Explain how hydrogen bonding between water molecules gives water a high latent heat of vaporisation, and outline one way this property is significant to living organisms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Hydrogen bonds form between water molecules because the O–H bond is polar, giving oxygen a partial negative charge and hydrogen a partial positive charge.
The many hydrogen bonds that hold the liquid together mean a large amount of energy is required to break them before the molecules can escape as vapour.
Therefore the evaporation of a small amount of water removes a large amount of heat energy from the surface or organism.
One biological significance is that sweating (or transpiration) uses this latent heat to cool the body (or leaf), helping to maintain a stable temperature.
The many hydrogen bonds that hold the liquid together mean a large amount of energy is required to break them before the molecules can escape as vapour.
Therefore the evaporation of a small amount of water removes a large amount of heat energy from the surface or organism.
One biological significance is that sweating (or transpiration) uses this latent heat to cool the body (or leaf), helping to maintain a stable temperature.
Examiner tips
- Use the term ‘hydrogen bond’ and explain the polar O–H bond; show the link to energy required to break bonds; mention heat removal; give a clear example of cooling such as sweating.
- Keep each point short and directly tied to the marks; use correct terminology and avoid unnecessary words.
Common mistakes
- Confusing hydrogen bonds with covalent bonds; not stating that energy is required to break bonds; giving an example that does not involve cooling (e.g. water transport) or omitting the heat removal aspect.
Mark scheme (4 marks)
- Hydrogen bonds form between water molecules due to the polar nature of the O–H bond / the partial negative charge on oxygen and partial positive charge on hydrogen.
- A large amount of energy is required to break the many hydrogen bonds holding water molecules together before vaporisation can occur.
- This means that the evaporation of a small amount of water removes a large amount of heat energy from the surface / organism.
- One biological significance outlined — e.g. sweating / transpiration / panting acts as an effective cooling mechanism, helping to maintain a stable body/leaf temperature (thermoregulation).
Key terms in this question
latent heat of vaporisation · hydrogen bond
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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