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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), author: qu Lijian

Water is the most common and abnormal substance in the world.

Write an article | qu Lijian

There are many "weird" things in our world, such as quantum, black hole, dark matter, dark energy, the origin of the universe, and so on. These things are a little far away from our daily life, but there is something as strange as these things in daily life, that is, water.

Weird water. Water is the weirdest liquid in science. Some scientists have listed at least 66 abnormal properties of effluent. Many of these strange properties are reflected in special scientific experiments, and some of them can be easily shown.

Throw a piece of ice-solid water-into cold liquid water and you will find that ice floats on the surface because it is less dense than liquid water. This is a strange thing. Liquids generally condense into solids and the density increases, because atoms or molecules are arranged more closely in solids than in liquids.

When the lake is freezing, use a thermometer to measure the temperature of the water at each depth. The temperature at the surface is 0 ℃, while the temperature at the bottom of the lake is 4 ℃. This is because the density of the water is the highest at 4 ℃.

The density of liquid water is higher than that of ice, and its density at freezing point is less than that at slightly higher temperature, otherwise lakes and rivers will freeze from the bottom up, and it will be difficult for life in the water to survive. This is of great significance to life, not to mention that they have survived many long ice ages in history.

The density of water at each temperature. Image source: CRC Handbook of Chemistry and Physics

The bottom temperature of the lake is 4 ℃. Image source: https://wtamu.edu/~cbaird / sq / images / lake_ temp. PG, on the other hand, to raise the temperature of water, the heat absorbed by water is surprisingly higher than that of ordinary liquids. Readers in kitchens often have life experience, and oil heats up faster than water. It also makes sense that water has a strong ability to absorb heat-if the heat absorption capacity is poor and the climate changes slightly, the ecosystem will be destroyed.

Water expands when it freezes, but shrinks when it dissolves. Water can form at least 17 kinds of crystals-ice.

The strange properties of several kinds of water will be listed later in this article.

We should be grateful for the strange nature of water, otherwise, complex life may not exist, and we will not have the opportunity to read this article and experience the magic of water.

Why does water behave strangely?

When scientists think about problems, they generally adhere to reductionist thinking, and the nature of matter comes from the structure of matter.

So, what kind of structure is water?

The story of Shuangshui dates back to 1976.

Austin Austen Angell and Robin Speedy of Purdue University in the United States cooled the water to see how low it could go.

You may ask, won't it freeze if it drops to 0 ℃?

Not necessarily, if the container is very clean and the water is very calm, it will remain liquid below 0 ℃, which is called "supercooled water".

A bottle of supercooled water freezes quickly after being disturbed. Image source: according to the Youtube video production Angel and Speedy found some strange phenomena: the lower the temperature, the more uneven the density distribution of supercooled water. Generally speaking, the lower the temperature is, the more uniform the density of water should be.

What happened in the water?

Limited to the experimental conditions at that time, it is impossible to observe in more detail.

In 1992, Peter Poole (Peter Poole) and Gene Stanley (Gene Stanley) of Boston University in the United States carried out a computer simulation of water (Nature 1992, 360,324-328), which reproduced a similar phenomenon in the experiment. More importantly, computer simulation can calculate the various properties of the system, and even the specific motion of molecules.

Based on their computer simulations, Poole and Stanley see that the behavior of supercooled water is very similar to that of ordinary water turning into steam. Under some special conditions, the density distribution of ordinary water will also become extremely uneven. Let's first briefly introduce the process of changing water from a liquid to a gas.

The dividing line between gas and liquid-vaporization line. Source: "Edge Miracle: phase transition and critical phenomenon" as shown in the above figure, the liquid pressure is maintained at P0 and the temperature rises, that is, the liquid state moves according to the midline LQ of the diagram. When it reaches point Q, part of the liquid begins to vaporize, that is, it becomes a gas. At this time, although the heating continues, the temperature no longer rises, but remains at T0. The temperature does not continue to rise along the QG until all the liquid turns into a gas. By doing experiments under various pressures, a series of gas-liquid coexistence points can be obtained. if these points are connected, a curve-vaporization line can be obtained.

