Water is not one liquid, but two

Water is the most common, yet the most anomalous substance in the world.

Written by | Qu Lijian

There are many "weird" things in our world, such as quantum, black holes, dark matter, dark energy, the origin of the universe, etc. These things are a bit far from our daily life, but there are also things in our daily life that are as weird as these things, that is - water.

Weird water

Water, which is commonplace, is the most bizarre liquid in science. Scientists have listed at least 66 abnormal properties of water. Many of these strange properties are reflected in special scientific experiments, and some properties can be easily demonstrated.

If you drop a piece of ice—solid water—into cold liquid water, you'll find that the ice will float on the surface of the water because ice is less dense than liquid water. This is a strange thing, because normally when a liquid condenses into a solid, its density increases because the atoms or molecules are packed more tightly together in a solid than in a liquid.

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

Liquid water is denser than ice, and its density at freezing point is less than at slightly higher temperatures. Otherwise, lakes and rivers would freeze from the bottom up, making it difficult for aquatic life to survive. This has important implications for life, not to mention its ability to survive many long ice ages throughout history.

Density of water at different temperatures. Source: CRC Handbook of Chemistry and Physics

The temperature of the lake bottom is 4°C. Image source: https://wtamu.edu/~cbaird/sq/images/lake_temp.png

On the other hand, to raise the temperature of water to a certain level, the amount of heat required is surprisingly higher than that of ordinary liquids. Readers who often cook have experience that oil heats up faster than water. Water has a strong ability to absorb heat, which also makes sense - if the ability to absorb heat is poor, the ecosystem will suffer a catastrophe if the climate changes slightly.

When water freezes, it expands, but when it melts, it contracts. Water can form at least 17 types of crystals - that is, ice.

We’ll list a few more of water’s weird properties later in this article.

We should be grateful for the strange properties of water. Otherwise, complex life might not exist, and we would 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 adopt a reductionist mindset, believing that the properties of matter originate from its structure.

So, what is the structure of water?

A Tale of Two Waters

The story dates back to 1976.

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

You may ask, wouldn’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℃. This is called "supercooled water".

A bottle of supercooled water freezes quickly after being disturbed. Image source: Based on a Youtube video

Angell and Speedy found something strange: the lower the temperature, the more uneven the density of supercooled water became. Normally, the lower the temperature, the more uniform the density of water should be.

What happened in the water?

Due to the limitations of the experimental conditions at the time, it was impossible to observe more detailed information.

In 1992, Peter Poole and Gene Stanley of Boston University conducted a computer simulation study on water (Nature 1992, 360, 324–328), reproducing similar phenomena in the experiment. More importantly, computer simulation can calculate various properties of the system and even the specific movement of molecules.

Based on their computer simulations, Poole and Stanley saw that supercooled water behaves very similarly to ordinary water turning into water vapor. Under certain conditions, the density distribution of ordinary water can also become extremely uneven. Let's first briefly introduce the process of water changing from liquid to gas.

The boundary between gas and liquid - the vaporization line. Image source: "Miracle on the Edge: Phase Transition and Critical Phenomenon"

As shown in the figure above, the liquid pressure is kept at P0, and the temperature is increased, that is, the liquid state moves along the line LQ in the figure. When it reaches point Q, part of the liquid begins to vaporize, that is, it becomes gas. At this time, although the heating continues, the temperature no longer rises, but remains at T0. The temperature continues to rise along QG until all the liquid turns into gas. Experiments under various pressures can obtain a series of points where gas and liquid coexist. Connecting these points will obtain a curve - the vaporization line.

If the temperature or pressure continues to increase, will the vaporization line continue to extend or stop at a certain point?

Experiments have found that the vaporization line has an end point, which is called the critical point, which is the point in the figure below.

The vaporization line has an end, the critical point. Image source: "Miracle on the Edge: Phase Transition and Critical Phenomenon"

Beyond the critical point, is the substance in a gaseous or liquid state?

This question is meaningless, because outside the critical point, the difference between gas and liquid no longer exists. If you do the experiment along the dotted line in the figure, the substance can continuously change from the liquid point to the gas state.

Near the critical point of the gas-liquid phase transition, the density distribution is also extremely uneven. A related experimental phenomenon is critical opalescence, as shown in the figure below. (Editor's note: Please refer to "200th Anniversary of Critical Phenomenon, Who First Discovered This Physical Phenomenon?")

When heated ethanol is illuminated with light, Figure 1 shows the coexistence of gas and liquid, and Figure 2 shows the critical opalescence phenomenon, that is, the light scattered by the substance is white, which means that at a scale as small as the wavelength of light, the density of the substance is uneven, and the substance becomes opaque and turbid. Figure 3 shows a supercritical fluid. Source: Wikipedia

Generally speaking, matter has three states: gas, liquid and solid. However, the word "phase" is more commonly used in physics rather than "state".

There are many more types of "phases" of matter than the types of "states" generally referred to. 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 change. The change of water from liquid (or liquid phase) to gas (or gas phase) is a phase change.

