why hot water freezes fast than cold water ???

Answer 1

It's called the Mpemba Effect, and the best thing it proves is that a high school student can discover new science! See the first reference, and the other articles listed there, for more details.

The process of freezing water is the process of removing heat energy from it, first the kinetic energy of the water molecules to get the temperature down to the freezing point, and then the enthalpy of fusion, allowing the transition from liquid to solid state (second reference). The faster you remove heat, the less time it takes to freeze.

How long does it take to get people off a football field? It takes longer to get 200 people off than 100 people, right? Not necessarily. If the 200 are standing near the perimeter and just need to step across the line, you can clear the field in 2 seconds. If the 100 people need to walk through a narrow gate, it can take 2 minutes.

With hot water, you have a higher temperature difference, therefore higher heat flow. You have more water evaporating, releasing yet more heat. You have more conduction and more convection. You are starting up a strong 'river' of heat flow out of the water. That flow tends to continue, reducing the time to convey that last heat energy out of the water.

Answer 2

This is another misbegotten wives’ tale. Mildly cold water-say about 40 degrees will freeze faster than mildly hot water (125 degrees). But, very hot water (195+) will freeze faster than mildly hot water.

Why? Because near the boiling point, heat escapes quickly, shrinking the volume. So less water means faster boiling. Also, the very fact that the hot molecules are escaping so quickly means that slower, colder ones are left behind. Thus, the evaporation process itself is a boon to the freezing process. Weird, but true.

Answer 3

Because hot water is less likely to contain tiny gas bubbles. Gas bubbles form from dissolved gasses as the water cools. When the hot water was heated, these dissolved gasses may have been driven out. In cold water, ice crystals use the tiny bubbles as starting points for formation (in physics, we call them nucleation points). But in the hot water, there are no bubbles, so there aren't as many starting points for the ice crystals.

Dissolved gasses also lower the freezing point. Since heated gas is less likely to contain dissolved gasses, it's more likely to freeze first.

Answer 4

I was told in school it was because the way the molecules were moving.

Answer 5

Never heard of such a thing.

Answer 6

that is BS