Which of the following statements about convection of water is correct? *
A Hot water sinks and cold water rises because hot water has higher density than cold water.
B Hot water rises and cold water sinks because hot water has higher density than cold water.
C Hot water rises and cold water sinks because hot water has lower density than cold water.
D Hot water sinks and cold water rises because hot water has lower density than cold water.
The correct answer and explanation is :
Correct Answer: C
Hot water rises and cold water sinks because hot water has lower density than cold water.
Explanation (300 words):
Convection is the transfer of heat through fluids (liquids or gases) by the movement of the fluid itself. In the case of water, convection occurs when there is a temperature difference within the fluid, such as when heating water in a pot.
When water is heated, the molecules within it gain energy and begin to move faster. As they move faster, they spread apart from each other, causing the water to become less dense. Density is a measure of how much mass is contained in a given volume; when molecules spread apart, the same amount of mass occupies more space, and density decreases.
Because hot water is less dense than cold water, it becomes lighter and rises to the top. In contrast, colder water has molecules that move more slowly and are packed more closely together, making it denser. As a result, the colder, heavier water sinks to the bottom. This movement of hot water rising and cold water sinking creates a convection current.
This process is crucial in many natural systems. For example, in the oceans and atmosphere, convection helps regulate temperature and drives weather patterns. In homes, convection is used in heating systems, where warm air rises and cooler air descends, allowing for more even temperature distribution.
The incorrect options can be ruled out based on their misunderstanding of density:
- A and D are incorrect because hot water does not sink—it rises.
- B is incorrect because although it correctly describes the direction of movement, it gives the wrong reason—hot water does not have a higher density.
Thus, C is correct because it accurately states both the direction of the movement and the reason—hot water rises and cold water sinks because hot water has lower density than cold water.