AP Biology: Unit 1, Water Properties & Hydrogen Bonding (Drill 2)

Question 1. Which property of water best explains why water rises higher in the 0.5 mm capillary tube than in the 1.0 mm tube in Experiment 1?

• A) Cohesion, because water molecules bond to each other and pull the water column upward as a unit, and this force is proportionally greater in narrower tubes than the adhesive pull between water and the glass surface, rather than contact with the tube wall.
• B) Adhesion, because water molecules form hydrogen bonds with the polar glass surface and are pulled upward; the narrower tube provides a greater surface-area-to-volume ratio, so adhesive forces dominate over gravity in a smaller cross section. ✓
• C) High specific heat capacity, because narrower capillary tubes retain more thermal energy than wide ones, and this extra heat drives the water column upward.
• D) Universal solvent properties, because dissolved minerals reduce water density in narrow tubes and cause the water column to rise.

Explanation: Correct: (B) Capillary action depends on both adhesion (water sticking to the glass surface) and cohesion (water molecules pulling each other upward). The key to why water rises higher in the narrower tube is the surface-area-to-volume ratio: a narrower tube has proportionally more glass surface relative to the volume of water it contains, so adhesive forces are stronger relative to the weight of the water column. Option A describes cohesion correctly but misidentifies which force drives the height difference. Options C and D describe unrelated properties.

Question 2. In Experiment 2, drops of water on the waxed (hydrophobic) surface bead up rather than spreading out. Which of the following best explains this observation?

• A) Wax increases the specific heat capacity of the water droplet, causing it to retain a spherical shape through thermal expansion on the surface.
• B) Water molecules on the waxed surface lose hydrogen bonds with each other, causing the drop to expand outward into a flattened shape that maximizes contact.
• C) Water molecules cannot form hydrogen bonds with the nonpolar wax surface, so cohesive forces among water molecules dominate and the drop pulls inward to minimize its surface area. ✓
• D) The high heat of vaporization of water prevents the drop from spreading because energy is required to break hydrogen bonds as the drop flattens.

Explanation: Correct: (C) On a hydrophobic surface, water cannot form hydrogen bonds with the surface material, making adhesive forces negligible. Cohesive forces between water molecules then dominate, causing the molecules to pull inward toward each other. The drop adopts a rounded, beaded shape that minimizes surface area. On the glass surface, adhesion competes with cohesion and the drop spreads. This demonstrates surface tension, which is a direct consequence of cohesion.

Question 3. A freshwater lake in a temperate climate remains unfrozen beneath a layer of surface ice throughout the winter. Which property of water is most directly responsible for allowing aquatic organisms to survive in the liquid water below?

• A) High specific heat capacity, which prevents the lake from losing heat rapidly as air temperatures drop.
• B) High heat of vaporization, which releases large amounts of heat energy into the lake as the surface water cools during winter.
• C) The fact that ice is less dense than liquid water, so ice forms at the surface and insulates the liquid water below from further heat loss. ✓
• D) Cohesion, which keeps liquid water molecules bonded together at low temperatures and prevents the entire lake from freezing at once under the conditions described.

Explanation: Correct: (C) Water is unusual in that its solid form (ice) is less dense than its liquid form. When water freezes, hydrogen bonds arrange into a regular crystalline lattice that spaces molecules farther apart than in liquid water, reducing density. Because ice is less dense, it floats on the surface. This floating layer acts as an insulating barrier, slowing heat loss from the liquid water below. Aquatic organisms survive in this liquid layer even when surface temperatures are well below 0°C. High specific heat capacity (A) and heat of vaporization (B) both contribute to temperature moderation but do not explain why liquid water persists beneath ice.

Question 4. During intense exercise, human body temperature rises. One mechanism that prevents dangerous overheating is evaporative cooling through sweating. Which property of water makes evaporative cooling particularly effective?

• A) Water's high heat of vaporization, because a large amount of heat energy must be absorbed from the body's surface to break the hydrogen bonds holding water molecules together in liquid form, carrying that heat away as sweat evaporates. ✓
• B) Water's high specific heat capacity, because a large amount of energy is stored in water molecules and released as they evaporate from skin.
• C) Water's cohesion, because water molecules on the skin surface pull liquid water up through sweat glands, removing heat from deeper tissues.
• D) Water's role as a universal solvent, because dissolved salts in sweat substantially increase the amount of heat energy released as the sweat evaporates.

Explanation: Correct: (A) Water has an exceptionally high heat of vaporization (~2,260 J/g). When water molecules evaporate from the skin surface, they must absorb a large amount of thermal energy from the skin and underlying tissues to break the hydrogen bonds holding them in the liquid phase. This heat is carried away with the evaporating water molecules, cooling the body's surface. High specific heat capacity (B) describes how much energy is required to raise water's temperature, a related but distinct property that does not directly drive evaporative cooling.

Question 5. A student claims that the high specific heat capacity of water is primarily responsible for the mild temperatures experienced by coastal cities compared to inland cities at the same latitude. Which of the following best evaluates this claim?

• A) The claim is incorrect because coastal cities are moderated mainly by ocean breezes, which depend on water's cohesion rather than its high specific heat capacity.
• B) The claim is incorrect because it is water's high heat of vaporization, not specific heat capacity, that moderates coastal temperatures by releasing heat energy as water condenses over coastal areas within the parameters described here.
• C) The claim is partially correct but incomplete because water's universal solvent properties also contribute to temperature moderation by allowing dissolved salts to absorb heat.
• D) The claim is correct because water's high specific heat capacity means oceans absorb large amounts of heat during summer and release it slowly in winter, moderating temperature extremes for nearby coastal regions. ✓

Explanation: Correct: (D) Water's high specific heat capacity means a large amount of heat energy must be added or removed to change water's temperature. Large bodies of water therefore act as thermal buffers: they warm slowly in summer and cool slowly in winter. Coastal areas adjacent to these bodies of water experience less extreme temperature fluctuations than inland areas surrounded by land, which has a much lower heat capacity. Heat of vaporization (B) contributes to evaporation and precipitation cycles but is not the primary explanation for coastal temperature moderation.