Free Liquids MCQs with Answers

11. Ice is less dense than liquid water because in ice each water molecule

• A. loses one of its hydrogen atoms
• B. is hydrogen bonded to four others in an open tetrahedral arrangement
• C. moves faster than in the liquid
• D. is compressed closer to its neighbours

Explanation: Every molecule uses its two hydrogens and its two lone pairs, so a rigid open cage with large empty spaces is formed, and this occupies more volume than the same molecules jostling in the liquid. Water is therefore anomalous in expanding on freezing, which is why pipes burst in winter and why ice floats. Almost every other substance contracts as it solidifies.

12. Water has its maximum density at

• A. 0 degrees Celsius
• B. 100 degrees Celsius
• C. 4 degrees Celsius
• D. minus 4 degrees Celsius

Explanation: Cooling water from room temperature increases density as normal, but below 4 degrees the open hydrogen bonded structure begins to form and the density falls again, so the maximum occurs at 4 degrees. This is why the bottom of a deep lake stays at about 4 degrees while ice forms at the surface, allowing aquatic life to survive the winter. At 0 degrees water is already less dense than at 4.

13. Water has an unusually high specific heat capacity because

• A. it is a very light molecule
• B. it is a good electrical conductor
• C. a large amount of energy is used breaking hydrogen bonds before the temperature rises
• D. it has a low boiling point

Explanation: Much of the heat supplied goes into breaking hydrogen bonds rather than into increasing the kinetic energy of the molecules, so the temperature rise per joule added is small. This makes water an excellent coolant in car radiators and gives coastal areas milder climates than inland regions. Its high latent heat of vaporisation has the same underlying cause.

14. Capillary rise of water in a narrow glass tube occurs because

• A. the adhesive forces between water and glass are stronger than the cohesive forces within water
• B. the cohesive forces are stronger than the adhesive forces
• C. the air pressure inside the tube is greater
• D. water is denser than glass

Explanation: Water wets glass because it is attracted to the silica surface more strongly than to itself, so it climbs the walls and drags a column up, forming a concave meniscus. Mercury shows the opposite behaviour, with stronger cohesion, so it is depressed in the tube and forms a convex meniscus. The narrower the tube, the higher the rise.

15. The energy required to convert one mole of a liquid into vapour at its boiling point is called the

• A. molar heat of fusion
• B. molar heat of vaporisation
• C. lattice energy
• D. activation energy

Explanation: Molar heat of vaporisation measures the energy needed to overcome the intermolecular forces completely, so it is large for hydrogen bonded liquids such as water at about 40.7 kJ per mole. Heat of fusion is the smaller quantity needed to melt a solid, because melting loosens the lattice without separating the particles. Neither is related to lattice energy, which concerns ionic solids.

16. As the temperature of a liquid rises, its vapour pressure

• A. decreases
• B. remains constant
• C. increases
• D. first increases and then decreases

Explanation: Higher temperature means a larger fraction of molecules have enough kinetic energy to escape the surface, so more vapour is present at equilibrium and the pressure rises, following an exponential rather than a straight line relationship. The boiling point is simply the temperature at which this rising curve meets the external pressure. This is why a liquid in a sealed container can burst it when heated.

17. Which pair of liquids is most likely to be completely miscible?

• A. Water and petrol
• B. Water and ethanol
• C. Oil and water
• D. Mercury and water

Explanation: Ethanol has a hydroxyl group that hydrogen bonds with water, so the two mix in all proportions, illustrating the rule that like dissolves like. Petrol and oil are non polar hydrocarbons that cannot form such interactions with water and therefore separate into layers. Mercury is a liquid metal held by metallic bonding and is immiscible with almost everything.

18. The meniscus of mercury in a glass tube is convex because

• A. mercury is a metal
• B. mercury evaporates rapidly
• C. cohesive forces between mercury atoms exceed the adhesive forces to glass
• D. glass is polar and mercury is non polar

Explanation: Mercury atoms are held to each other by strong metallic bonding far more strongly than they are attracted to the glass, so the liquid pulls away from the walls and the surface bulges upwards. Water shows a concave meniscus for the opposite reason. When reading a burette or a thermometer the eye must be level with the bottom of a concave meniscus and the top of a convex one.

19. A pressure cooker cooks food faster because increasing the pressure inside

• A. lowers the boiling point of water
• B. removes all the air from the vessel
• C. increases the amount of steam produced
• D. raises the boiling point of water above 100 degrees Celsius

Explanation: A liquid boils when its vapour pressure equals the external pressure, so raising that external pressure means a higher temperature is needed, and water inside a pressure cooker can reach about 120 degrees Celsius. The food cooks faster because the reaction rate rises with temperature, not because there is more steam. At high altitude the reverse problem arises and cooking takes longer.

20. Which statement about liquids is INCORRECT?

• A. Liquids diffuse more slowly than gases
• B. Liquids are almost incompressible
• C. The molecules of a liquid are held in fixed positions
• D. Liquids have a definite volume but no definite shape

Explanation: Molecules in a liquid are in constant random motion and slide freely past one another, which is exactly what allows a liquid to flow and to diffuse, however slowly. Fixed positions belong to the solid state. The other three statements are all correct, and incompressibility follows from the molecules already being nearly in contact.