Will hot water freeze faster than cold water?
Yes and no, hot water can freeze faster than cold water, it certainly drops it’s temperature faster than cold water does, but if it drops fast enough to beat cold water in a race to frozen is kind of up in the air. The actual temperature matters as well, water at 1 degree Celsius will freeze faster than water at 99 degrees Celsius. Let’s assume we’re dealing with cold water at 30 degree Celsius and hot water at 70 degrees Celsius. The hot water could freeze first. People assume that if the cold water takes ten minutes to freeze the hot water will get to 30 degrees and then take ten more minutes to freeze. Unfortunately the properties of the two temperature waters will differ, as well as their surroundings. There is also no great scientific discovery related to this phenomenon.
Let’s examine some of the factors that can and will affect hot water. Firstly there could be some evaporation of the hot water making its volume less than it was at the beginning, and making the freeze time quicker, less water takes less time to freeze. Hot water will also contain less dissolved gasses. These gases that will exist in cold water more commonly than hot water may lower the freezing point, at the rate of temperature change. If you’re using tap water who knows what’s in that water, spring water won’t change much with all the minerals in it. There are two explanations, read on for one more.
Convection is the act heat transfer in liquids and gases. Heat is the speed of the movement of molecules and atoms in any given substance. The stove gets hot because the atoms are moving very quickly, the stove top and the air around the stove get hot because as the atoms fling around at high speeds they impact other atoms and cause them to increase speed, thus increasing their heat. That’s why your spoon handle gets hot when you put it in hot soup. A wooden handle will not get hot because the molecules will not as easily take the energy (heat) from the hot liquid. How does this relate to the question? I’ll tell you. As the hot water molecules fling around faster than the cold molecules they create a current circling the in the water. The top of the water will be warmer then the bottom and the heat will be lost out the top quicker than the bottom. This increases the speed of heat loss and thus decrease the time it takes to freeze the water.
There are other miscellaneous factors that can help the hot water freeze faster. For example it may melt some frost under the container, that melted frost may refreeze into ice and help with the cooling process. There are many situations that can contribute to the time it takes to freeze hot water, both speeding up the process and slowing it down.
A combination of these factors and possibly others will sometimes cause hot water to freeze faster than cold water, but I wouldn’t claim your own freezer will freeze hot water faster than cold, but I won’t claim the opposite either.
Here’s a more detailed explaination, it even names the effect after some guy.