H2Oh! 10 mysteries of water

Water has a host of unusual properties – many of which are essential for life. Here we round up ten of the most peculiar

Weird water: The Rhone river meets the silty Arve in Geneva, Switzerland

Henryk Sadura / Alamy Stock Photo

No liquid behaves quite as oddly as water. It exhibits a raft of unusual behaviours, many of which are essential for life as we know it. In our cover feature, we look at how a controversial new theory could finally explain water’s weird behaviour. Here we explain how the theory could explain 10 of water’s behaviours – and then take a quick look at its many other peculiarities.

Read more: H2Oh! Water is actually two liquids disguised as one

Water’s mysteries

Picturing water as a liquid that can form two types of structure, one tetrahedral and the other disordered, could explain many of its unusual properties. Here are 10 of them.

1. Water is most dense at 4 °C

EXPLANATION: Heating reduces the number of ordered, tetrahedral structures in favour of a more disordered arrangement in which molecules are more densely packed. However, the heat also agitates the molecules in the disordered regions, causing them to move further apart. Above 4 °C, this effect takes precedence, making the water less dense

2. Water has an exceptionally high specific heat capacity: it takes a lot of heat energy to raise water’s temperature by a given amount

EXPLANATION: Much of the extra heat energy is used to convert more molecules from the tetrahedral structures to the disordered structures, rather than into increasing the kinetic energy of the molecules, and hence the temperature.

3. Specific heat capacity is at a minimum at 35 °C but increases as the temperature falls or rises, whereas the heat capacity of most other liquids rises continuously with temperature.

EXPLANATION: Between 0 and 35 °C, increasing the temperature steadily removes regions of ordered, tetrahedral structure, reducing water’s ability to absorb heat. Above 35 °C, so few of the tetrahedral regions are left that water behaves like a regular liquid.

4. Water’s compressibility drops with increasing temperature until it reaches a minimum at 46 °C, whereas in most liquids, the compressibility rises continuously with temperature

EXPLANATION: As the temperature rises, the dense, disordered regions become more prevalent, and these are more difficult to compress. However, rising temperature also forces molecules within these regions further apart and hence makes them more compressible. This effect takes precedence beyond 46 °C.

5. Water is particularly difficult to compress

EXPLANATION: The strong attraction between water molecules keeps them more closely packed than the molecules of many other liquids.

This effect is particularly marked when the higher-density disordered structure dominates

6. The speed of sound in water increases with temperature up to 74 °C, after which it starts to fall again

EXPLANATION: This is the result of the interplay between water’s unusual density and compressibility profiles, which directly stem from the changing balance between the two types of structure.

7. Water molecules diffuse more easily, not less easily, at higher pressures

EXPLANATION: High pressure converts more molecules to the disordered structure, in which they are more mobile.

8. Unlike many liquids, water becomes less viscous, not more viscous, at higher pressures

EXPLANATION: Molecules are freer to move when in the disordered structures, which are favoured at higher pressures, than when they are in the ordered, tetrahedral structure.

9. Increasing the pressure increases the amount by which water expands on heating

EXPLANATION: Rising temperature causes disordered regions to expand more rapidly than ordered, tetrahedral ones, and high pressure favours fluctuations to the disordered regions.

10. Properties such as viscosity, boiling point and melting point are significantly different in “heavy” water – made from the heavier hydrogen isotopes deuterium and tritium – compared with their equivalents in normal water.

EXPLANATION: The heavier isotopes change the quantum mechanical properties of water molecules, altering the balance of the disordered and tetrahedral regions.

By David Robson and Michael Marshall