Free Properties of 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. 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.

5. 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.

6. 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.

7. 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.