With constant heating or pressure, will the vaporization line continue or will it stop abruptly at some point?

The experiment found that the vaporization line has an end point, which is called the critical point, that is, the point in the following picture.

The vaporization line has an end, that is, the critical point. Image source: beyond the critical point of "marginal Miracle: phase transition and critical phenomena", is matter gaseous or liquid?

This problem is meaningless, because beyond the critical point, the difference between gas and liquid no longer exists. By experimenting along the dotted line in the diagram, matter can change continuously from a liquid point to a gaseous state.

Near the critical point of gas-liquid phase transition, the density distribution is also extremely uneven. A related experimental phenomenon is critical opacification, as shown in the following figure. (editor's Note: see the 200th Anniversary of the critical phenomenon, who first discovered this physical phenomenon? ")

When the heated ethanol is irradiated with light, figure 1 shows the coexistence of gas and liquid. In figure 2, the critical emulsion phenomenon occurs, that is, the light scattered by the matter is white, which shows that the density of the substance is not uniform on a scale as small as the wavelength of light. the substance becomes opaque and turbid. Figure 3 is a supercritical fluid. Image source: Wikipedia generally has three states of matter: gaseous, liquid and solid. However, the more common word in physics is "phase" rather than "state".

There are much more types of "phases" of matter than what is commonly known as "states". In other words, there can be many different "phases" corresponding to the same state. For example, the solid state of water is ice, but ice has many different ways of crystallization, which correspond to different "phases".

The transformation of matter from one phase to another is called phase transition. The change of water from liquid (or liquid phase) to gaseous (or gaseous phase) is a kind of phase transition.

Going back to Poole and Stanley's experiments, they found through computer simulations that the density of supercooled water would also become extremely uneven near a certain temperature, similar to the situation near the critical point of gas-liquid phase transition. As a result, Poole and Stanley imagine that there is a tipping point, and supercooled water can also undergo phase transition, which is low-density water and high-density water.

Poole and Stanley's idea is supported by the simulation results of subsequent more accurate water models, showing that their conjecture is reliable that water has a tipping point for supercooled water in addition to the critical point of the vaporization line.

Supercooled water will have high and low density water phase transition, which is similar to the gas-liquid phase transition of ordinary water. Note that in this figure, the ordinate is the temperature and the Abscissa is the pressure. Figure source: can you see this tipping point in the Chemistry World experiment?

It's hard. The tipping point is-45 ℃. Water freezes easily at such a low temperature.

Many outstanding research groups around the world have carried out research for 26 years. In 2017 and 2018, two independent exquisite experiments (Science 2017, 358,1589; Science 2018, 359, 1127) determined that the second critical point exists and that supercooled water can undergo phase transition under appropriate conditions, that is, there are two kinds of water.

What kind of structure is it?

Professor Anders Nilsson of Stockholm University in Sweden and his collaborators have done systematic work in this area, and we directly introduce their conclusions.

The structure of one water and two structures of water is determined by the interaction between water molecules.

The water molecule consists of two hydrogen atoms and one oxygen atom. The two hydrogen atoms are closely bound to the oxygen atom to form a V-shaped structure, which chemists call the "covalent bond".

Oxygen atoms and hydrogen atoms combine through covalent bonds to form water molecules. Picture source: Chinese water molecules are electrically neutral as a whole, but within the molecules, the electric charge distribution is uneven, with oxygen atoms slightly negatively charged and two hydrogen atoms slightly positively charged. When the oxygen atom in one water molecule is close to the hydrogen atom in another, there is an attraction between the two water molecules, which chemists call "hydrogen bonding".

