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AS · Practice questions · Water

Property, reason, role.

Six original Cambridge-style questions on the polarity of water, hydrogen bonding and the three properties the syllabus asks you to link to roles in organisms.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Always start from the hydrogen bond.

01
Recall
[3 marks]

Explain how hydrogen bonds form between water molecules.

  • Oxygen is more electronegative than hydrogen, so electrons in the O-H bonds are pulled towards O. ✓
  • The O gains a partial negative charge (δ−) and each H a partial positive charge (δ+); the molecule is polar. ✓
  • A δ+ H of one molecule is attracted to a lone pair on the δ− O of another molecule: a hydrogen bond. ✓
02
Explain
[3 marks]

Explain why water is a good solvent and give two roles of this property in living organisms.

  • Water is polar: δ− O is attracted to cations and δ+ H to anions, separating and surrounding the ions; polar molecules form hydrogen bonds with water. ✓
  • Role: a transport medium, for example glucose and ions in blood plasma or mineral ions in xylem. ✓
  • Role: metabolic reactions take place in solution in the cytoplasm (also accept removal of urea in urine). ✓
03
Calculation
[2 marks]

The specific heat capacity of water is 4.2 J g−1 °C−1. Calculate the energy needed to raise the temperature of 500 g of water by 3.0 °C.

  • Energy = mass × specific heat capacity × temperature change. ✓
  • = 500 × 4.2 × 3.0 = 6300 J (6.3 kJ). ✓
04
Calculation
[2 marks]

During a long run an athlete evaporates 800 g of sweat. The latent heat of vaporisation of water at skin temperature is about 2.4 kJ g−1. Calculate the heat removed from the body.

  • Heat removed = mass evaporated × latent heat of vaporisation. ✓
  • = 800 × 2.4 = 1920 kJ (about 1.9 MJ). ✓
05
Explain
[3 marks]

Explain why the high specific heat capacity of water is important to organisms.

  • A lot of energy is needed to raise the temperature of water because hydrogen bonds must be broken first. ✓
  • So the temperature of cells and bodies, which are mostly water, changes only slowly, keeping conditions stable for enzymes. ✓
  • Large bodies of water have stable temperatures, providing stable aquatic habitats. ✓
06
Data
[3 marks]

Over 24 hours the air above a lake varies from 6 °C to 24 °C, while the water 1 m below the surface varies from 15 °C to 16 °C. Describe and explain the difference.

  • The air temperature range is 18 °C; the water temperature range is only 1 °C. ✓
  • Water has a high specific heat capacity: much of the energy absorbed is used to break hydrogen bonds, so the temperature rises little. ✓
  • So the lake is a thermally stable habitat for aquatic organisms. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.