Water Potential
- Water potential (Ψ) can be defined as follows:
The potential energy of water, per unit volume, relative to pure water
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- Potential energy is the energy stored in an object due to its position in relation to other objects
- In this instance it is the energy stored in water molecules due to their position in relation to other molecules, e.g. molecules of a solute in a solution
- The unit of water potential is usually kilopascals, or kPa
- Water potential is always stated relative to pure water at atmospheric pressure and 20 °C:
- The water potential of pure water is given a value of 0 kPa
- Note that the water molecules in pure water do technically have potential energy, but it is impossible to determine, and this designated value of zero allows for a simple comparison with solutions
- As solutes are added to a solution, the water potential decreases into negative values; solutions with a high solute concentration have a lower water potential
- Energy is stored in hydrogen bonds between solute molecules and water molecules in a solution, meaning that less energy is available as potential energy
- Water molecules in a solution with a higher solute concentration therefore have less potential energy
- The water potential of pure water is given a value of 0 kPa