Explain how hydrogen bonding between water molecules gives water a high specific heat capacity and outline the significance of this property for organisms living in aquatic environments.

IB DP Biology Higher Level (2023 syllabus) — A1.1 Water · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Water molecules are polar; the O–H bonds give a partial negative charge on O and a partial positive on H. This polarity allows each molecule to form hydrogen bonds with neighbouring molecules. A large amount of heat energy is required to break or disrupt these hydrogen bonds before the kinetic energy of the molecules – and hence the temperature – can rise. Consequently water has a high specific heat capacity.

Because of this high specific heat, the temperature of water bodies changes only slowly even when air temperatures fluctuate widely. This thermal stability keeps the internal environment of aquatic organisms relatively constant, allowing their enzymes and metabolic processes to operate efficiently and supporting survival.

Examiner tips

  • Use the term ‘hydrogen bonds’ and explain they arise from polarity of O–H bonds. Show that energy is needed to break bonds before temperature rises. Mention thermal stability of water bodies. Link stability to enzyme activity and organism survival.

Common mistakes

  • Forgetting to mention the polarity of O–H bonds. Saying water has a high specific heat without explaining the role of hydrogen bonds. Not connecting the thermal stability to the benefit for aquatic organisms.

Mark scheme (4 marks)

  1. Water molecules form hydrogen bonds with neighbouring water molecules due to the partial charges arising from the polarity of the O–H bonds.
  2. A large amount of heat energy must be absorbed to break / disrupt these hydrogen bonds before the kinetic energy of molecules (and therefore temperature) increases, resulting in a high specific heat capacity.
  3. The high specific heat capacity means the temperature of aquatic environments changes only slowly / remains relatively stable despite large fluctuations in air temperature.
  4. This thermal stability allows aquatic organisms to maintain relatively constant internal conditions / enzyme-catalysed reactions to proceed efficiently, supporting survival and metabolic activity.

Key terms in this question

hydrogen bond · specific heat capacity

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