Question

Why does water become cold if we add some ice cubes in it?

Answer 1

Water becomes cold if we addsome ice cubes to it because of conservation of energy. Heat is a form of energy. The temperature of water is more than ice cubes, so the heat flows from the waterto ice cubes. The heat gained by the melting of ice cubes is equal to the heat lost by liquid water.

REXPLANATION

Heat moves from areas of greater heat to areas of lesser heat. This is defined under the second law of thermodynamics: The second law of thermodynamicsstates that the entropy of any isolated system always increases.

Isolated systems spontaneously evolve towards thermal equilibrium—the state of maximum entropy of the system.

What is going on at the atomic level is that atoms and molecules are bouncing around faster when hot as opposed to slower when they are cold. As the faster moving H2O molecules in the water bang into the slower moving molecules in the ice they jostle and speed up some of the ice molecules while losing some of their speed/energy.

Also, some of the ice will undergo a phase change, from solid to liquid. This melting phase change is an endothermic process requiring and absorbing energy drawn the the temperature of the water.