Is it true that cold water boils faster than hot water?

The common advice is that if you need to boil water faster, it is better to boil cold than hot. We decided to check whether such a recommendation has scientific basis.

Questionwhat kind of water boils faster - cold or hot, interested many. About this regularly ask on websites questions And answers And are discussing it's in Media. worries users and question, Maybe whether salt speed up boiling water, and some culinary resources definitely claimthat salt water will boil faster than fresh water.

From a physics point of view, boiling is the process of vaporization within the entire volume of a liquid, in contrast to evaporation, in which vapor is formed only on the surface. The temperature at which a liquid boils is called boiling point. This parameter depends on atmospheric pressure and type of liquid. The higher the pressure, the higher the boiling point. That is why on the tops of mountains, where the pressure is lower than at sea level, water boils not at 100 ° C, but at lower temperatures - for example, at 90 ° C. Water at the top of Everest boils at a temperature of only 68 °C. 

However, at any altitude, the boiling process occurs according to the same laws: water absorbs heat from a stove, fire or heating element of an electric kettle, gradually becoming hotter. It is logical that water with a temperature of 60–75 ° C - and according to the rules it is such there must be water in the hot water supply system in Moscow - it will need to heat up fewer degrees to boil than water at room temperature (18–20 ° C). However, the colder the water, the more effective it is. absorbs warm, and therefore cold water will warm up faster at first. But this benefit will be short-lived, and once the hot water reaches temperature, it will heat up at the same rate as the original hot water. As a result, the boiling time for cold water will be the sum of the time required to heat it to hot temperature and the time it takes for hot water to boil. 

Presumably, the misconception could arise due to the fact that in the case of freezing hot and cold water, Mpemba effect. It lies in the fact that hot water, under certain conditions, can freeze faster than cold water. Named after Tanganyika schoolboy Erasto Mpemba, the effect has not yet received a sufficient scientific explanation, and scientists continue to debate whether it really exists, trying to replicate the original experiments student and physics professor Dennis Osborne. In 2016, after conducting their observations, British scientists came to the conclusion that hot water does not cool faster than cold water, that is, they refuted the existence of the effect. But already in 2017, their conclusions were immediately questioned by two groups of scientists: Chinese-Singaporean and American. First discovered, that in theory, heating a cooled system that is far from equilibrium should take less time than another system that was initially closer to equilibrium, but they did not provide a practical implementation of their theory. Second predicted the appearance of the direct and reverse Mpemba paradox for some granular solids.

In addition to pressure, the boiling point is affected by the amount of impurities in the water. Various chemical compounds are added to hot tap water to protect pipes from corrosion. According to Raoult's second law, the more impurities in the solution, the higher the temperature required for boiling. Mosvodokanal declaresthat hardness, that is, the amount of salts dissolved in water, in Moscow is between 1.9 and 5.7 mEq/l. First of all, these are magnesium and calcium salts. By requirements Rospotrebnadzor, the permissible limit of water hardness for centralized water supply is 7 mEq/l. This amount of impurities has very little effect on the boiling point. However, in an electric kettle, which is rarely cleaned from the inside, there is a large layer of limescale and flakes of impurities left over from numerous previous boilings - this amount can become significant. This is why manufacturers advise pour into such devices or pre-fill filtered water, or regularly clean. They draw attention to the fact that water in a kettle with a large amount of sediment will boil more slowly, and the device itself will consume more electricity.

Table salt, or sodium chloride, also increases the boiling point of water, so salted water will boil at a higher temperature than fresh water. Every 29 g of salt (approximately heaped tablespoon) per liter of water increase boiling point by 0.5 °C. However, if you try to conduct such an experiment at home, the results may be surprising. Density of salt water higher, than fresh, and the heat capacity is slightly lower. That is, the bubbles that appear before the boiling process itself will appear first at the bottom of a pan of salted water. Fresh water will heat up evenly, so bubbles will appear later, but the boiling point will be reached faster. 

Thus, hot water still boils faster than cold water, and fresh water boils faster than salty water. However, for cooking, the difference between the boiling points and boiling rates of salted and unsalted water is extremely insignificant - you will need to add two heaped tablespoons of salt to increase the boiling point by just one degree. What can really speed up the boiling of water is a lid that covers the pan: this will keep the heat inside the pan and the water will boil faster.

Cover image: Image by Three-shots from Pixabay

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