Explain hydrogen bonding and water's anomalies
Identify syllabus hydrogen-bond donors and use hydrogen bonding to explain water's physical properties and ice density.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the supplied Pauling values when given, show particle and bond reasoning, and distinguish intramolecular bonds from intermolecular forces. Written practice is self-checked
Before you begin
- Read element symbols and simple molecular formulae; use the Periodic Table.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In the syllabus examples, which group can make a hydrogen-bond donor?
What you will learn
- Describe hydrogen bonding for molecules with N–H or O–H groups
- Explain water's high melting and boiling points, surface tension and ice density
Hydrogen bonding is a strong intermolecular attraction in this syllabus context
For this topic, hydrogen bonding is considered between molecules containing N–H or O–H groups. The hydrogen bonded to a highly electronegative atom is strongly attracted to a lone pair on a nitrogen or oxygen atom of a neighbouring molecule. Ammonia and water are the simple examples.
A hydrogen bond is intermolecular in these examples; do not confuse it with the covalent N–H or O–H bond inside one molecule.
Water's hydrogen-bond network changes its properties
Many hydrogen bonds between water molecules require energy to overcome, helping explain water's relatively high melting and boiling points and relatively high surface tension compared with similar-sized molecules.
In ice, hydrogen bonds hold water molecules in an open arrangement. On melting, some of this open structure collapses so molecules can pack more closely; liquid water is therefore denser than ice near the melting point. The solid ice floats because its density is lower.
Pause and explain
Why is ice less dense than liquid water near melting?
Worked example
Explain why ice is less dense than liquid water near its melting point.
Show the worked solution
- In ice, hydrogen bonds hold molecules in an open lattice arrangement.
- This structure occupies more volume for a given amount of water.
- When some structure collapses on melting, the molecules pack more closely.
Answer Ice has an open hydrogen-bonded structure, so the same amount occupies more volume and has lower density than liquid water near its melting point.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why water has a relatively high boiling point using the intermolecular model.
Give me a hint
Compare energy needed to separate water molecules with a similar-sized molecule lacking this network.
Compare my reasoning
- Water molecules form many hydrogen bonds.
- Boiling separates molecules by overcoming intermolecular attractions.
- The hydrogen-bond network requires substantial energy to disrupt.
Water has a relatively high boiling point because energy is needed to overcome many intermolecular hydrogen bonds between its molecules.
Look for these in your work
- Hydrogen bonds identified
- Intermolecular nature stated
- Energy and boiling linked
Explain the high surface tension of water and state the relevant attraction.
Give me a hint
Surface molecules experience cohesive attraction from neighbouring water molecules.
Compare my reasoning
- Water molecules form hydrogen bonds with neighbours.
- These intermolecular attractions create cohesion at the surface.
- Relate this cohesion to the relatively high surface tension.
Intermolecular hydrogen bonding gives water strong cohesion, contributing to its relatively high surface tension.
Look for these in your work
- Intermolecular attraction identified
- Cohesion explained
- Property linked
Compare the density of ice with liquid water and explain the difference in molecular packing.
Give me a hint
Track what the open lattice does to volume on melting.
Compare my reasoning
- Ice has an open hydrogen-bonded lattice.
- For a fixed mass, the open structure occupies more volume.
- Melting allows closer packing, so liquid water near melting is denser.
Ice is less dense. Its open lattice occupies more volume for the same mass; some of this structure collapses on melting, allowing closer packing in liquid water.
Look for these in your work
- Relative densities correct
- Open solid structure named
- Volume and packing linked
Common mistakes
- Calling an O–H covalent bond a hydrogen bond
- Claiming hydrogen bonds are stronger than the covalent O–H bond
- Saying ice is denser because its molecules are fixed
- Explaining high surface tension using only covalent bonds inside one molecule
What can you explain now?
In the syllabus examples, which group can make a hydrogen-bond donor?
Compare with the explanation
N–H or O–H
The hydrogen is bonded to a highly electronegative nitrogen or oxygen.
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Make it stick
Which water properties are explained by intermolecular hydrogen bonding, and what makes ice less dense?
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