IB Biology HL – WATER (Topic 1.1 Core Concepts)

1. What is the molecular composition of water?

H₂O → one oxygen atom covalently bonded to two hydrogen atoms.

2. Why is water considered polar?

Because oxygen pulls electrons more strongly, giving O a partial negative charge and H atoms partial positive charges.

3. What type of bonds form within a water molecule?

Polar covalent bonds.

4. What type of bonds form between water molecules?

Hydrogen bonds (intermolecular, weaker than covalent).

5. Why does water have positive and negative poles?

Unequal electron sharing → oxygen becomes δ– and hydrogens become δ+.

6. What property allows water molecules to stick to each other?

Cohesion (due to hydrogen bonding).

7. Biological significance of cohesion?

Allows water transport up xylem; creates surface tension.

8. What property allows water to stick to other surfaces?

Adhesion.

9. Biological significance of adhesion?

Helps water climb xylem walls against gravity.

10. Why is water a good solvent?

It dissolves polar molecules and ions because its polarity surrounds and separates them.

11. Biological significance of water as a solvent?

Allows metabolic reactions, transport in blood, enzyme function.

12. What is buoyancy?

Upward force exerted by water on immersed objects.

13. Biological significance of buoyancy?

Aquatic animals float more easily → lower energy cost.

14. What is viscosity?

Resistance to movement through a fluid.

15. How does viscosity affect aquatic organisms?

Higher drag → more energy needed to move.

16. Why does water have high thermal conductivity?

Hydrogen bonds allow efficient heat transfer.

17. Biological significance of thermal conductivity?

Stabilizes temperature in aquatic environments.

18. Why does water have high specific heat capacity?

Hydrogen bonds require lots of energy to break.

19. Biological significance of high specific heat capacity?

Prevents rapid temperature changes → stable habitats.

20. Why do aquatic animals need more energy than terrestrial animals?

Higher viscosity + higher thermal conductivity → more energy for movement and maintaining body temperature.

21. What is the Goldilocks Zone?

Region around a star where liquid water can exist.

22. Why does Earth retain water?

Gravity prevents escape; distance from Sun keeps temperature suitable.

23. What is the hypothesis for the origin of Earth’s water?

Delivered by collisions with water‑rich asteroids.