Water (H2O) differs from similar small molecules (e.g. ammonia (NH3), methane (CH4), nitrogen dioxide (NO2)) in being a liquid as opposed to a gas at ‘normal’ temperatures because the H2O molecule is strongly polarized, making the molecules cling together through weak hydrogen bonding (Fig. W1). The energy required to separate the molecules so that they can escape by evaporation is the heat of vaporization, λ. Its value for water is one of the highest known (λ = 2.48 MJ kg−1 at 20°C), and this means that evaporation has a powerful cooling effect.
The hydrogen bonding between the oxygen and hydrogen of adjacent molecules of water accounts for the high tensile strength of water in capillaries enabling water to exist as a continuum in stems of vascular plants, and to be drawn through the roots, stem and leaves when evaporation at the leaf surface occurs.
Water is an excellent solvent for mineral salts. Ions such as Na+ and Cl− are attracted to each other. When placed in water, their attraction is reduced. This capacity of a solvent to influence the attraction between ions is expressed as the dielectric constant. The value for water is 80 whilst that of non-polar organic solvents is less than 2. Thus mineral ions needed for plant growth, and originating from the soil, accompany the transpiration stream to all parts of the plant, and become available for metabolic uptake in the growing regions of all organs. The solutes in the transpiration stream are not only the mineral elements. Most biochemically important substances in plants are charged, and readily soluble in water. Water molecules become associated with polar groups of organic biochemical constituents, including proteins, polysaccharides and other macromolecules as water of hydration.
Water is a better electrical conductor than other non-metallic liquids as a result of dissociation of H2O into H+ and OH−. At 25°C there are 10−7 mol dm−3 of each species.
The non-compressible nature of water makes it useful as a hydroskeleton; leaves of many land plants owe their rigidity to the water pressures inside them, which are often in the range 0.3–1 MPa (3–10 atmospheres). This pressure is vital during the cell-expansion phase of growth.
The rate at which oxygen and Carbon dioxide (CO2) diffuse in water is very low indeed, some 10 000 times slower than these molecules diffuse in air, although both gases are sparingly soluble in water (Table W1). Water is more viscous than most other solvents as a result of the attraction between its molecules which tends to prevent layers of the liquid sliding over each other. Viscosity declines markedly with temperature (Table W2). Thus the tensions during transpiration, when water is drawn through the fine capillaries that constitute the cellulose cell walls and xylem tissues of the plant, are considerable. Water participates directly and fundamentally in biochemical processes. In Photosynthesis the water molecule is split, the H being used to reduce CO2 to carbohydrates (CH2O)n, whilst O is evolved as oxygen gas. This is the origin of the oxygen of the Earth's atmosphere. See also water (hydrological) cycle. [J.G.]
Fig. W1 Adjacent molecules of H2O clinging together as a result of electrostatic attraction between the positively charged hydrogen and the negatively charged oxygen. Many molecules are associated in this way as ‘clusters’ in liquid water and they form crystals in ice. (After Jones, H.G. (1992) Plants and Microclimate. Cambridge University Press, Cambridge.)
Table W1Solubilities and diffusivities in water in the range 0–40°C. S, solubility of gas (m3gas per m3of water); D, diffusivity of gas and heat in water (mm2s−1).
|Density, ρ||kg m−3||1000*||999.7||998.2||995.6||992.2|
|Saturation vapour pressure, e||kPa||0.61||1.23||2.34||4.24||7.38|
|Latent heat of vaporization, λ||MJ kg−1||2.50||2.48||2.45||2.43||2.41|
|Specific heat capacity, cp||J g−1 K−1||4.22||4.19||4.18||4.18||4.18|
|Viscosity, η||mPa s||1.79||1.30||1.00||0.80||0.65|
|Surface tension||mN m−1||75.6||74.2||72.7||71.2||69.6|
|Thermal conductivity||mW K−1 m−1||561||580||598||615||630|
|* Water reaches its maximum density at 3.98°C.|
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