Free Liquids MCQs with Answers

1. A liquid takes the shape of its container but has a fixed volume because its molecules

• A. are held in fixed positions in a lattice
• B. are close together but free to slide past one another
• C. are very far apart and move independently
• D. have no kinetic energy at all

Explanation: Intermolecular forces in a liquid are strong enough to keep the molecules in contact, which fixes the volume, but not strong enough to hold them in place, so they flow and take the shape of the vessel. Fixed positions in a lattice describe a solid, and wide separation with independent movement describes a gas. A liquid is therefore intermediate in both structure and freedom of movement.

2. Surface tension in a liquid arises because

• A. molecules at the surface are pulled inwards and sideways only, with no upward pull
• B. the surface molecules move faster than those inside
• C. air presses down on the surface
• D. the liquid evaporates from the surface

Explanation: A molecule inside the liquid is pulled equally in all directions, but one at the surface has no neighbours above it, so the net inward pull makes the surface behave like a stretched skin and contract to the smallest possible area. This is why free drops are spherical and why a needle can float on water. Surface tension falls as temperature rises, because the molecules are harder to hold together.

3. The viscosity of a liquid

• A. increases as the temperature rises
• B. is independent of temperature
• C. decreases as the temperature rises
• D. is a measure of how quickly it evaporates

Explanation: Viscosity is resistance to flow and it depends on the intermolecular forces, so heating a liquid gives the molecules more kinetic energy to overcome those forces and the liquid flows more freely. This is why engine oil thins in a hot engine and why honey pours easily when warmed. Gases behave in the opposite way, becoming more viscous when heated.

4. Evaporation of a liquid causes cooling because

• A. the surface of the liquid contracts
• B. cold air replaces the escaping vapour
• C. the vapour formed is colder than the liquid
• D. the fastest moving molecules escape, lowering the average kinetic energy of those left behind

Explanation: Only molecules with above average kinetic energy can break free from the surface, so the average energy of the remaining liquid falls and the temperature drops with it. This is why sweating cools the body and why water in an earthen pot stays cool. Evaporation happens at all temperatures, unlike boiling, which occurs only at one specific temperature for a given pressure.

5. The vapour pressure of a liquid is the pressure exerted by its vapour when

• A. the liquid has completely evaporated
• B. the vapour is in dynamic equilibrium with the liquid in a closed container
• C. the liquid is boiling in an open vessel
• D. the container is completely empty of air

Explanation: In a closed vessel evaporation and condensation eventually occur at the same rate, and the constant pressure of vapour at that point is the equilibrium vapour pressure of the liquid at that temperature. It depends only on the liquid and the temperature, never on the volume of the container or the amount of liquid present. In an open vessel equilibrium is never reached because vapour escapes.

6. A liquid boils when its vapour pressure becomes equal to

• A. zero
• B. its surface tension
• C. the external atmospheric pressure
• D. the vapour pressure of water

Explanation: Bubbles of vapour can only form and rise through the bulk of the liquid when the vapour pressure inside them matches the pressure pushing down from outside. Because atmospheric pressure falls with altitude, water boils below 100 degrees Celsius on a mountain, which is why food takes longer to cook there. A pressure cooker uses the same relationship in reverse.

7. Which of the following liquids would be expected to have the highest vapour pressure at room temperature?

• A. Water
• B. Glycerol
• C. Ether
• D. Mercury

Explanation: Ether has weak dispersion forces between its molecules, so a large fraction escape the surface easily and it is highly volatile, which is why it evaporates almost as soon as it is spilt. Water and glycerol are held by extensive hydrogen bonding and mercury by metallic bonding, all of which are far stronger. Vapour pressure and boiling point always run in opposite directions.

8. A hydrogen bond forms when hydrogen is covalently bonded to

• A. carbon, silicon or sulphur
• B. any halogen
• C. a metal
• D. a small, highly electronegative atom such as fluorine, oxygen or nitrogen

Explanation: The electronegative atom pulls electron density away, leaving the hydrogen nucleus unusually exposed so that it attracts a lone pair on a neighbouring molecule. Only fluorine, oxygen and nitrogen are small and electronegative enough for the effect to be strong. Carbon is not electronegative enough, which is why hydrocarbons show no hydrogen bonding and are gases or volatile liquids.

9. The boiling point of water is far higher than that of hydrogen sulphide, although sulphur lies below oxygen in the same group, because

• A. water molecules are hydrogen bonded to one another
• B. water has a larger molar mass
• C. hydrogen sulphide is an ionic compound
• D. water molecules are non polar

Explanation: Extensive hydrogen bonding between water molecules must be overcome before they can escape, so a great deal of energy is needed and water is liquid at room temperature. Hydrogen sulphide has a larger molar mass yet boils at minus 60 degrees Celsius, which proves that molar mass is not the deciding factor. Sulphur is too large and not electronegative enough to support hydrogen bonding.

10. Hydrogen bonding is responsible for all of the following EXCEPT

• A. the high boiling point of ammonia compared with phosphine
• B. the solubility of ethanol in water
• C. the low density of ice compared with water
• D. the electrical conductivity of molten sodium chloride

Explanation: Molten sodium chloride conducts because it is an ionic compound whose ions become mobile when the lattice melts, and there is no hydrogen in the compound at all. The other three are classic consequences of hydrogen bonding: raised boiling points, mutual solubility with water and the open lattice of ice. Recognising when a substance simply cannot hydrogen bond is the quickest route to the answer.