Q.What are the properties of water?

Water's unusual physical properties — high specific heat, high latent heat of vaporisation, cohesion, adhesion, high surface tension, and excellent solvent ability — arise from hydrogen bonding between its molecules, and these properties are exactly what make it central to plant water relations (transpiration, ascent of sap, turgidity).

Water is uniquely suited to its biological roles because of several distinctive physical properties, largely a consequence of hydrogen bonding between its polar molecules:

1. High specific heat capacity: water can absorb/release a large amount of heat with only a small change in its own temperature, helping buffer plant tissues against rapid temperature fluctuations.

2. High latent heat of vaporisation: a large amount of energy is needed to convert liquid water to vapour; this is exploited during transpiration, where evaporative water loss from leaf surfaces carries away significant heat, cooling the plant.

3. Cohesion: water molecules are strongly attracted to one another via hydrogen bonding, giving water high tensile strength; this cohesion allows an unbroken, continuous column of water to be pulled up through the narrow conduits of the xylem (as in the cohesion-tension theory of ascent of sap) without the column breaking under tension.

4. Adhesion: water molecules are also attracted to polar surfaces such as the walls of xylem vessels/tracheids (which are hydrophilic), which helps water climb along these walls and contributes to capillary movement, counteracting the pull of gravity.

5. High surface tension: the cohesive attraction between water molecules at a surface creates high surface tension, which along with cohesion and adhesion contributes to capillary rise in narrow xylem elements.

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