Why did a person start boiling water before drinking it? Correctly, 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 to the country and introduced the rite of unhurried evening tea drinking. In fact, our people used a kind of samovar in ancient Russia to make drinks from herbs, berries and roots. Boiling was required here mainly for the extraction of useful plant extracts, rather than for disinfection. Indeed, at that time it was not even known about the microcosm where these bacteria and viruses live. However, thanks to boiling, our country was bypassed by global pandemics of terrible diseases such as cholera or diphtheria.
Celsius scale
The great meteorologist, geologist and astronomer from Sweden, Anders Celsius, originally used the 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 itdeath in 1744, a no less famous person, the botanist Carl Linnaeus and the successor of Celsius Morten Strömer, turned this scale upside down for ease of use. However, according to other sources, Celsius himself did this shortly before his death. But in any case, the stability of the readings and the understandable graduation influenced the widespread use of its use among the most prestigious scientific professions at that time - chemists. And, despite the fact that the upside down 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 P-T or P-S coordinates (entropy S is a direct function of temperature), we see how closely temperature and pressure are related. The boiling point of water also changes with pressure. And any climber is well aware of this property. Everyone who at least once in his life comprehended heights over 2000-3000 meters above sea level knows how hard it is to breathe at altitude. This is because the higher we go, the thinner the air becomes. Atmospheric pressure falls below one atmosphere (below n.a., that is, below "normal conditions"). The boiling point of water also drops. Depending on the pressure at each altitude, it can boil at both eighty and sixty degrees Celsius.
Pressure cookers
However, it should be remembered that although the main microbes die at temperatures above sixty degrees Celsius, many can survive at eighty degrees or more. That is why we achieve boiling water, that is, we bring its temperature to 100 ° C. However, there are interesting kitchen appliances that allow you to reduce time and heat the liquid to high temperatures, without boiling it and losing mass through evaporation. Realizing that the boiling point of water can change depending on pressure, engineers from the United States, based on a French prototype, introduced the world to a pressure cooker in the 1920s. The principle of its operation is based on the fact that the lid is tightly pressed against the walls, without the possibility of steam removal. Increased pressure is created inside, and water boils at higher temperatures. However, such devices are quite dangerous and have often led to explosions 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 distribution of heat is uniform (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, at n. y. boiling in a 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.
Reality
Let's find out what the initial boiling point of water is. 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 will see uneven edges and sharp frequent peaks protruding above the main surface. The heat to the surface of the pan, we will 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 unevenly distributed on the stove, and there are always local overheating zones responsible for the early boiling of water. How many degrees are at the same time at the peaks of the surface and in its lowlands? The peaks of the surface with an uninterrupted supply of heat warm up faster than the lowlands and the so-called depressions. Moreover, surrounded on all sides by water with a 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.
Temperatures
That is why the initial boiling point of water is about eighty degrees Celsius. At this value, the surface peaks supply enough heat to instantly boil the liquid and form the first bubbles visible to the eye, which timidly begin to rise to the surface. What is the boiling point of water atnormal pressure - many ask. The answer to this question can be easily found in the tables. At atmospheric pressure, a stable boil is established at 99.9839 °C.
