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Chemistry · Unit 4

Liquids: MDCAT Notes and MCQs

Liquids by kinetic theory

  • Molecules are close together (little space) → almost incompressible; definite volume, no definite shape.
  • Intermolecular forces stronger than in gases; molecules slide past each other → flow, slower diffusion than gases.
  • Expand slightly on heating; KE increases with temperature.

Evaporation, vapour pressure, boiling point

  • Evaporation: high-energy molecules escape from the surface at all temperatures → cooling effect. Increases with temperature, surface area, and weaker intermolecular forces.
  • Vapour pressure: pressure exerted by vapour in equilibrium with its liquid at a given temperature. Depends on temperature and nature (intermolecular forces) — NOT on surface area or amount of liquid. Water at 25 °C ≈ 23.8 torr; at 100 °C = 760 torr.
  • Boiling point: temperature at which vapour pressure = external pressure. Lower external pressure → lower b.p. (water boils at ≈ 69 °C on Mount Everest); higher pressure → higher b.p. (pressure cooker ≈ 120 °C).

Hydrogen bonding

  • Electrostatic attraction between H bonded to a highly electronegative atom (F, O, N) and a lone pair on another such atom. Weaker than covalent bonds, stronger than other dipole forces.
  • HF: strongest individual H-bond (F most electronegative), forms zig-zag chains. H2O: each molecule forms up to 4 H-bonds → highest b.p. (100 °C) among the three. NH3: weakest (N less electronegative, one lone pair).
  • Boiling points: H2O (100 °C) > HF (19.5 °C) > NH3 (−33 °C). H2O vs H2S (−60 °C) — water is anomalously high due to H-bonding.
  • Other effects: solubility of alcohols in water, DNA base pairing, protein structure, dimer of acetic acid.

Anomalous behaviour of water

On cooling, water contracts down to 4 °C (maximum density 1.000 g/cm3) and then expands until it freezes. Ice has an open, cage-like hexagonal structure held by H-bonds → ice is less dense (≈ 0.917 g/cm3) and floats, insulating aquatic life below.

PMDC learning outcomes for this unit
  • Properties of Liquids: 4.1 Describe simple properties of liquids e.g. diffusion, compression, expansion, motion of molecules, spaces between them, intermolecular forces and kinetic energy based on kinetic molecular theory.
  • Properties of Liquids: 4.2 Explain physical properties of liquid such as evaporation, vapour pressure, boiling point
  • Properties of Liquids: 4.3 Describe the hydrogen bonding in H₂O, NH₃ and HF molecules.
  • Properties of Liquids: 4.4 Anomalous behaviour of water when its density shows maximum at 4 degrees centigrade.

Liquids MCQs with answers

Practise and track your score →
Q1
The vapour pressure of a liquid depends on:
AShape of container
BAmount of liquid
CSurface area
DTemperature
Show answer
Answer: D (Temperature). Vapour pressure depends only on temperature and the nature of the liquid.
Q2
A liquid boils when its vapour pressure becomes equal to:
AIts critical pressure
B1 torr
CThe external pressure
DZero
Show answer
Answer: C (The external pressure). Boiling point is the temperature where vapour pressure equals external pressure.
Q3
Water boils at a lower temperature at the top of a mountain because:
AWater is purer
BExternal pressure is low
CTemperature is low
DAir is thin in oxygen
Show answer
Answer: B (External pressure is low). Lower atmospheric pressure is reached by vapour pressure at a lower temperature.
Q4
Which has the highest boiling point?
ANH₃
BH₂S
CH₂O
DHF
Show answer
Answer: C (H₂O). Water forms up to 4 hydrogen bonds per molecule, giving the highest b.p.
Q5
The strongest individual hydrogen bond is formed in:
AH₂O
BNH₃
CHF
DHCl
Show answer
Answer: C (HF). Fluorine is the most electronegative element, so F–H···F bonds are strongest.
Q6
Ice floats on water because:
AIce has an open cage-like structure due to H-bonding
BIce is denser
CWater has no H-bonds
DIce has higher mass
Show answer
Answer: A (Ice has an open cage-like structure due to H-bonding). The hexagonal H-bonded network in ice has more empty space → lower density.
Q7
Evaporation causes cooling because:
ALow-energy molecules escape
BHigh-energy molecules escape, lowering average KE
CVolume decreases
DPressure increases
Show answer
Answer: B (High-energy molecules escape, lowering average KE). Escape of energetic molecules lowers the average kinetic energy of the remaining liquid.
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