A lot of people know that if you have a cup of hot water and a cup of cold water, you put them all in the refrigerator, and the cup of hot water will freeze first.
There are also ice flower videos in the Great Northeast, which are such a phenomenon.
Is there any factual basis for this seemingly counterintuitive claim? Does boiling water really freeze faster than cold water?
To get a short and quick answer, skip to the bottom of this article.
The answer is... It depends.
If you have two glasses of water with the same chemistry, the colder cup will freeze first because it will reach the freezing point faster.
However, the answer is a little more complicated, because the mere act of heating the water changes the chemical composition of the sample. Since the two samples are no longer the same, their freezing points will also be different.
It's like adding salt to a road to keep it from freezing, and why freshwater freezes faster than seawater — adding or subtracting minerals, chemicals, and other pollutants can reduce or increase the drop in freezing point.
The freezing point is reduced
Freezing point reduction is the act of adding solute to a solvent to lower its freezing point.
This is a phenomenon that we have been exploiting. A common example is the radiator of a car. The water in the radiator is used to keep the car engine cool, but in winter, the water will almost certainly freeze into a large chunk of ice, what should we do? We add antifreeze (usually glycol), which lowers the freezing point of the water and prevents it from freezing during the harsh winter.
Even animals take advantage of the freezing point to descend. The spring peeping frog is able to survive temperatures below freezing points, thanks to the large amounts of glucose it releases into the bloodstream when it detects the freezing point temperature. Because the frog has extra glucose in its blood, its freezing point is significantly lower than that of water, which allows it to survive the harsh winter. Several Arctic fish species also take advantage of this phenomenon.
The same thing happens when the water is boiled or heated. Magnesium carbonate and calcium carbonate precipitate out of the water, free oxygen escapes. As a result of removing these components, the freezing point of the water is raised. If you use hard water (mineral-rich water), this effect will be more pronounced.
When trying to freeze the hot water, the first ice crystals that begin to form exclude the solute, which causes the ion concentration of the remaining liquid to increase, resulting in a larger reduction in the freezing point of the remaining water (if ions are present in the sample). Colder water may begin to show signs of rapid freezing, but eventually, the hot water will freeze first.
Remember, distilled water, or water filtered by reverse osmosis, will not occur due to a lack of solutes and contaminants.
This is known as the Mpemba effect when hot water wins the race and freezes completely before a similar cold water sample.
The Mpemba effect
In 1963, a Tanzanian Ilasto Mba described this unusual phenomenon when he noticed that the hotter ice cream he made was colder and the ice cream froze first. A few years later, in a lecture, Mbaba asked the lecturer, "If you have two similar containers of the same volume, one with a temperature of 95 degrees Fahrenheit (95 degrees Fahrenheit) and the other with a temperature of 212 degrees Fahrenheit (212 degrees Fahrenheit), put them in the refrigerator, and the higher temperature will freeze first." "Why is this happening?"
Instead of getting answers or explanations, he was ridiculed by his classmates and teachers. The lecturer (Dennis G. Osborne) decides to do an experiment to verify Mbaba's claims and eventually confirms Mbaba's observations. Mmba and Osborne published their findings in 1969.
Short answer
Simply put, if a cold water sample contains solutes (contaminants or trace minerals), then the hot water will freeze in the first place. Solutes increase the reduction of freezing point, which means that water takes longer to freeze. The act of heating or boiling water removes many of these solutes, contaminates and lowers the freezing point, causing the hot water to freeze in the first place.