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. We list water's peculiarities below.
In The strangest liquid, 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: Martin Chaplin of London South Bank University has posted a much more detailed and technical discussion of these anomalies.
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.
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
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.
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.
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.
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
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.
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.
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.
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.
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.
Water has an unusually high melting/freezing point.
Water has an unusually high boiling point.
Water has an unusually high critical point. This is the temperature at which the distinct liquid and gas states cease to exist. Instead, there is only a supercritical fluid, which can diffuse through solids just like a gas but also dissolve things just like a liquid. Water's critical point is at a temperature of 374 °C and a pressure of 217 atmospheres: above this temperature, it is a supercritical fluid.
Solid water exists in a wider variety of stable (and metastable) crystal and amorphous structures than other materials.
The thermal conductivity of ice falls with increasing pressure.
The structure of liquid water changes at high pressure.
Supercooled water – that is, water that has been cooled below its freezing point without it becoming a solid – behaves strangely. It has two phases and a second critical point at about -91°C.
Liquid water is easy to supercool, but difficult to turn into a glass-like solid.
Liquid water exists at very low temperatures and freezes on heating.
Liquid water may be easily superheated: that is, heated to a temperature above its boiling point without it boiling.
Hot water may freeze faster than cold water – the Mpemba effect.
Warm water vibrates longer than cold water.
The density of ice increases on heating (up to a temperature of -203 °C). Normally, solids expand and become less dense when heated.
Water shrinks on melting, when most substances expand.
Pressure reduces ice's melting point, when it normally increases it: pressure normally encourages a substance to become a solid.
Liquid water has a high density that increases on heating (up to 3.984 °C). Heating a liquid normally causes it to expand, reducing its density.
The surface of water is denser than the bulk. This may be because the density of the surface water does not vary with temperature as the density of the bulk does.
Pressure reduces the temperature of maximum density.
There is a minimum in the density of supercooled water.
Water has a low thermal expansivity: for a given increase in temperature, it does not expand as much as it might be expected to.
Water's thermal expansivity decreases at low temperatures. Below 4 °C, it becomes negative – so if you heat water that is below this temperature, it will shrink.
The number of nearest neighbours that each water molecule has increases on melting. Normally, because the molecules of a liquid are moving around so much more, any one molecule is likely to have fewer nearest neighbours than if it were part of a solid.
The number of nearest neighbours increases with temperature. This happens because the increasing temperatures break down the hydrogen bond network holding the molecules in place, allowing them to move closer to each other.
There is a maximum in the compressibility-temperature relationship, probably near the temperature at which the density is lowest.
The speed of sound may show a minimum.
High-frequency sounds travel as "fast sound", because for these frequencies water behaves as if it is a glassy solid rather than a liquid. Water also shows a discontinuity at higher pressure, probably as a result of the water molecules rearranging themselves.
Nuclear magnetic resonance (NMR) spin-lattice relaxation time is very small at low temperatures. In other words, if the nuclei of the atoms making up water are excited to a higher energy level – for instance by a magnetic field – they return to their previous, lower energy level unusually fast.
The NMR shift increases to a maximum at low (supercool) temperatures.
The refractive index of water – that is, how much light is slowed down, and thus deflected, when it enters water – has a maximum value at just below 0 °C.
The change in volume as liquid water changes to gas is unusually large.
No aqueous solution is ideal. In other words, there is no substance that can be dissolved in water without any heat being absorbed or released. This is because dissolving a substance in water always involves disrupting the clustering of the water molecules.
The mean kinetic energy of water's hydrogen atoms increases at low temperature.
When different substances are dissolved in water, they have varying effects on properties such as density and viscosity.
The solubilities of non-polar gases in water decrease with temperature to a minimum and then rise.
The dielectric constant of water is high.
The dielectric constant shows a temperature maximum.
Proton and hydroxide ion mobilities are anomalously fast in an electric field. This may be because the protons can use quantum tunnelling to travel rapidly between neighbouring water molecules.
The electrical conductivity of water rises to a maximum at about 230 °C.
For weak acids in water, the acidity constants (a measure of the strength of the acid when dissolved) show temperature minima.
X-ray diffraction shows an unusually detailed structure.
Under high pressure, water molecules move further away from each other with increasing pressure.
Water's heat of fusion – the amount of heat energy that 1 mole of it must absorb to melt – is at a maximum at -17 °C.
Liquid water has over twice the specific heat capacity of ice or steam. In other words, it takes almost twice as much energy to increase its temperature by the same amount.
The specific heat capacity also has a maximum, at about -45 °C.
The specific heat capacity has a minimum with respect to pressure.
The heat capacity also has a maximum.
Water's heat of vaporization – the energy required to transform it from a liquid into a gas – is unusually high.
Its heat of sublimation, the energy needed to change it from a solid directly into a gas – without becoming a liquid in between – is also unusually high.
Water's entropy of vaporization – the increase in disorder caused by changing it from a liquid to a gas – is high.
The thermal conductivity of water is high and rises to a maximum at about 130 °C. In other words, energy is transferred unusually fast from regions of hot water to regions of cooler water, and this rate of transfer reaches a maximum at around 130 °C.
Water is surprisingly viscous: although it is "thin", it is surprisingly resistant to force.
Water's viscosity decreases with pressure at temperatures below 33 °C.
At low temperatures, the self-diffusion of water increases as the density and pressure increase.
The thermal diffusivity, water's ability to adjust its temperature to that of the surroundings, rises with pressure until it reaches a maximum at a pressure of about 7900 atmospheres.
Water has unusually high surface tension.
Some salts exhibit the Jones-Ray effect when dissolved in water: when the salt is at a very low concentration, the surface tension of the water reaches a minimum.
Some salts stop small bubbles from coalescing.