Practice Questions

Water has a high dielectric constant (~80 at 20°C). This means it significantly weakens the electrostatic force of attraction between the oppositely charged Na⁺ and Cl⁻ ions in the crystal lattice, allowing them to dissociate and become surrounded by hydration shells.

Anabolic polymer synthesis (e.g., peptide bond, glycosidic bond, phosphodiester bond) involves the removal of a water molecule (dehydration synthesis). The hydroxyl group is removed from one monomer and a hydrogen from another, forming water and a new covalent bond.

Water potential quantifies the tendency of water to move from one area to another. It is the difference between the chemical potential of water in a system and that of pure water at the same temperature and atmospheric pressure. It comprises solute potential (ψs) and pressure potential (ψp).

In liquid water, hydrogen bonds constantly break and reform, allowing close packing. Upon freezing, water molecules lock into a stable, tetrahedral arrangement where each molecule is H-bonded to four others, creating large spaces. This expanded lattice results in lower density.

The reaction CO₂ + H₂O ⇌ H₂CO₃ is catalyzed by carbonic anhydrase. Here, water is not just a solvent but a substrate that chemically participates in the reaction by combining directly with carbon dioxide.

Large bodies of water absorb solar energy during the day and release it slowly at night without large temperature swings. This property, due to the extensive hydrogen bonding network, protects aquatic organisms from thermal shock and provides a stable environment.

In ice, each water molecule forms a maximum of four hydrogen bonds in a rigid, crystalline hexagonal lattice. This structure holds molecules farther apart on average than in the liquid state, reducing the density. This lattice is an open, ordered arrangement, maximizing H-bonding.

Osmosis is the net passive diffusion of water across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). It does not require energy and is central to water relations in cells.

In the Krebs cycle, the enzyme fumarase catalyzes the addition of a water molecule across the double bond of fumarate. This hydration reaction converts fumarate to malate. Water is a direct reactant in this specific step, not just a solvent.

When sweat evaporates, the phase change from liquid to gas requires a large amount of heat energy (latent heat of vaporization). This heat is absorbed from the skin's surface, effectively cooling the body. The cooling is due to heat removal, not heat release.