AP Biology Unit 1 Topic 1.1: Properties of Water, Hydrogen Bonding, Polarity Explained for AP Exam

Question 1

Why is water considered a polar molecule?

Accepted Answer

Water is polar because oxygen has a much stronger pull on electrons than hydrogen does. This unequal sharing creates a partial negative charge near the oxygen atom and partial positive charges near the two hydrogen atoms. This uneven charge distribution gives water its dipole, which is the foundation of nearly all of its biological properties.

Question 2

What is a hydrogen bond?

Accepted Answer

A hydrogen bond is a weak attraction between a partially positive hydrogen atom in one molecule and a partially negative atom, usually oxygen or nitrogen, in another molecule. In liquid water, each molecule can form up to four hydrogen bonds with neighbors. Although individually weak, these bonds collectively give water many of its emergent properties.

Question 3

What is cohesion in water and why does it matter biologically?

Accepted Answer

Cohesion is the attraction between water molecules due to hydrogen bonding. It allows water to form droplets, creates surface tension, and enables water to be pulled up tall plant stems against gravity through xylem. Without cohesion, transpiration in trees would be impossible.

Question 4

What is adhesion and how does it differ from cohesion?

Accepted Answer

Adhesion is the attraction between water molecules and other polar substances, while cohesion is the attraction between water molecules themselves. Together they produce capillary action, which helps water climb narrow tubes. Both are essential for water transport in plants from roots to leaves.

Question 5

Why does water have a high specific heat?

Accepted Answer

Water has a high specific heat because hydrogen bonds must be broken before water molecules can move faster, which is what increases temperature. This means water absorbs a lot of heat with only a small temperature change. Biologically, this stabilizes ocean temperatures, regulates body temperature in organisms, and protects coastal climates.

Question 6

How does evaporative cooling work in water?

Accepted Answer

When water evaporates, the molecules with the highest kinetic energy escape as vapor, leaving behind cooler molecules. This is why sweating cools mammals and transpiration cools plant leaves. The high heat of vaporization of water makes this process particularly effective at removing excess heat.

Question 7

Why is ice less dense than liquid water?

Accepted Answer

When water freezes, hydrogen bonds lock the molecules into a crystalline lattice that holds them farther apart than they are in the liquid. This makes ice about 9 percent less dense than water, so ice floats. This property allows aquatic organisms to survive winter beneath an insulating ice layer.

Question 8

Why is water called the universal solvent?

Accepted Answer

Water is called the universal solvent because its polarity allows it to dissolve a wide range of polar and ionic substances. Water molecules surround ions and polar molecules, separating them and keeping them in solution. This is why aqueous solutions are the medium of nearly all biochemical reactions in living cells.

Question 9

What is the difference between a hydrophilic and a hydrophobic substance?

Accepted Answer

Hydrophilic substances are polar or charged and dissolve readily in water because they form favorable interactions with water molecules. Hydrophobic substances are nonpolar and do not dissolve because water molecules form hydrogen bonds with each other rather than with the nonpolar molecule. This distinction explains membrane structure and protein folding.

Question 10

What does pH measure and why does it matter for living systems?

Accepted Answer

pH measures the concentration of hydrogen ions in a solution on a logarithmic scale from zero to fourteen. A pH of seven is neutral, below seven is acidic, and above seven is basic. Living systems require narrow pH ranges because enzymes denature outside their optimal pH, which is why blood and cells use buffers to maintain stable pH.

Question 11

What is a buffer and how does it stabilize pH?

Accepted Answer

A buffer is a substance that resists changes in pH by absorbing excess hydrogen ions or releasing them when needed. Buffers usually consist of a weak acid and its conjugate base. In humans, the bicarbonate buffer system in blood keeps pH near 7.4, which is essential for enzyme function and oxygen transport.

Question 12

How does hydrogen bonding explain water's surface tension?

Accepted Answer

Surface tension arises because water molecules at the surface have no neighbors above, so they form stronger hydrogen bonds with the molecules beside and below them. This creates a film-like layer at the surface. Surface tension is what allows insects like water striders to walk on water without sinking.

Question 13

Why is water essential for all known life?

Accepted Answer

Water is essential because it serves as a solvent for biochemical reactions, regulates temperature, transports nutrients, participates directly in reactions like hydrolysis and dehydration synthesis, and provides the medium in which proteins fold and enzymes function. No known organism can survive without liquid water in some form.

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