The boiling point of liquid water is related to pressure. The boiling point of water is 100℃ under standard atmospheric pressure. If we increase the pressure, the boiling point of water will also increase. the air pressure reaches 10 MPa, the boiling point of water will reach 300°C, and the air pressure exceeds 22.064 MPa, it becomes supercritical water.
We know that it is difficult to cook rice by boiling water in a regular pot on the tray, because when the atmospheric pressure decreases, the boiling point of water also decreases, for example at an altitude of 1,000 meters. At 4,000 meters, the boiling point of water is only 88℃, the boiling point of water at the top of Mount Everest is only about 70℃. It is best to use a pressure cooker to cook the food at this time.
The pressure inside the pressure cooker is designed to be approximately 1.5 to1.8 atmospheres, which corresponds to the boiling point of water being 110°C to 117°C. So it is faster to cook food in a pressure cooker. pressure cooker because the temperature inside is higher.
In physics, crystalline substances often come in three forms: solid, liquid, and gas. We use a three-phase diagram to describe the relationship between material shape and pressure. water.
We can see from the image that water is generally divided into three regions: the solid region on top, the liquid region in the middle, and the gas region below. The intersection of the three regions is called. the three phases. Point, the point data of the water three-phase diagram is:
273.16K (0.01℃), 611.73Pa.
Standard atmospheric pressure is 101.3 Kpa. The three-phase diagram of water shows that the solid-liquid boundary point at this time is freezing point 0°C and gas-liquid. the junction pointion is the boiling point 100°C.
As pressure increases, below 50 MPa (500 atmospheres), the freezing point temperature of water does not change much, but the boiling point of water increases considerably, such as:
( 1 ) At 0.15 MPa, the boiling point of water is 111 ℃;
(2) At 1 MPa, the boiling point of water is 180
(3) At 5 MPa, the boiling point of water is of 264 ℃;
(4) At 10 MPa, the boiling point of water is 311 ℃;
(5) At 20 MPa, the boiling point of Boiling water is 366℃.
In thermal power plants, water must be heated and evaporated under high pressure, then the steam drives the turbine to do its work. For subcritical units, the pressure inside the steam drum can reach 16~. 19 MPa and the internal boiling water temperature just above 300℃.
As the pressure increases, the difference between the statesgaseous and liquid ts of water become smaller and smaller, until reaching the supercritical point (647K, 22.064 MPa), the two can no longer be distinguished, except in the solid state region. The water is collectively called supercritical water. For example, in the supercritical unit of a thermal power plant, the main steam pressure reaches 25 MPa and the temperature can reach more than 500 degrees Celsius.
In an underwater crater 5,000 meters deep, the pressure reaches nearly 50 MPa (500 atmospheres). The water near the crater at this time is supercritical water, with a water temperature of up to 450 degrees Celsius.
But the temperature of water has no upper limit. When the temperature reaches a certain level, the chemical bonds in water molecules break, forming a plasma state of atoms d hydrogen and oxygen atoms. At this point, the substance can no longer be called water in nature.