Physicochemical properties of water and the influence of phase transitions on agroecosystems

Phase states of water and thermal balance

Water acts as the primary natural thermoregulatory agent in soil and plants. Due to its exceptionally high heat capacity and thermal conductivity, it smoothes out sharp fluctuations in ambient temperature and preserves the vital functions of living organisms. The molecular mass of water is 18.02, and the substance itself can exist in liquid, solid, and gaseous states.

For accurate calculation of thermal processes, the indicators of water phase transitions are important. At an atmospheric pressure of 101.3 kPa (760 mm Hg), melting or crystallization correspond to point D on the phase diagram, boiling — to point E, and heating or cooling — to the segment DE. The physical parameters of these transitions are provided in the tables below.

• Molecular mass of water — 18.02
• Triple point temperature — 0.0099 °C
• Heat of melting at 0 °C — 6.01 kJ/mol
• Heat of vaporization at 100 °C — 40.63 kJ/mol

On the phase diagram, stable phases are separated by three equilibrium curves. Curve OA (boiling) shows the equilibrium of liquid water and vapor, curve OC (melting) — of ice and liquid, and curve OB (sublimation) — of ice and vapor. All three lines intersect at the triple point O, where all three phases are in equilibrium simultaneously. Sublimation represents the transition of a substance upon heating from a solid state to a gaseous state, bypassing the liquid phase.

Density anomalies and water behavior during freezing

Water has a unique feature: when transitioning from a solid state to a liquid state, its density increases. When heated from 0 to 4 °C, the density of water also continues to rise. The substance has the highest density at a temperature of 4 °C, and with further heating, the density decreases.

This density anomaly protects water bodies from freezing through during the winter. Cooling to 4 °C, surface layers sink to the bottom as they are heavier, displacing warmer masses upward. When the entire column of water cools to 4 °C, the colder and lighter water remains on the surface and turns into ice. Ice covers the water body from above like a blanket and, thanks to low thermal conductivity, protects it from freezing, facilitating the preservation of life.

At positive temperatures, ice on the surface melts faster, and layers of water with a temperature below 4 °C sink downward, ensuring mixing. Along with water, dissolved nutrients circulate. Thanks to this process, water bodies and soil horizons are inhabited by living organisms to a significant depth.

Thermophysical properties: how water protects plants from temperature stress

Water acts as the primary thermoregulatory agent in the field and greenhouse. Since crops are forced to develop under conditions of constant fluctuations in ambient temperature, their survival depends directly on the unique thermophysical properties of the liquid phase. The high specific enthalpy of vaporization (Δ H_vap = 40.683 kJ/mol at 373 K) allows crops to effectively protect themselves from overheating through transpiration. Rapidly moving water molecules transition into vapor and carry away excess thermal energy, cooling the leaf blade.

Due to the high thermal conductivity of water, heat released during biochemical reactions is instantly distributed throughout the volume of cellular moisture. This prevents the appearance of local "hot spots" capable of destroying membranes or denaturing proteins. The specific heat capacity of water is 5–30 times higher than that of most other chemical substances, with the exception of hydrogen and ammonia. When changing the temperature from 0 to 35 °C, the heat capacity of water first drops and then begins to rise; the value at 16 °C is conventionally taken as unity.

For an agronomist, the high heat capacity of water is important when assessing the thermal balance of the soil. Sand heats up and cools down 5 times faster than water. Moist soil warms up more slowly in spring than dry sandy soil, but it retains heat significantly longer and protects the root system from sharp frosts. On a global scale, this ability to retain heat smoothes out climatic fluctuations: the temperature difference in the World Ocean from the equator to the pole is only 30 K.

The specific latent heat of fusion of ice is also abnormally high. This property smoothes out seasonal temperature fluctuations in spring and autumn in middle and high latitudes, stretching phase transitions of water over time. The freezing of water is accompanied by a massive release of heat, which reduces the rate of cooling of the cell sap and the surrounding soil solution.

Capillary phenomena and chemical activity in soil processes

The movement of moisture in soil is regulated by the forces of cohesion (the adhesion of water molecules to each other by hydrogen bonds) and adhesion (the sticking of water to solid soil particles). The result of cohesion is high surface tension (Q = 71.9·10⁻³ J/m² at 298 K). Ripples quickly appear on open water under the influence of wind, which multiplies the contact area of the water surface with the atmosphere and activates the processes of evaporation and heat exchange.

In soil pores, the combination of adhesion to the capillary walls and the cohesion of the boundary layer forms a concave meniscus. The resulting capillary force causes water to rise against the force of gravity. Through this mechanism, moisture is able to rise through fine pores to a height of up to 10–12 meters from the water table. This ensures the constant replenishment of the root zone from deep soil horizons.

Water is a universal solvent for salts and mineral compounds, and is also capable of dissociating into ions. The process of molecular decomposition involves the release of protons, which instantly bind to neighboring water molecules, forming hydroxonium ions (H₃O⁺). Schematically, this reaction is written as H₂O → H⁺ + OH⁻ or 2H₂O → H₃O⁺ + OH⁻.

In a real soil solution, more complex interactions occur with the decomposition of water associates into heavy ions: 8H₂O → H₉O₄⁺ + H₇O₄^–. The degree of dissociation is extremely low — only one molecule out of half a billion decomposes. At a temperature of 25 °C, the concentration of hydrogen and hydroxyl ions is 1·10⁻⁷ mol/l, which determines the neutral reaction of pure water (pH 7).

• Specific latent heat of fusion of ice — about 335 J/g
• Heat release during the freezing of 1 m³ of water — equivalent to 10 kg of coal
• Surface tension of water at 298 K — 71.9·10⁻³ J/m²
• Maximum capillary rise in soil — 10–12 m
• Concentration of hydrogen ions at 25 °C — 1·10⁻⁷ mol/l

Any mineral nutrition or plant treatment product starts working only after dissolution. Water acts as the main and universal solvent, thanks to which nutrients penetrate into cells and are absorbed. In dry form or without sufficient moisture in the soil, even the most effective fertilizers remain unavailable to the root system.

• Dielectric permittivity of water (at 298 K) — 78.3
• Reduction of mutual ion attraction in water — by 80 times

The high dissolving capacity of water is explained by the polarity of its molecules. Due to this, oppositely charged salt ions are attracted to each other in an aqueous environment 80 times more weakly than in air. The ordinary thermal motion of molecules easily breaks the bonds between them, converting solid fertilizer crystals into a solution through ion hydration.

Behavior of organic compounds and the role of cell membranes

Organic substances dissolve in water depending on their chemical structure. Compounds containing carboxyl, hydroxyl, and carbonyl groups easily transition into solution, forming strong hydrogen bonds with water. Non-polar substances, such as lipids, do not mix with water and repel its molecules. This property lies at the basis of creating barriers and compartmentalization within a living cell.

Mutual repulsion forces non-polar molecules to group and attract each other, similar to merging oil droplets. Such hydrophobic interactions perform a crucial structural function at the micro-level. They ensure the stability of cell membranes, protein molecules, and nucleic acids, protecting the internal environment of the cell from destruction.