As we all know, all macro substances in nature exist in three states: solid, liquid and gas, just like water. Generally speaking, the material state of traditional substances is determined by temperature and pressure. When temperature and pressure are under certain conditions, three states of the same substance can exist in balance, and this point is called triple point.
The point where gas and liquid exist is called the critical point. The critical point of different substances is different, and the critical temperature and critical pressure are also different. This critical phenomenon was discovered by Andrews when he was studying the gas-liquid density difference of carbon dioxide in 1869. Take water as an example. When water is at the critical point, the critical temperature is 374 and the critical pressure is 2 1.7 MPa (about 2 18 times of the standard atmospheric pressure).
When the temperature and pressure of water exceed the critical value, water will be in a supercritical state, and the properties of gas and liquid will be very similar, which is difficult to distinguish, just like liquid and gas.
Supercritical state, which is the fourth form of matter besides solid, liquid and gas. Water in this state is called supercritical water.
Under the standard atmospheric pressure (0. 10 13 MPa), the freezing point of pure water is 0 and the boiling point is 99.975.
As the pressure rises, the boiling point of water will also rise, and people who have used pressure cookers should understand. In order to change ordinary water into supercritical water, high temperature (above 374℃) and high pressure (above 22. 1 MPa) must be maintained. In the laboratory, this is easy to achieve.
Although humans discovered and made supercritical water a long time ago, and believed that it must exist in nature, it was not until the beginning of the 20th century that the 65438th supercritical water was observed.
In 2008, when German scientists inspected a high-temperature hydrothermal vent in the mid-Atlantic ridge, they found that the highest water temperature near the vent actually reached 464℃. This discovery made scientists ecstatic, because it was not only the highest temperature liquid found by human beings in nature, but also the first time to find naturally occurring supercritical water.
The submarine hydrothermal vent, also known as the "submarine black chimney", is formed by the expansion and separation of the submarine crust, the infiltration of seawater into the ground and the encounter with hot magma. The hydrothermal vent above was first discovered in 2005 by Professor Andrea, a scientist from Jacob University in Bremen, Germany, and her research team.
There is so much water on the earth that scientists discovered natural supercritical water so late because the existence of supercritical water needs to meet the requirements of high temperature and high pressure at the same time. This kind of situation is not common on earth, and we didn't have the ability or opportunity to observe it before. For example, although the deep-sea pressure is sufficient, only the water temperature in a few areas such as the black chimney on the seabed can meet the supercritical conditions.
Supercritical water is an unusual fluid with the characteristics of both liquid and gas. You can think of it as a dense gas with a density close to that of liquid water.
Although the density of supercritical water is close to that of water, its viscosity is low, far exceeding that of ordinary liquid, and its diffusivity is high, similar to that of gas. It can penetrate into the pores of solids like air, and it can also make other substances diffuse rapidly in supercritical water.
In addition, supercritical water also has strong oxidizing ability and dissolving ability. Many substances insoluble in liquid water can be dissolved quickly in supercritical water, most metals can be slowly corroded in supercritical water, and some substances can even be burned in supercritical water. The physical and chemical properties of supercritical water can change with temperature or pressure.
It is these characteristics that enable supercritical water to be used as solvent, extractant and catalyst in the chemical field, and has applications in accelerating chemical reaction process, separation and purification. Secondly, using the characteristics of supercritical water, it can also be used to treat sewage and some pollutants, which has broad application prospects in the field of environmental protection.
Many countries in the world, such as Japan and Germany, are studying technologies related to supercritical fluids. Supercritical fluids, like superconductors, must be kept at a certain temperature when used, which leads to high application cost and limits its application space. But because of its many advantages, it is still worth studying.