Question

A beaker of water is heated and thermal energy travels through the water by convection.
What happens to the density of water when it is heated and how does the water move?
$A.$ The density decreases and the heated water moves downwards
$B.$ The density decreases and the heated water moves upwards
$C.$ The density increases and the heated water moves downwards
$D.$ The density increases and the heated water moves upwards

Answer 1

As heating starts, the water molecules at the bottom start vibrating and moving in random motion. This means they need to escape elsewhere so they move upward. With the movement of molecules the number of molecules present in a given volume after heating will be lesser than the molecules before heating.

Complete step by step answer:
When water starts heating the temperature of water at the bottom increases rapidly. Density of water is maximum at 4$^{\circ }C$ and then decreases with heating. As the density of water at the bottom is lesser than that at the top so it moves upwards.
Therefore the density decreases and the heated water moves upwards.
The correct option is B.

Additional information:
There are three types of heat transfer process:

1. Conduction: The process in which a rapidly moving atom or particle transfers energy to their adjacent atom and sets it in motion leading to transfer of heat is known as conduction. Conduction in metals happens when electrons are transferred from one atom to another. Heat is transferred from one body to another only when there is a temperature difference.
2. Convection: The process in which transfer of heat takes place due to the movement of fluids. There are a lot of factors affecting heat transfer due to convection. Density of the fluid, size of the molecules are some of the examples.
3. Radiation: The process in which heat transfer takes place by electromagnetic waves emitted by matter. They required no medium for its transfer. Thermal radiation can also occur in vacuum.

Note:
In a body of fluid that is heated from underneath its container, both conduction and convection can take place. Both would try to outsmart each other. If heat conduction is too great, fluid moving down by convection is heated by conduction so fast that its downward movement will be stopped due to its buoyancy. Meanwhile fluid moving up by convection is cooled by conduction so fast that its driving buoyancy will diminish.