Properties of Water and Acid-Base Chemistry in Biology

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Water molecules exhibit unique cohesive and adhesive properties due to their molecular structure and hydrogen bonding.

Cohesion: Water molecules are attracted to each other via hydrogen bonds, resulting in high surface tension.
Adhesion: Water molecules can also be attracted to other substances, such as glass, which is important for processes like capillary action.
Example: Water droplets forming on a leaf due to cohesion; water climbing up a thin tube due to adhesion.

Water has a high capacity to absorb and retain heat, which is crucial for maintaining stable temperatures in biological systems.

Thermal energy: The kinetic energy of molecules; higher thermal energy means higher temperature.
Specific heat: Water can absorb a large amount of heat with only a small increase in temperature.
Calorie: The amount of energy needed to raise 1 gram of water by 1 degree Celsius.
High heat of vaporization: Water requires significant energy to change from liquid to gas, helping organisms cool down via evaporation.
Expansion upon freezing: Water forms a crystalline structure when frozen, making ice less dense than liquid water.
Hydrogen bonds: These bonds are not broken when water freezes, contributing to the unique properties of ice.

Water's polarity makes it an excellent solvent, capable of dissolving a wide variety of substances.

Solution: A homogeneous mixture of two or more substances.
Solvent: The substance that dissolves the solute (water is often called the "universal solvent").
Solute: The substance that is dissolved.
Polarity: Water's polar nature allows it to dissolve ionic and polar compounds effectively.
Example: Table salt (NaCl) dissolving in water due to the attraction between water molecules and the ions.

The pH of a solution is determined by the concentration of hydrogen ions (H+), which affects biological processes.

Water autoionization: Water molecules can dissociate into H+ and OH- ions.
pH scale: Measures the concentration of H+ ions; lower pH means higher acidity.
Acid: A substance that increases the H+ concentration in a solution.
Base: A substance that decreases the H+ concentration, often by increasing OH-.
Strong acids and bases: Dissociate completely in water.
Weak acids and bases: Dissociate only partially.

Buffers help maintain stable pH in biological systems by neutralizing excess acids or bases.

Buffer: A solution that resists changes in pH when acids or bases are added.
Buffer solution: Typically consists of a weak acid and its conjugate base.
Example: Blood contains buffers to maintain a stable pH necessary for physiological functions.

The following table illustrates how buffers work to maintain pH stability when acids or bases are added:

Condition	Buffer Action	Result
Addition of OH-	Weak acid neutralizes added base, forming conjugate base	pH remains stable
Addition of H3O+	Conjugate base neutralizes added acid, forming weak acid	pH remains stable

Additional info: Buffer systems are essential in biological fluids, such as blood, to prevent harmful changes in pH.