When you mix a tumbler of hot water with one bucket of normal water, what will be the direction of heat flow? Justify.

Explanation

The water in the tumbler is at a higher temperature than the bucket of normal water. But the bucket of normal water has larger internal energy than the hot water in the tumbler. This is because the internal energy is an extensive variable and it depends on the size or mass of the system.
Even though the bucket of normal water has larger internal energy than the tumbler of hot water, heat will flow from water in the tumbler to the water in the bucket.
This is because heat flows from a body at higher temperature to the one at lower temperature and is independent of internal energy of the system.
Once the heat is transferred to an object it becomes internal energy of the object. The right way to say is .object has certain amount of internal energy.. Heat is one of the ways to increase the internal energy of a system as shown in the Figure.