Heat transfer by convection in a liquid is due to the

Step 1: Analyze the question We are asked what causes heat transfer by convection in a liquid.

Step 2: Explain the concept Convection is heat transfer through the actual movement of the fluid (liquid or gas). It works like this:

1. The part of the liquid near the heat source is heated.
2. It expands, so its density decreases (the same mass now occupies a larger volume).
3. The less dense, warmer liquid rises.
4. The cooler, denser liquid from above sinks to take its place.
5. This creates a continuous circulation called a convection current, which carries heat through the liquid.

The key driver of the movement is the difference (variation) in density between the hot and cold parts of the liquid.

Step 3: Check each option

• A. Latent heat of vaporization: This concerns the change of state from liquid to gas. Convection happens without boiling, so this is wrong.
• B. Increased vibration of molecules about their mean position: This describes conduction, where heat passes from molecule to molecule without the liquid moving as a whole. Wrong.
• C. Variation of density of the liquid: Correct. Density differences cause the warm liquid to rise and the cool liquid to sink, setting up convection currents.
• D. Expansion of the liquid as it is heated: Expansion is part of the process, but on its own it does not cause the circulation. It is the resulting density difference that makes the liquid move. So this is a less complete answer than C.

Step 4: Answer The correct option is C.

Exam tip: Remember the three modes of heat transfer with a keyword each:

• Conduction → molecular vibration (solids)
• Convection → density variation and fluid movement (liquids and gases)
• Radiation → electromagnetic waves (no medium needed)