The formation of hydrogen bonds between water molecules. Picture source: popular science China hydrogen bond is much weaker than covalent bond and is easy to be destroyed. Someone vividly said: "hydrogen bond is equivalent to two people holding hands, can be pulled or divided." covalent bonds connect your own hands and feet and cannot be separated. "

According to their experimental results, Nilsson proposed that under the influence of hydrogen bonds, water molecules can be arranged in two ways: ordered tetrahedral arrangement or random disordered arrangement to form low-density water and high-density water, respectively.

Water has two structures. Picture source: the above theory of New Scientists can explain many abnormal properties of water. Here are a few examples.

The density of abnormal ice is lower than that of water.

The arrangement of water molecules in ice is the same as that in low-density water, that is, tetrahedral structure, and there is high-density water with disordered structure in water, so the average density of water is greater than that of ice.

The density of water is the highest at 4 ℃.

At 0 °C, water molecules are more in the ordered phase of tetrahedral structure, that is, low density water is dominant. In extreme cases, if there is no disordered high-density water at all, the liquid water will freeze. As the temperature increases, the irregular thermal motion of the molecule becomes more intense, the ordered structure decreases, and the higher the density of water, the more dominant it is. However, when the temperature is above 4 °C, the molecular thermal motion makes the distance between water molecules increase with the increase of temperature, and the density of water decreases.

Molecules have been doing irregular thermal motion, the higher the temperature, the more intense the molecular thermal motion, the more difficult it is to maintain the ordered structure. Image source: www.tec-science.com. The specific heat capacity of water is significantly larger than that of most liquids.

Heat a substance to raise its temperature by a certain amount, but water requires more heat than other liquids, that is, greater heat capacity, because water needs some heat to destroy the tetrahedral structure of low-density water.

The specific heat capacity of water first decreases and then increases with the increase of temperature, and there is a minimum at 35 °C, while the specific heat capacity of most liquids increases with the increase of temperature.

Between 0 and 35 °C, the increase of temperature leads to the continuous destruction of the tetrahedral structure in water, which is convenient for the disordered movement of water molecules; with the increase of temperature, the tetrahedral structure becomes less and less, and the heat absorption capacity of water decreases. When the temperature reaches 35 °C, the tetrahedral structure in the water is destroyed and the specific heat capacity of water begins to behave similar to that of ordinary liquids.

The specific heat capacity and temperature of water. Image source: the compression ratio of Lawrence Berkeley National Laboratory water-the ratio of the reduced volume to the original volume after pressure-first decreases and then increases with the increase of temperature, with a minimum at 46 °C, while the compression ratio of most liquids increases with the increase of temperature.

As the temperature increases, water becomes difficult to compress before 46 °C, because the structure of low-density water gradually disintegrates and the proportion of high-density water becomes higher and higher. After the temperature reaches 46 °C, there is almost only disordered high-density water in the water, which behaves like an ordinary liquid, and the higher the temperature, the easier it is to be compressed.

The specific heat capacity reflects the change of the number of microstructure, and the compression ratio reflects the tightness of molecular accumulation. It is normal that the two minimums do not fall at the same temperature.

Water is harder to compress than most liquids.

This is caused by the strong attraction between hydrogen-bonded water supply molecules, especially for high-density water.

Water molecules are more likely to spread under high pressure.

High pressure can destroy the ordered structure of tetrahedron, and the more disorderly the arrangement of water molecules is, the easier it is to spread.

Water expands, pressurizes, expands more when heated.

Pressurization makes the water more disorderly, so it is easy to expand.

No more abnormal properties of water are listed to explain. In short, water molecules are arranged in two ways, which is consistent with the experiment and can explain the abnormal properties of water consistently.

The mystery of the strange nature of water began to surface, but it made it even weirder.

Main reference materials

New Scientists, 2018, 238, 3180, 26-29

New Scientists, 2010, 205, 2746, 32-35

Chem. Rev. 2016, 116, 7463 − 7500

The weirdness of water https://www.chemistryworld.com/features/the-weirdness-of-water/4011260.article

Physics Today, 2017, 70, 18-21

Physics, 2010, 39, 79-84

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