Water - discover its seven strange and unique properties

Water - discover its seven strange and unique properties

Water, so common and abundant on Earth, is an almost unique substance: it has astonishing chemical and physical properties and behaves in a bizarre way. Ice, liquid, or gas: discover the mysteries of H2O.

Do you think you know everything about the H2O molecule? Recent studies have just shown that liquid water can coexist in two different states! Have you ever seen an iron bar floating on molten iron? Ice cubes, on the other hand, float on top of liquid water! In short, H2O is far from being a random matter.

Ice is less dense than liquid

While almost all materials are denser in their solid state than in their liquid form, icebergs float on the ocean, and ice cubes float in a glass.

Usually, when a substance freezes, its molecules come together. But water has an anomaly: its maximum density is reached at 4°C and decreases below that. This means that, for the same volume, there are fewer molecules in an ice cube than in liquid: it is therefore lighter.

Three states: liquid, solid, and gas

Ice, liquid water, and gas are easy to observe in nature. However, you are unlikely to find gold vapor or dry ice (the solid form of carbon dioxide) while walking around outside!

One reason for this strange property is the particularly high melting temperature of H2O: oxygen alone evaporates at, say, -183°C, but the hydrogen bonds between water molecules require much more energy to change to a gas state.

Hot water freezes faster than cold

Place an ice cube tray filled with hot water in the freezer, and your ice cubes will be ready faster than with cold water. This curious phenomenon called the "Mpemba effect," could have several explanations. It could be due to evaporation which reduces the mass and consumes heat, which favors its cooling. Hot liquid would also be less sensitive to the effect of supercooling, which allows a liquid to remain solid even when its temperature has dropped below its solidification threshold.

Other hypotheses are based on heat transfers (convection currents would be more important in hot water) or the modification of chemical bonds between water molecules (shrinking of covalent bonds between oxygen and hydrogen atoms).

There are two forms of liquid water

Recently, several studies have suggested that liquid water actually coexists in two forms: one of low density, in which molecules form numerous hydrogen bonds with their neighbors, keeping them at a distance from each other, and another of high density, where the molecules "pile up" on top of each other and can therefore move closer together.

If it is relatively common to find several forms of solids for the same material, it is much rarer for a liquid. The only known case to date is helium, whose liquid form helium II can crawl along surfaces!

Excellent solvent

You only have to watch a lump of sugar melt in your coffee or powdered salt dissolve in your saucepan to realize that H2O is an excellent solvent. It is impossible to dissolve salt in oil or petroleum: it will remain solid at the bottom of the container.

This ability to dissolve is explained by the polar nature of water which tends to attract ionic molecules (with positively or negatively charged ions). For example, when salt (NaCl) is put in water, the H2O molecules dissociate the chloride ions and the sodium ions; an aqueous solution Na+ + Cl- is formed.

Similarly, water easily dissolves other polar molecules such as sugars or ethanol.

Water can be both liquid and solid

Researchers have recently demonstrated the existence of "superionic water," semi-liquid and solid water that becomes conductive like a metal.

Under very high pressure and temperature conditions, the hydrogen bonds are destroyed, leaving the hydrogen ions to flow freely in the crystal lattice of oxygen atoms, which remain organized as if they were "frozen." H2O can also be found in supercooled form, where it remains liquid down to -48°C before suddenly freezing on contact with another surface.

Water shrinks as it melts

The vast majority of substances take up more space in the liquid state, when the molecules are disordered, than in the solid state, where the molecules have an organized structure. Water is a particular case: it occupies about 10% more volume when it freezes.

This oddity called "dilatometric anomaly" is explained by the specific structure of the H2) molecule, where the hydrogen bonds are assembled in a hexagonal network, leaving a lot of space between the molecules.

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