Is it true that cold water boils faster than hot?

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

Questionwhat water boils Faster - cold or hot, Interested many. About this regularly They ask On sites questions And answers And Discuss This is in Media. Excites users and question, Maybe Lee salt accelerate boiling water, a some Culinary resources are definitely They saythat salty water will boil faster than fresh.

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

However, at any height, the boiling process occurs according to the same laws: water absorbs heat from the plate, fire or heating element of an electric kettle, gradually becoming hot. It is logical that water with a temperature of 60–75 ° C - and according to the rules precisely such There should be water in the hot water supply system in Moscow - you will need to warm up to fewer degrees to boil than water temperature water (18–20 ° C). However, the colder the water, the more effective it absorbs Heat, and therefore cold water will first heat up faster. But this advantage will be short -term, and, having reached the temperature of hot water, it will heat up at the same speed as the initially hot water. As a result, the time of boiling for cold water will take shape from the time necessary to heat it to the hot temperature, and the time for which hot water boils. 

Presumably, the misconception could arise due to the fact that in the case of freezing of hot and cold water may occur The effect of mpemba. It consists in the fact that under some conditions, hot water can freeze faster than cold. Named after the Tangagnik schoolboy Erasto Mpemba, the effect has not yet received a sufficient scientific explanation, and the scientists continue to argue whether he actually exists, trying to repeat the original Experiments Schoolchman and professor of physics Dannis Osborne. In 2016, having made his observations, British scientists They came To the conclusion that hot water does not cool faster than cold, that is, they refuted the existence of the effect. But already in 2017, their conclusions were in doubt at once by two groups of scientists: Sino-Singapore and American. The first discoveredthat in the theory of heating the chilled system, far from equilibrium, should take less time than another system, which was initially closer to equilibrium, but did not provide practical implementation of their theory. The second Propheted The appearance of a direct and reverse paradox of MPEMBA for some bulk bodies.

In addition to pressure, the amount of boiling is affected by the amount of impurities in water. Various chemical compounds are added to hot tap water to protect the pipes from corrosion. According to The second law of RaulThe larger in a solution of impurities, the higher you need a boiling temperature. Mosvodokanal declaresWhat is stiffness, that is, the number of salts dissolved in water in the water is in Moscow between 1.9 and 5.7 m-EKV/L. First of all, it is magnesium and calcium salts. By requirements Rospotrebnadzor, the permissible limit of the stiffness of water for centralized water supply is 7 m-EKV/l. Such a number of impurities on the boiling temperature affects extremely slightly. However, in the electric kettle, which is rarely cleansed from the inside, there is a large layer of lime plaque and the impurities remaining from numerous previous boiling water in the form of flakes - this amount can become significant. That is why manufacturers Consult pour into such devices or preliminary filtered water, or regularly clean. They pay attention to the fact that the water in a teapot with a large number of deposits will boil more slowly, and the device itself will consume a larger amount of electricity.

Salted salt, or sodium chloride, also increases the boiling temperature of water, so salted water will boil at higher temperatures than fresh. Every 29 g of salt (approximately tablespoon with a slide) per liter of water Increase A boiling temperature of 0.5 ° C. However, if you try to conduct such an experiment at home, then the results may be surprised. The density of salty water higherthan fresh, and heat capacity is slightly lower. That is, the bubbles that appear before the boiling process itself will first appear at the bottom of the pan with salted water. Fresh water will heat up evenly, so the bubbles will appear later, but the boiling point will be achieved faster. 

Thus, hot water boils nevertheless faster than cold and fresh-faster than salty. However, for cooking food, the difference between boiling temperatures and the boiling rate of salty and unsolved water is extremely insignificant - it will be necessary to add two tablespoons with a hill of salt so that the boiling temperature increases by only one degree. What can really accelerate boiling water is a cover that covers the pan: so warmly will remain inside the pan, and the water will boil faster.

Image on the cover: Image by Three-Shots from Pixabay

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