Edexcel A-Level Chemistry AS Paper 1, June 2017: Question 2
6 marks · Medium difficulty · Short Open Response
Answer questions about water regarding its polarity, hydrogen bonding in ice, the low density of ice, and the solubility of Group 2 hydroxides.
Practise this questionQuestion
Question text
2 This question is about water.
(a) Water is a polar covalent molecule. The strongest intermolecular forces between
water molecules are hydrogen bonds.
(i) The O–H bond in water is polar because, when compared with the
hydrogen atom, the oxygen atom has
(1)
A a higher mass number
B a larger atomic radius
C greater electronegativity
D more electrons
(ii) Draw a diagram of a hydrogen bond between two water molecules in ice.
Show the value of the H–O–H angle within a molecule and the value of the
O–H–O angle between the two molecules.
(2)
(iii) Explain why hydrogen bonding causes ice to be less dense than liquid water.
(2)
(b) Liquid water is a good solvent for many, but not all, ionic compounds. Which is
least soluble in water?
(1)
A barium hydroxide *P49858A0424*
B calcium hydroxide
C magnesium hydroxide
D sodium hydroxide
(Total for Question 2 = 6 marks)
Mark scheme
Show the mark scheme
Question
Answer Mark
Number
2(a)(i) 2(a)(i). The only correct answer is C (1)
A is not correct because oxygen does have a higher mass number but it is not the cause of polarity
B is not correct because oxygen does have a larger atomic radius but it is not the cause of polarity
D is not correct because oxygen does have more electrons but this is not the cause of polarity
Question
Acceptable Answer Additional Guidance Mark
Number
2 (a)(ii) (2)
correct shape of two water molecules and hydrogen Allow about 10° tolerance by eye.
bond show at about 180 o but not necessarily labelled
(1)
o o Allow 104 – 105o
HOH bond angle 104.5 and OHO angle 180 (1)
Question
Acceptable Answer Additional Guidance Mark
Number
2 (a)(iii) An explanation that makes reference to two of the (2)
following points:
Do not award MP1 if the gaps are full
more open / more space between molecules (making it of air molecules
less dense) (1)
May be shown as a diagram
due to (3 Dimensional) lattice / ring structure in ice (1)
Allow reverse arguments for liquid
hydrogen bonds longer than the covalent bonds (1) water
Question
Answer Mark
Number
2 (b) 2(b). The only correct answer is C (1)
A is not correct because barium hydroxide is the most soluble group 2 hydroxide
B is not correct because calcium is below magnesium in the Periodic Table so this is more soluble
D is not correct because group 1 hydroxides are very soluble compared to group 2 hydroxides
(Total for Question 2 = 6 marks)
How to answer it
Properties of Water and Solubility
What this question tests
This question assesses your understanding of bonding, molecular shape, electronegativity, hydrogen bonding in ice, and the solubility trends of Group 2 hydroxides in aqueous solution.
Why is the O-H bond polar?
✅ Correct Answer
C: greater electronegativity
💡 Key Knowledge
- Electronegativity is the ability of an atom to attract bonding electrons in a covalent bond.
- Oxygen is significantly more electronegative than hydrogen, pulling the shared pair of electrons closer to itself.
❌ Common Errors
Students often confuse electronegativity with atomic radius, mass number, or total number of electrons. While oxygen has all of these in greater amounts than hydrogen, polarity is exclusively caused by a difference in electronegativity.
Drawing Hydrogen Bonds in Ice
✅ Correct Answer Requirements
- Correct molecular shape for both water molecules (bent/V-shape with 2 lone pairs implied or drawn).
- A dashed/dotted line representing the hydrogen bond running directly from a lone pair on oxygen to the hydrogen atom of the adjacent water molecule.
- Bond angles labelled accurately: 104.5° (or 104°–105°) for the intra-molecular H-O-H angle, and 180° for the linear O-H-O hydrogen bond arrangement.
🧠 Exam Technique
Make sure the 180° angle is clearly shown across the covalent bond, the hydrogen bond, and the oxygen atom of the second molecule to secure the second mark.
Explaining the Density of Ice
✅ Correct Explanation (Any 2 points)
- Ice has a more open / more spaced-out structure (or 3D lattice / ring structure) compared to liquid water.
- Hydrogen bonds hold water molecules further apart in the solid state than they are on average in liquid water.
❌ Common Errors
Do not state that the gaps in the ice lattice are filled with air molecules. Examiners heavily penalise this misconception; the spaces are empty space within the molecular framework.
Solubility of Group 2 Hydroxides
✅ Correct Answer
C: magnesium hydroxide
💡 Key Knowledge
- Solubility of Group 2 hydroxides increases down the group (Mg(OH)₂ is sparingly soluble/least soluble, while Ba(OH)₂ is much more soluble).
- Group 1 hydroxides (like NaOH) are extremely soluble compared to Group 2.
🧠 Exam Technique
Remember the opposite trends for Group 2: hydroxide solubility increases down the group, whereas sulfate solubility decreases down the group.
Topics
Physical Chemistry · Inorganic Chemistry · Topic 2: Bonding and Structure · Topic 4: Inorganic Chemistry and the Periodic Table
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.