MCQs

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.

Wax is a non-polar, hydrophobic surface. The polar water molecules are more strongly attracted to each other via hydrogen bonding (cohesion) than they are to the wax surface (adhesion). This causes the water to minimize its contact with the wax and form spherical beads.

Imbibition is the physical adsorption of water onto the hydrophilic surfaces of macromolecules (proteins, polysaccharides) and cell walls inside the seed. This matric potential is an extremely negative component of water potential, creating a massive driving force for water uptake.

Evaporation is a physical change, not a chemical one. The covalent bonds within the molecule are not broken. The energy (latent heat of vaporization) is used to overcome the attractive intermolecular hydrogen bonds holding the water molecules together in the liquid phase.

Water is the solvent in which buffers operate, and its ionization constant (Kw) is fundamental to the pH scale. A buffer's equilibrium (e.g., H₂CO₃ ⇌ H⁺ + HCO₃⁻) involves the release or absorption of H⁺ ions produced by water's interaction with the buffer components, resisting pH change.

Wilting occurs when transpiration exceeds water uptake. Cells lose water, decreasing their water potential and turgor pressure. The protoplast no longer presses firmly against the cell wall, and the non-woody tissues lose their rigidity, causing the plant to droop.

The hydration of CO₂ (CO₂ + H₂O → H₂CO₃) produces carbonic acid, which dissociates into H⁺ and HCO₃⁻. The release of H⁺ lowers the pH. This is a major source of acidity, not just the dissolution of pre-formed lactic acid.

Water, as a major component of synovial fluid and serous fluid, forms a cohesive, slippery film. The high cohesion of water molecules allows the fluid to resist being squeezed out from between surfaces under pressure, providing an excellent lubricating layer.

An increase in plasma solute (NaCl) concentration decreases its water potential. Water moves by osmosis from the area of higher water potential (inside cells) to the area of lower water potential (blood plasma), leading to cellular dehydration and increased blood volume/pressure.

Water molecules form highly ordered "cage-like" structures around exposed non-polar groups, which decreases entropy. To minimize this, water forces non-polar groups to aggregate, freeing the caged water and increasing overall entropy. This entropic force is the hydrophobic effect.

While water's polarity and geometry are contributing factors, the direct reason for its abnormally high boiling point is the strength and number of intermolecular hydrogen bonds. A large amount of thermal energy is required to disrupt these attractive forces to allow molecules to escape as vapor.