OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 3

1 mark · Easy difficulty · Multiple Choice

Identify the correct statement explaining why ice is less dense than water based on hydrogen bonding.

Practise this question

Question

Multiple choice question 3 asks which statement explains why ice is less dense than water, with options A (Hydrogen bonds are stronger in ice than in water), B (Hydrogen bonds hold H2O molecules apart in ice), C (Ice is a solid but water is a liquid), and D (Ice contains hydrogen bonds, but water does not contain hydrogen bonds). A box for the answer is provided, worth 1 mark.
Question text

3 Which statement explains why ice is less dense than water?

A Hydrogen bonds are stronger in ice than in water.

B Hydrogen bonds hold H2O molecules apart in ice.

C Ice is a solid but water is a liquid.

D Ice contains hydrogen bonds, but water does not contain hydrogen bonds.

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme shows question number 3 with the correct answer as B.

3 B 1 1.1

How to answer it

Explaining the Density of Ice

📌 What this question tests

This question assesses your understanding of intermolecular forces, specifically hydrogen bonding, and how the 3D spatial arrangement of molecules in solid crystal lattices (ice) differs from the more random, closely packed arrangement found in liquids (water).

Question 3 (Multiple Choice)

Which statement explains why ice is less dense than water?

✅ Correct Answer: Option B

Hydrogen bonds hold H₂O molecules apart in ice.

In the solid ice lattice, hydrogen bonds force molecules into an open, tetrahedral crystal structure (similar to diamond), creating large empty spaces between molecules. This increases volume and decreases density.

💡 Key Knowledge

  • Water molecules are held in an open lattice structure in ice due to directional hydrogen bonding.
  • Each H₂O molecule forms up to four hydrogen bonds in ice, fixing them at greater average distances than in liquid water.
  • When ice melts, some hydrogen bonds break, allowing molecules to pack closer together, which is why liquid water is denser.

🧠 Exam Technique

Read through multiple-choice options carefully to eliminate scientifically inaccurate statements. Look for specific structural descriptions regarding states of matter rather than vague generalisations about bond strength.

❌ Common Errors

  • Choosing A: Assuming hydrogen bonds are physically "stronger" in ice. Their bond energy remains broadly similar; it is the geometry and rigidity of the lattice that matters.
  • Choosing D: Incorrectly thinking liquid water lacks hydrogen bonds entirely. Water molecules remain hydrogen-bonded in the liquid state, just less rigidly arranged.
Mark Allocation: [1 mark] awarded for selecting B.

Topics

Module 2: Foundations in chemistry · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.