In addition to solid, liquid and gas, water can actually burn in this special form!

Matter in the world can appear in many different forms. There are six main known forms of matter, namely solid, liquid, gas, plasma, Bose-Einstein condensate and fermion condensate. Among them, solid, liquid and gas are the three forms we are most familiar with.

Most of the substances we see in daily life have three forms: solid, liquid and gas. Under different temperatures and pressures, these three forms can be transformed into each other. The most typical one is water. Under standard atmospheric pressure, the boiling point of water is 100 degrees Celsius, which means that at 100 degrees Celsius, water will turn into gaseous form, that is, water vapor. And 0 degrees Celsius is the freezing point of water. At 0 degrees Celsius, water will condense into ice. If the air pressure increases, the freezing point and boiling point of water will also increase accordingly, and vice versa.

Does water only have three forms? No, not only water, but many substances have a special form different from the three forms under special circumstances. This state is called "supercritical state".

What is the supercritical state?

Take water as an example. Water has a critical temperature and critical pressure. The so-called critical temperature refers to the highest temperature at which water changes from gas to liquid. What does it mean? We know that under standard atmospheric pressure, the boiling point of water is 100 degrees Celsius, but as the pressure increases, the temperature will also increase. However, once the temperature exceeds the critical temperature, no matter how much pressure is increased, gaseous water will no longer turn into liquid. The critical pressure is the minimum pressure required for water to change from gas to liquid at the critical temperature.

The critical temperature of water is 374.15 degrees Celsius, that is, once the temperature exceeds 374.15 degrees Celsius, no matter how the pressure increases, water will not change from gas to liquid.

The critical pressure of water is 22.05 MPa, which means that at 374.15 degrees Celsius, the minimum pressure required for water to transform from gas to liquid is 22.05 MPa. So, what happens when the temperature and pressure of water far exceed the critical value? Water will present a special state - supercritical state, and water in this state is called "supercritical water".

Supercritical water is a special fluid. Compared with ordinary liquid water, the compressibility of supercritical water is significantly enhanced, and it also has the unique ability to dissolve other substances. Specifically, the difference between supercritical water and ordinary liquid water is mainly reflected in density, dielectric constant and viscosity.

Under standard atmospheric pressure, the dynamic viscosity of liquid water at room temperature is 0.001 Pascal seconds, while the viscosity of supercritical water is 0.298x10∧-2 Pascal seconds. What does this mean? It means that supercritical water can become a substance with high fluidity.

Having said so much, what is the use of supercritical water? The unique properties of supercritical water determine that it must have a wide range of uses, such as the treatment of organic waste. How is traditional organic waste treated? It is through incineration. As the name suggests, the incineration method is to decompose organic waste through high-temperature incineration, but this treatment method will cause secondary pollution. However, the use of "supercritical water combustion technology" can effectively solve this problem. After the fuel or organic waste meets the oxidant, an oxidation reaction can occur in supercritical water, and this violent oxidation reaction will produce a "hydrothermal flame."

"Hydrothermal flame" is a very interesting phenomenon. To us, it looks like there is fire in the water, as if water can really burn.

Using supercritical water combustion technology to treat organic waste not only does not cause secondary pollution due to high temperature, but also can achieve a removal rate of 99%, which can be completed in one go without the need for secondary rework. In addition to its combustible properties, supercritical water is also a high-quality "extractant". Because supercritical water has good solubility, it can extract flavors and oils from many natural substances. It is precisely because supercritical water has a wide range of uses and great development potential that scientists have been conducting research on supercritical water.

Early supercritical water was obtained in the laboratory, so scientists have always had a question, that is, "Does supercritical water exist in nature?" Later, this question finally had an answer.

German scientists accidentally discovered the existence of natural supercritical water at a hydrothermal vent in the central Atlantic Ocean. The reason is that the temperature here is far higher than the critical temperature of water. According to measurements, the water temperature near the hydrothermal vent reached 464 degrees Celsius, far higher than the critical temperature of water, 374.15 degrees Celsius. This is a discovery that makes scientists very excited, because it confirms that supercritical water is not artificially created, but objectively exists in nature. Of course, supercritical water is not the only substance that can be in a supercritical state. As long as the conditions are right, many substances can have a supercritical state.

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