What are two kinds of fluids

Physics: is water made up of two liquids?

Water is as ubiquitous as it is unusual. It differs drastically from most other liquids in terms of a number of properties, such as surface tension, heat capacity, compressibility, and melting and boiling points. Some scientists explain the strange behavior by viewing water not as a complicated substance, but rather as a mixture of two simple substances with a complicated relationship. Other experts vehemently contradict: Such a statement does not fit the basic principles of physical chemistry.

Over the past decade, emotions have risen in the academic dispute. The renowned water researcher Anders Nilsson of Stockholm University says the question brings out "very strong, almost religious views". Scientists generally attribute the strange properties of water to the bonds created by what are known as hydrogen bonds. However, what exactly is going on is controversial, especially when the water is in a supercooled state. Then it should actually freeze, but it doesn't come to that.

This article is included in Spectrum of Science 2/2021

Like many chemists, Nilsson thought early in his career that the structure of water should be well understood. Then he realized it wasn't. For many years he has devoted himself entirely to the peculiarities of the substance. His more recent work lends weight to a hypothesis that is controversial in specialist circles: two different fluid structures exist separately from one another in water.

  1. Water behaves differently from most substances in many ways. Its density does not increase more and more as it cools, but rather it expands again below four degrees Celsius.

  2. According to computer simulations, this may be due to the coexistence of two molecular orders in water: It forms individual liquids with different densities.

  3. In order to check this, researchers are gradually penetrating an area that has been experimentally inaccessible for a long time. Initial data seem to support the idea, but the interpretation is controversial.

"With regard to their simple thermodynamic and kinetic properties, all liquids behave the same way when the pressure and temperature change," explains Nilsson. When it cools down, the density increases and the heat capacity and compressibility decrease. “Almost every liquid on earth is like that. Just not water. «Water has its greatest density at four degrees Celsius, at lower temperatures it decreases again. Anyone who studies chemistry learns the reason for this: The molecules in liquids are disordered and constantly realign themselves, whereas water forms a network of relatively stable hydrogen bonds. Their strength lies - in comparison to the usual intermolecular interactions - between the stronger covalent bonds and the weaker dipole forces. In contrast to the latter, they are directed; each hydrogen atom turns to an electron pair of an oxygen atom.

While water behaves like most liquids at higher temperatures, this special type of bond plays a more important role when it cools. "That's a lot of hydrogen bonds for such a small molecule," says chemist and water researcher Martin Chaplin of London South Bank University. "They enforce a structured, orderly state."

Do molecules packed with different densities create two completely different liquids?

For a long time there have been hypotheses about the exotic properties of water. In the 1960s, Soviet chemists even suspected that its molecules could form particularly stable polymer structures in thin capillary tubes. They came to be known as polywater; Western scientists also took the idea seriously. Laboratories around the world worked intensively on this until about ten years later it turned out that the results were probably due to contamination.