Ethanol (C₂H₅OH) boils at 78 °C while dimethyl ether (CH₃OCH₃), with the same molecular formula, boils at about −24 °C. Why?
Correct answer B. Ethanol molecules form hydrogen bonds with each other
Topic / 40 questions
Liquids MCQs
Liquids MCQs are multiple-choice questions on the properties of liquids explained by the kinetic molecular theory, including evaporation, vapour pressure, boiling point and hydrogen bonding. They follow the MDCAT chemistry curriculum, and every question on MCQs360 shows its correct answer instantly so you can spot gaps in your revision quickly.
- Gases 40
- Liquids 40
- Solids 40
- Chemical Equilibrium 40
- Reaction Kinetics 40
- Thermochemistry and Energetics 40
- Electrochemistry 40
- Fundamental Concepts of Chemistry 40
- Atomic Structure 39
- Chemical Bonding 39
- Alcohols and Phenols 38
- Industrial Chemistry 37
- Aldehydes and Ketones 37
- Alkyl Halides 37
- s- and p-Block Elements 37
- Chemistry of Hydrocarbons 36
- Fundamental Principles of Organic Chemistry 36
- Carboxylic Acids 36
- Transition Elements 35
- Macromolecules 35
At a given temperature, which factor makes the vapour pressure of a liquid lower?
Correct answer D. Stronger intermolecular forces
The two strands of a DNA double helix are held together by ____
Correct answer C. hydrogen bonds between complementary bases
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Why do water pipes sometimes burst in severe winter?
Correct answer D. Water expands when it freezes into ice
Hydrogen bonding occurs when a hydrogen atom is covalently bonded to ____
Correct answer B. a small, highly electronegative atom such as F, O or N
How does the strength of a hydrogen bond compare with that of a typical covalent bond?
Correct answer A. It is much weaker
Which hydrogen halide has the highest boiling point?
Correct answer B. HF
In liquid and solid hydrogen fluoride, the HF molecules are linked by hydrogen bonds into ____
Correct answer B. zig-zag chains
How much heat is released when 4.5 g of steam condenses at 100 °C (ΔHvap = 40.7 kJ mol⁻¹, H₂O = 18 g mol⁻¹)?
Correct answer A. 10.2 kJ
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Why does water have an unusually high enthalpy of vaporisation for a molecule of its size?
Correct answer A. Strong hydrogen bonds must be broken to separate the molecules
Why are liquids almost incompressible?
Correct answer D. Their molecules are already very close together
How does the viscosity of a liquid change when its temperature is raised?
Correct answer A. It decreases
Why is glycerol much more viscous than water?
Correct answer D. Each molecule has three –OH groups that form extensive hydrogen bonds
Which liquid has the lowest boiling point?
Correct answer C. Diethyl ether
Why is glycerol purified by distillation under reduced pressure?
Correct answer B. It decomposes at its normal boiling point, and lower pressure lowers the boiling point
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When a liquid is placed in a closed container, what is true once a constant vapour pressure is reached?
Correct answer B. Rate of evaporation equals rate of condensation
Which statement correctly distinguishes evaporation from boiling?
Correct answer C. Evaporation occurs at all temperatures from the surface; boiling occurs at a fixed temperature throughout the liquid
How does the surface tension of a liquid change as its temperature rises?
Correct answer A. It decreases
Why does water boil below 100 °C at a hill station such as Murree?
Correct answer A. Atmospheric pressure there is lower than at sea level
Why is ethanol completely miscible with water?
Correct answer C. Ethanol molecules form hydrogen bonds with water molecules
About Liquids MCQs
Questions in this topic explain everyday properties of liquids in terms of molecular motion and intermolecular forces. You will meet evaporation and its cooling effect, the factors that affect vapour pressure, the definition of boiling point and how it changes with external pressure, and simple heat of vaporisation calculations. Surface tension, viscosity and diffusion in liquids also appear. A large share of the questions focus on hydrogen bonding in H₂O, NH₃ and HF and its effect on boiling points compared with the other hydrides of the same group. The anomalous behaviour of water, such as its maximum density at 4 °C and the open structure that makes ice float, is a favourite examiner theme.
Key facts to remember
- A liquid boils when its vapour pressure equals the external pressure; water boils at 100 °C at 1 atm.
- Vapour pressure rises with temperature and is higher for liquids with weaker intermolecular forces; it does not depend on the amount of liquid or its surface area.
- Hydrogen bonding gives H₂O, HF and NH₃ much higher boiling points than the other hydrides of their groups.
- Water has its maximum density at 4 °C; ice is less dense than liquid water because of its open, hydrogen-bonded structure.
- Surface tension and viscosity of a liquid decrease as temperature rises.
- Evaporation is a surface process that causes cooling, because the most energetic molecules escape first.
Exams that include Liquids MCQs
Pakistan
- MDCAT
- ECAT
- NUMS
- AKU
- FSc boards
Bangladesh
- HSC
- Medical Admission Test
How to prepare for Liquids MCQs
- Link every property (vapour pressure, boiling point, surface tension, viscosity) to the strength of intermolecular forces.
- Compare the boiling points of group hydrides in a small table to see the effect of hydrogen bonding.
- Revise why ice floats and why water is densest at 4 °C, as these appear often.
- In heat of vaporisation questions, convert grams to moles before multiplying by ΔHvap.
Frequently asked questions
What does the MDCAT syllabus include in Liquids?
The PMDC MDCAT curriculum covers the properties of liquids explained by the kinetic molecular theory, evaporation, vapour pressure and boiling point, hydrogen bonding in H₂O, NH₃ and HF, and the anomalous behaviour of water, including its maximum density at 4 °C.
Which exams ask Liquids MCQs?
Questions on liquids, vapour pressure and hydrogen bonding appear in MDCAT, ECAT, NUMS and AKU entry tests and in FSc board papers in Pakistan. In Bangladesh, the topic is part of HSC chemistry and is useful for the medical admission test.
Why does ice float on water?
When water freezes, hydrogen bonds hold each molecule in an open, cage-like arrangement with more empty space than in the liquid. Ice is therefore less dense than liquid water, so it floats and insulates the water below it.
Last reviewed October 2026