Practice Questions

Water molecules form highly ordered, cage-like structures (clathrates) around non-polar solutes to maintain hydrogen bonding. This organization represents a local decrease in entropy (ΔS < 0), which is thermodynamically unfavorable and drives the hydrophobic effect.

The high heat of vaporization (latent heat) means a large amount of thermal energy is required to convert liquid sweat to vapor. This energy is absorbed from the skin, lowering its temperature. This is a highly effective cooling mechanism.

In a glass cylinder, the adhesive force between polar water and the glass is stronger than the cohesive force between water molecules. Water climbs the glass wall, creating a concave meniscus. In contrast, mercury (non-polar) has stronger cohesion than adhesion to glass, forming a convex meniscus.

The insect's mass is distributed over its long hydrophobic legs so that the force per unit area is less than the surface tension. The cohesive hydrogen bonds at the water-air interface create a strong film that resists being broken by the light insect.

Water is relatively transparent to wavelengths of visible light, the spectrum used for photosynthesis. This allows aquatic plants and phytoplankton to carry out photosynthesis in the upper layers (photic zone), forming the base of the aquatic food web.

Adhesion is the attraction of water to the xylem walls (cellulose is polar with many -OH groups). This adhesion helps to counteract gravity and, combined with cohesion, allows for a continuous capillary column. The transpiration-cohesion-tension mechanism relies on both adhesion and cohesion.

Most liquids contract upon cooling, becoming densest at their freezing point. Water behaves anomalously; it reaches its maximum density at 4°C. Below 4°C, it expands. This is due to the formation of transient, expanded ice-like clusters of hydrogen bonds as it approaches the freezing point.

The reaction center P680, upon excitation by light, donates an electron to the primary electron acceptor and becomes a strong oxidant (P680⁺). It extracts electrons from water molecules via the oxygen-evolving complex, splitting water and returning P680 to its ground state.

Water drives membrane formation. The hydrophobic fatty acid tails are excluded from water to minimize the ordering of water molecules. This entropic force causes the tails to aggregate, while the polar heads interact favorably with water, self-assembling into a bilayer.

The dielectric constant is a measure of a solvent's ability to insulate opposite charges from each other. Water's high value (~80) means it weakens the electrostatic attraction between dissolved ions, enabling their dissociation and hydration. This is central to its role as a solvent for salts.