How does the boiling point of water change depending on the pressure

Why did a person start boiling water before using it directly? Correctly, to protect yourself from many pathogens and viruses. This tradition came to the territory of medieval Russia before Peter the Great, although it is believed that it was he who put the first samovar into the country and introduced the rite of unhurried evening tea drinking. In fact, some people used our samovars in ancient Russia to make drinks from herbs, berries and roots. Boiling was required here mainly to extract useful plant extracts, rather than for disinfection. After all, at that time, it was not even known about the microcosm, where these bacteria with the viruses live. However, thanks to boiling, our country was avoided by world pandemics of terrible diseases, such as cholera or diphtheria.

Celsius

The great meteorologist, geologist and astronomer from Sweden Anders Celsius initially used a value of 100 degrees to indicate the freezing point of water under normal conditions, and the boiling point of water was taken as zero degrees. And even after his death in 1744, no less famous person, botanist Carl Linnaeus and Celsius receiver Morten Stremer, turned this scale for the convenience of its use. However, according to other sources, Celsius himself did this shortly before his death. But in any case, the stability of the testimony and understandable graduation influenced the widespread distribution of its use among the most prestigious at that time scientific professions - chemists. And, despite the fact that in an inverted form the mark of the scale of 100 degrees established the point of stable boiling of water, and not the beginning of its freezing, the scale began to bear the name of its primary creator, Celsius.

Below the atmosphere

However, not everything is as simple as it seems at first glance. Looking at any state diagram in PT or PS coordinates (the entropy S is temperature-dependent in direct dependence), we will see how closely the temperature and pressure are closely related. Similarly, the boiling point of water, depending on the pressure, changes its values. And any mountaineer is familiar with this property. Everyone who has at least once in his life comprehended heights above 2000-3000 meters above sea level, knows how difficult it is to breathe at altitude. This is due to the fact that the higher we rise, the air becomes more rarefied. Atmospheric pressure falls below one atmosphere (below the N, ie, below the "normal conditions"). The temperature of the boiling water also drops. Depending on the pressure at each of the heights, it can boil at both eighty and sixty degrees Celsius.

Pressure cookers

However, it should be remembered that even though the main microbes die at temperatures above sixty degrees Celsius, many can survive at eighty or more degrees. That's why we are trying to boil water, that is, we bring its temperature to 100 ° C. However, there are interesting kitchen appliances that allow you to reduce the time and heat the liquid to high temperatures, without boiling it and losing weight by evaporation. Realizing that the boiling point of water, depending on the pressure, can change, engineers from the United States on the basis of the French prototype presented the world in the 1920s with a pressure cooker. The principle of its operation is based on the fact that the lid tightly presses against the walls, without the possibility of removing steam. Inside, an increased pressure is created, and water boils at higher temperatures. However, such devices are quite dangerous and often lead to an explosion and serious burns to users.

Ideally

Let's look at how the process comes and goes. Imagine an ideally smooth and infinitely large heating surface, where the heat distribution occurs evenly (an equal amount of thermal energy is supplied to each square millimeter of the surface), and the surface roughness coefficient tends to zero. In this case, when n. Y. Boiling in the laminar boundary layer will begin simultaneously over the entire surface area and occur instantly, immediately evaporating the entire unit volume of liquid located on its surface. These are ideal conditions, in real life this does not happen.

In real

Let's find out what is the initial boiling point of water. Depending on the pressure, it also changes its values, but the main point here lies in this. Even if we take the smoothest, in our opinion, pan and bring it under a microscope, then in its eyepiece we see uneven edges and sharp frequent peaks protruding above the main surface. Heat to the surface of the pan, we will assume, is brought in uniformly, although in reality this is also not quite true statement. Even when the pan is on the largest burner, the temperature gradient is distributed unevenly on the slab, and there are always local overheating zones responsible for the early boiling of water. How many degrees in this case on the peaks of the surface and in its lowlands? The peaks of the surface with an uninterrupted supply of heat warm up faster than lowlands and so-called depressions. Moreover, surrounded on all sides by water with low temperature, they better give energy to water molecules. The coefficient of thermal diffusivity of the peaks is one and a half to two times higher than that of nisins.

Temperatures

That is why the initial boiling point of water is about eighty degrees Celsius. At this value of the peak of the surface, a sufficient amount of heat is provided to allow instant boiling of the liquid and the formation of the first bubbles visible to the eye, which timidly begin to ascend to the surface. And what is the boiling point of water at normal pressure? Many people ask. The answer to this question can easily be found in the tables. At atmospheric pressure, stable boiling is established at 99.9839 ° C.