Why did a person begin to boil water before using it directly? Right to protect yourself from many pathogenic bacteria and viruses. This tradition came to the territory of medieval Russia even before Peter the Great, although it is believed that it was he who brought the first samovar into the country and introduced the rite of unhurried evening tea drinking. In fact, some people used some semblance of samovars in ancient Russia to make drinks from herbs, berries and roots. Boiling was required here mainly for extracting useful plant extracts, rather than for disinfection. After all, at that time it was not even known about the microworld, where these bacteria with viruses live. However, thanks to boiling, our country was circumvented by world pandemics of terrible diseases, such as cholera or diphtheria.
Celsius
Anders Celsius , a great meteorologist, geologist and astronomer from Sweden, 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 after his death in 1744, a no less famous person, the botanist Karl Linney and the receiver of Celsius Morten Stremer, turned this scale over 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 grading influenced the widespread use of its use among the most prestigious scientific professions of the time - chemists. And, despite the fact that the inverted mark of the scale at 100 degrees set 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 the PT or PS coordinates (the entropy S is functionally dependent on temperature in direct relation), we will see how closely the temperature and pressure are related. Also, the boiling point of water, depending on pressure, changes its values. And any climber is familiar with this property. Everyone who at least once in his life comprehended altitudes above 2000-3000 meters above sea level knows how hard it is to breathe at altitude. This is due to the fact that the higher we rise, the thinner the air becomes. Atmospheric pressure drops below one atmosphere (below n.a., that is, below "normal conditions"). The boiling point of water also falls. Depending on the pressure at each altitude, it can boil both at eighty and at sixty degrees Celsius.
Pressure cooker
However, it should be remembered that although the main microbes die at temperatures above sixty degrees Celsius, many can survive at eighty or more degrees. That is why we achieve boiling water, that is, we bring its temperature to 100 ° C. However, there are interesting kitchen appliances that can reduce the time and heat the liquid to high temperatures, without boiling it and losing weight through evaporation. Realizing that the boiling point of water may vary depending on pressure, US engineers based on the French prototype introduced a pressure cooker to the world in the 1920s. The principle of its operation is based on the fact that the lid is firmly pressed against the walls, without the possibility of steam removal. An increased pressure is created inside and the water boils at higher temperatures. However, such devices are quite dangerous and often led to an explosion and serious burns to users.
Perfectly
Let's look at how the process comes and goes. Imagine a perfectly smooth and infinitely large heating surface, where the distribution of heat occurs evenly (the same 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. at. boiling in a laminar boundary layer will begin simultaneously over the entire surface area and will 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 hold it under the microscope, then in its eyepiece we will see uneven edges and sharp frequent peaks protruding above the main surface. The heat to the surface of the pan, we assume, is supplied evenly, although in reality this is also not a completely true statement. Even when the pan is on the largest burner, the temperature gradient is not evenly distributed on the stove, and there are always local overheating zones responsible for the early boiling of water. How many degrees are there at the peaks of the surface and in its lowlands? Surface peaks during uninterrupted heat supply warm up faster than lowlands and so-called troughs. Moreover, surrounded on all sides by water with low temperature, they better give energy to water molecules. The thermal diffusivity of the peaks is one and a half to two times higher than that of the lowlands.
Temperature
That is why the initial boiling point of water is about eighty degrees Celsius. With this value, the surface peaks bring in a sufficient amount of heat necessary for instant boiling of the liquid and the formation of the first bubbles visible to the eye, which timidly begin to rise to the surface. And what is the boiling point of water at normal pressure - many ask. The answer to this question can be easily found in the tables. At atmospheric pressure, stable boiling is established at 99.9839 ° C.