Let's go back to Poole and Stanley's experiment. Through computer simulation, they found that the density of supercooled water will become extremely uneven near a certain temperature, which is very similar to the situation near the critical point of gas-liquid phase transition. Therefore, Poole and Stanley imagined that there was a critical point there, and supercooled water could also undergo a phase transition, with the two phases being low-density water and high-density water.

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

Supercooled water will undergo a phase transition between high and low density water, similar to the gas-liquid phase transition of ordinary water. Note that the vertical axis in this figure is temperature and the horizontal axis is pressure. Image source: Chemistry World

Can this critical point be seen experimentally?

It's difficult. The critical point is -45℃. At such a low temperature, water can easily freeze.

Many outstanding research groups around the world have conducted research for 26 years. In 2017 and 2018, two independent and sophisticated experiments (Science 2017, 358, 1589; Science 2018, 359, 1127) confirmed that the second critical point exists and that supercooled water can undergo a phase change under appropriate conditions, that is, there are two structures of water.

What kind of structure is it exactly?

Professor Anders Nilsson of Stockholm University in Sweden and his collaborators have done systematic work in this regard. We will directly introduce their conclusions.

One Water, Two Structures

The structure of water is determined by the interactions between water molecules.

A water molecule is composed of two hydrogen atoms and one oxygen atom. The two hydrogen atoms are tightly bound to the oxygen atoms to form a V-shaped structure. The bonding method between them is called "covalent bond" by chemists.

Oxygen atoms and hydrogen atoms are combined through covalent bonds to form water molecules. Image source: Science Popularization China

The water molecule is electrically neutral as a whole, but the charge distribution inside the molecule is uneven, with the oxygen atom slightly negatively charged and the two hydrogen atoms slightly positively charged. When the oxygen atom in one water molecule and the hydrogen atom in another water molecule come close, an attraction is generated between the two water molecules, an effect chemists call "hydrogen bonding."

The formation of hydrogen bonds between water molecules. Image source: Science Popularization China

Hydrogen bonds are much weaker than covalent bonds and can be easily broken. Someone vividly said: "Hydrogen bonds are like two people holding hands, which can be pulled or separated. Covalent bonds connect your own hands and feet, which cannot be separated."

Based on their experimental results, Nilsson proposed that water molecules can be arranged in two ways under the influence of hydrogen bonds, either in an orderly tetrahedral arrangement or in a random disordered arrangement, forming low-density water and high-density water respectively.

Water has two structures. Image source: New Scientists

The above theory can explain many abnormal properties of water. Here are a few examples.

Correct answer is abnormal

• Ice is less dense than water.

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

• Water has the highest density at 4°C.

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, liquid water will freeze into ice. As the temperature rises, the irregular thermal motion of molecules becomes more intense, the ordered structure becomes less, and the high-density water becomes more dominant, that is, the density of water increases. However, when the temperature reaches above 4°C, the molecular thermal motion causes the distance between water molecules to increase with the increase in temperature, and the density of water decreases.

Molecules are constantly in random thermal motion. The higher the temperature, the more intense the molecular thermal motion becomes, and the more difficult it is to maintain an ordered structure. Image source: www.tec-science.com.

• The specific heat capacity of water is significantly greater than that of most liquids.

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

• The specific heat capacity of water first decreases and then increases with increasing temperature, reaching a minimum value at 35°C, while the specific heat capacity of most liquids continues to increase with increasing temperature.

Between 0 and 35°C, the rising temperature causes the tetrahedral structure in water to be continuously destroyed, facilitating the disordered movement of water molecules; as the temperature rises, the tetrahedral structure becomes less and less, and the water's ability to absorb heat appears to be decreasing. When the temperature reaches 35°C, the tetrahedral structure in water is completely destroyed, and the specific heat capacity of water begins to behave like that of ordinary liquids.

Specific heat capacity of water and temperature. Image source: Lawrence Berkeley National Laboratory

• The compressibility of water - the ratio of the reduced volume to the original volume after pressure is applied - first decreases and then increases with increasing temperature, reaching a minimum at 46°C, while the compressibility of most liquids continues to increase with increasing temperature.

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

The specific heat capacity reflects the change in the number of microstructures, and the compression rate reflects the tightness of molecular stacking. It is normal that their minimum values ​​do not fall at the same temperature.

• Water is more difficult to compress than most liquids.

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

• Water molecules diffuse more easily under high pressure.

High pressure can destroy the ordered tetrahedral structure. The more disordered the arrangement of water molecules is, the easier it is to diffuse.

• Water expands when heated, pressurized, and expands even more…

Pressurization makes the water more disordered, so it tends to expand.

I will not list more anomalous properties of water to explain them. In short, there are two ways of arranging water molecules. This theory is consistent with the experiment and can consistently explain the anomalous properties of water.

The mystery of water's weird properties began to emerge, except that it made water even weirder.

Main references

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

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.