Question

A beaker of water at 80 degrees celsius is placed in the center of a well-insulated room whose air temperature is 20 degrees celsius. Is the final temperature of the water:

A) 20 degrees celsius

B) Slightly above 20 degrees celsius

C)  50 degrees celsius

D) Slightly below 80 degrees celsius

E)  80 degrees celsius

Answer 1

Answer:-

The correct answer option B - Slightly above 20 degree celsius.

Explanation:-

The beaker placed in a well-insulated room. So, no heat energy will escape out of the room. All of the heat exchange happens between the water and the air inside the room. According to second law of thermodynamics, heat energy always flows from higher temperature to lower temperature. In this case heat flows from hot water to air until both have the same temperature.

Therefore, Final temperature of water = Final temperature of the air.

Option A and Option E are not the answers, because temperature will change when heat transfer occur between hot water and air since they are not at their phase changing temperature.

Option C obviously is not the answer. Beacuse water can not transfer so much heat to rise the entire temperature of air inside the room to 50ᵒC. For the same reason Option D is not the answer. Heat energy from hot water can't rise the final temperature up to a temperature slightly less than 80ᵒC.

Therefore the only option left is Option B.

We know that the heat energy lost by a body = heat energy gained by a body

in this case,
Mass of the water << mass of the air.
Hence water can not transfer enough amount of heat energy to rise the temperature of the entire air inside the room. As a result water lose heat energy continously to the air until the thermal equilibrium established. As a result final temperature of water is slightly above 20ᵒC.
i.e Option B