pressure potential is often positive in walled cells. Water entering plant cells increases pressure potential and dilutes solutes; water leaving plant cells lowers pressure potential and makes solute potential more negative.
Concept essentials
- Pure water has water potential of zero kilopascals at standard conditions.
- Adding solute lowers water potential.
- Pressure can raise water potential in walled cells.
- Water potential predicts the direction of net water movement.