AP Bio Chap 3

Chapter 3: Water & Life — Study Guide1. Water Molecules

  • Water has polar covalent bonds.

  • Oxygen is slightly negative (δ−).

  • Hydrogen is slightly positive (δ+).

  • Water molecules form hydrogen bonds with each other.

  • Water is polar because its charge is unevenly distributed.

Know:

Polar covalent bond → polarity → hydrogen bonding

2. Four Properties of Water

Know these 4:

  1. Cohesion

  2. Moderates temperature

  3. Expands when freezing

  4. Versatile solvent

Cohesion vs. Adhesion

  • Cohesion = water sticks to water

  • Adhesion = water sticks to other substances

  • Cohesion + adhesion help move water through plants.

Surface Tension

  • Water has high surface tension because of hydrogen bonds.

3. Temperature & Heat

  • Kinetic energy = energy of motion

  • Thermal energy = kinetic energy from random molecular movement

  • Temperature = average kinetic energy

  • Heat = thermal energy transferred between objects

Specific Heat

  • Water has a high specific heat.

  • This means water can absorb lots of heat without changing temperature very much.

  • Hydrogen bonds are responsible for much of this.

Important:
Hydrogen bonds break → absorb heat
Hydrogen bonds form → release heat

4. Evaporative Cooling

  • Evaporation = liquid → gas

  • Heat of vaporization = heat needed to turn liquid into gas.

  • Evaporative cooling = evaporation causes the remaining liquid to cool.

  • Helps organisms maintain temperature.

5. Ice & Density

  • Ice floats because its hydrogen bonds form a more ordered structure.

  • Ice is less dense than liquid water.

  • Water is most dense at 4°C.

  • Floating ice helps keep bodies of water from completely freezing.

💧 6. Water as a SolventVocabulary

  • Solution = homogeneous mixture

  • Solvent = does the dissolving

  • Solute = gets dissolved

  • Aqueous solution = water is the solvent

Why is water a good solvent?

Because water is polar.

  • Ionic compounds can dissolve in water.

  • Water molecules surround ions in a hydration shell.

  • Polar molecules can also dissolve in water.

7. Hydrophilic vs. Hydrophobic

Hydrophilic = attracted to water
Hydrophobic = not attracted to water

  • Oil is hydrophobic because it is mostly nonpolar.

  • Hydrophobic molecules are important parts of cell membranes.

Easy way to remember:

PHILIC = likes water
PHOBIC = avoids water

🧮 8. Moles & MolarityMole

1 mole = 6.02 × 10²³ molecules

This number is called Avogadro's number.

Molarity

Molarity (M) = moles of solute ÷ liters of solution

🧪 9. Acids & BasesWater can dissociate:

H₂O ⇌ H⁺ + OH⁻

More accurately:
H₂O ⇌ H₃O⁺ + OH⁻

  • H₃O⁺ = hydronium

  • OH⁻ = hydroxide

Acid

An acid increases H⁺.

Base

A base decreases H⁺.

  • Strong acids/bases = completely dissociate.

  • Weak acids/bases = only partially/reversibly dissociate.

📊 10. pH Scale — VERY IMPORTANT

pH = −log[H⁺]

Remember:

pH

Type

0–6

Acidic

7

Neutral

8–14

Basic

  • Acidic: more H⁺

  • Neutral: H⁺ = OH⁻

  • Basic: more OH⁻

At pH 7:
[H⁺] = 10⁻⁷

Also:
[H⁺][OH⁻] = 10⁻¹⁴

Easy memory trick:

LOW pH = ACID
HIGH pH = BASE

🛡 11. Buffers

  • Buffers help prevent big changes in pH.

  • They minimize changes in H⁺ and OH⁻.

  • Most buffers contain a weak acid + its corresponding base.

  • Most living cells have an internal pH close to 7.