What is the effect of temperature when warm water is put in the cold water?

Answer 1

Firstly, you have to know what temperature actually is. It's a measure of the amount of energy contained in a substance that contributes to its' movement. Atoms in a solid vibrate, and in a liquid or gas, they move around until they hit something - either way, they have a kinetic energy, and temperature is a way of detecting it. (And I have no idea how sensing heat or cold works - nerves are strange and magical constructs).

From this, it's fairly simple to imagine what is happening when warm and cold water mix. The cold water molecules are moving slowly, while the warm ones are moving quite rapidly. Over time, the warm molecules collide with the small ones, and their speed averages out. Warm molecules lose energy in the collision and slow down, cold molecules gain energy in the collision and speed up. The net result is water that is cooler than the warm water, but warmer than the cool water.

Neglecting other energy exchanges (between mixed water and air or container around it, or gains or losses due to chemical reactions), the final temperature of the mixed water will be (T_h*W_h+T_c*W_c)/W. T is temperature in degrees, W is weight or weight fraction of the water involved, and the _h and _c tags refer to hot and cold water respectively. The W without a suffix is the total weight or weight fraction of the water - the sum of W_h and W_c. This holds true for any other pure liquid or gas. Any mixtures of different substances will not behave this way because they have different amounts of energy needed to change the temperature.

Source: first year engineering physics class - thermodynamics unit, at UBC.