Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Oxygen is more electronegative than hydrogen, so the O–H bonds are polar covalent. The bent shape makes the whole molecule polar (δ⁻ on O, δ⁺ on each H) but not charged.
- A hydrogen bond is a weak attraction between a δ⁺ H on one molecule and a δ⁻ O on another. Each water molecule can make up to four. They constantly break and re-form.
- Hydrogen bonds also form within and between large biological molecules, such as DNA and proteins.
- The network of hydrogen bonds causes cohesion, adhesion, surface tension, high specific heat, high heat of vaporization, low density of ice and strong solvent action.
Key relationships
| Mechanism | Property | Biological outcome |
|---|---|---|
| H bonds between water molecules | cohesion, surface tension | water column pulled up xylem during transpiration |
| H bonds to polar surfaces | adhesion | water clings to xylem walls |
| energy breaks H bonds before raising temperature | high specific heat | stable body and lake temperatures |
| escaping molecule must break all its H bonds | high heat of vaporization | evaporative cooling (sweat, transpiration) |
| H bonds fix an open lattice | ice less dense than water | ice floats and insulates water below |
| δ⁻/δ⁺ ends surround ions and polar molecules | good solvent | transport of ions and sugars in cells and blood |
| Equation | Symbols and units |
|---|---|
| q = m × c × ΔT | q in J, m in g, c in J g⁻¹ °C⁻¹ (water: 4.18), ΔT in °C |
| ΔT = q ÷ (m × c) | rearranged to compare temperature changes |
Assumptions behind the numbers
- q = mcΔT assumes no energy is lost to the surroundings and no change of state during warming.
- Values of c apply to the stated phase (liquid water, not ice or steam).
- Small experiments (few trials) support or do not support a hypothesis; they do not prove it.
Mistakes to avoid
- Calling the O–H bond inside water a hydrogen bond.
- Saying high specific heat means water heats quickly.
- Saying cohesion alone lifts water; evaporation provides the pull.
- Explaining floating ice with trapped air.
- Using the final temperature instead of ΔT in q = mcΔT.
- Treating a correlation as proof of cause without ruling out confounding variables.
Quick self-check
- Why is a water molecule polar but not an ion? (Unequal sharing in O–H bonds and a bent shape give δ⁻ and δ⁺ ends, but the charges sum to zero.)
- Energy to warm 100 g of water by 5.0 °C? (100 × 4.18 × 5.0 = 2090 J, about 2.1 kJ)
- Why does sweating cool the body? (The most energetic molecules break their hydrogen bonds and escape, lowering the average energy of what remains.)
Next: practice questions.