Properties of Water

Cohesion and Adhesion

Water exhibits unique properties due to its molecular structure and hydrogen bonding. Two important characteristics are cohesion and adhesion, which play vital roles in biological systems, especially in plants.

• Cohesion: The attraction between water molecules due to hydrogen bonding. This property allows water molecules to stick together, enabling phenomena such as the transport of water in plants through xylem vessels.

• Adhesion: The attraction between water molecules and other types of molecules or surfaces. Adhesion helps water "climb" up plant cell walls and other surfaces, aiding in capillary action.

• Example: Water droplets clinging to the surface of a leaf are due to cohesion, while water adhering to the walls of a plant vessel is due to adhesion.

Hydrogen Bonds

Hydrogen bonds are weak interactions that occur between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another. These bonds are responsible for many of water's unique properties.

• Key Point: Hydrogen bonds create strong cohesive forces between water molecules.

• Example: The ability of water to form droplets and its high boiling point compared to similar-sized molecules.

Surface Tension

Surface tension is the measure of how difficult it is to stretch or break the surface of a liquid. Water has a high surface tension due to hydrogen bonding among its molecules at the surface.

• Key Point: Surface tension allows small insects, such as water striders, to walk on water without sinking.

• Example: The formation of a meniscus in a glass of water and the ability of water droplets to maintain a spherical shape.

Capillary Action

Capillary action is the ability of water to flow in narrow spaces without the assistance of external forces. It results from the combination of cohesion and adhesion.

• Key Point: Capillary action is essential for the movement of water from roots to leaves in plants.

• Example: Water rising in a thin tube or moving up the stem of a plant.

Thermal Properties of Water

Heat and Temperature

Water's thermal properties are crucial for life. Heat is the total energy of molecular motion in a substance, while temperature measures the average kinetic energy of molecules.

• Key Point: A large body of water can store more heat than a small body, even if both are at the same temperature, due to the greater number of molecules.

• Example: A lake at 20°C contains much more heat than a cup of water at 20°C.

Specific Heat of Water

Specific heat is the amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius. Water has a high specific heat, meaning it can absorb or release large amounts of heat with only a slight change in temperature.

• Key Point: The specific heat of water is approximately 1 calorie per gram per degree Celsius ().

• Example: Water helps moderate Earth's climate by absorbing heat in the summer and releasing it in the winter.

• Formula: where is heat energy, is mass, is specific heat, and is change in temperature.

Evaporative Cooling

Evaporative cooling occurs when the molecules with the highest kinetic energy evaporate from a liquid, lowering the temperature of the remaining liquid.

• Key Point: This process helps organisms regulate temperature, such as sweating in humans or transpiration in plants.

• Example: After sweating, the evaporation of water from the skin cools the body.

Temperature Scales: Celsius and Kelvin

In biology, temperature is commonly measured in degrees Celsius. The Celsius scale is based on the freezing point (0°C) and boiling point (100°C) of water at standard atmospheric pressure. The Kelvin scale is used in scientific contexts and starts at absolute zero.

• Key Point: Celsius is preferred in biology for its relevance to water's phase changes.

• Formula:

Summary Table: Water Properties

Macromolecules and Water

Water is a universal solvent, meaning it can dissolve a wide variety of substances, including many biological macromolecules. This property is essential for biochemical reactions and transport within cells.

• Key Point: Water's polarity allows it to dissolve ionic and polar substances.

• Example: Dissolved salts, sugars, and proteins in the cytoplasm of cells.

Additional info: Some explanations and examples have been expanded for clarity and completeness, based on standard biology curriculum.