A liquid is said ‘normal’ if it decreases its volume and increases its density upon freezing. Similarly, if a liquid increases volume and decreases its density upon melting or being heated, it’s a normal liquid. But a liquid is said ‘abnormal’ or its behavior is called ‘anomalous’ if it does the exact opposite of what a normal liquid does. The ‘anomalous behavior of water is what makes water, a unique liquid with properties beyond the expectations of what we see in other normal liquids.
These anomalies do not just limit to the expansion upon freezing phenomenon but, extend to other properties as well. Some of such properties are written below.
The general aspect of Anomalies of Water
- Hydrides of the sixth group elements are gases at room temperatures, like dihydrogen sulfide (H2S), but water (H2O) is a liquid.
- Unlike its other family members, water remains liquid over a very wide range of temperatures (100 °C).
- It takes an unexpectedly large amount of heat to vaporize water at its boiling point due to its high latent heat of vaporization.
- The existence of ‘cubic ice’ as we move further away from earth.
- And last but not the least, the obvious one; it expands on freezing, in between a temperature range of 4 °C to 0 °C. It is sometimes referred to as, ‘the density anomaly’.
The existence of such a large number of anomalies makes water, the most puzzling substance known to mankind. The understanding of such anomalies has not only baffled scientists for generations but also plays a key role in the development of a unified understanding of this liquid.
Many plausible explanations have been built up over the years for these anomalous behaviors. In all these explanations, the main role has been attributed to the hydrogen bonding present in water. As the increase in temperature weakens these hydrogen bonds, water is most anomalous at low temperatures.
Detailed Anomalies of water
The detailed anomalies of water can be summarized as:
- Density anomaly
- Isobaric specific heat (Cp)
- Isothermal compressibility (KT)
- Coefficient of thermal expansion (αp)
Density anomaly is the oldest and the easiest one to understand. The temperature of maximum density (TMD) of water is 277 °K or 4 °C at about 1 bar pressure. As the pressure is increased, the TMD point shifts to a lower temperature. It means that, at higher pressures, this anomaly of water is less seen.
When water goes below 20 °C, 2 and 3 coordinated water molecules, which are already present, tend to change into 4 and 5 coordinated water molecules. At 10 °C, 70% of the molecules are 4 coordinated, while the rest 14% constitute 5 and 3 coordinated structures. However, as we approach 4 °C, all water molecules start to favor 4 coordinated states due to energetic reasons which makes the density of water, to rise to its maximum limit. By further decreasing temperature, water molecules adopt 5 and 6 coordination geometries which makes their densities lower than the 4 coordinated ones.
This makes the density of ice (≤ 0 °C) less dense than the density of water at 4 °C. As a result, ice rises above the water bodies. Now, as we know that decreased density or increased size makes spaces between water molecules, making ice an insulator. This is how life underneath ice exists.
Isobaric specific heat (Cp)
A typical liquid decreases its heat capacity with decreasing temperature, as the Debye theory predicts. However, the case of water is not the same. Its heat capacity decreases according to the Debye theory up to a certain temperature i.e. ∼35 °C, but below this temperature, the specific heat capacity of water increases instead.
Isothermal compressibility (KT)
The isothermal compressibility of a liquid gives a general perspective of that liquid’s change in volume according to a change in pressure applied at a constant temperature.
Similar to heat capacity, the isothermal compressibility of a typical liquid also decreases with a decrease in temperature. Water does not follow this regular trend and increases its compressibility near its freezing point.
Coefficient of thermal expansion (αp)
The coefficient of thermal expansion provides us with a measure of volume change in a system according to changes in temperature at constant pressure.
For simple liquids, the thermal expansion increases with an increase in temperature which is also the case in water till TMD (temperature of maximum density) i.e. 4 °C. Below this temperature, it decreases rapidly, such that it even goes below zero. This suggests that, on a decrease in temperature below 4 °C, the entropy of water increases.
Significance of Anamolous Behavior of Water
As the famous saying goes;
“If you are the only sane one, you are the insane one”
The behavior of water can be called abnormal, anomalous, or whatever we like, but the reality is that it is the only liquid that has extensive capacity to dissolve almost anything (Universal solvent). It is one of the reasons behind life, by providing a transport media for transport of everything in any living organism at the cellular or tissue level. Its polarity, cohesion, adhesion, surface tension, high specific heat, evaporative cooling, abnormal expansion and compressibility, and many other properties make it a unique liquid.
Due to the anomalous behavior of water, many otherwise impossible processes exist. Like;
- Existence of aquatic life, even under ice.
- Weathering of rocks, to create soil and the starting of the primary succession of life.
- Transpiration in plants, which is dependent on cohesive forces, i.e. hydrogen bonding.
- The existence of water in the liquid state, due to its high heat capacity.
- Floating of ice on water, (solid over liquid), which is not seen otherwise.
What is meant by anomalous behavior of water?
By anomalous behavior of water, the expansion on cooling, irregular expansion, and compressibility are meant.
What causes the anomalous behavior of water?
The anomalous behavior of water is mainly caused by the hydrogen bonding present among water molecules.
What are the anomalous properties of water?
The anomalous properties of water include;
- Expansion upon cooling
- Irregular specific heat
- Irregular compressibility
- Irregular thermal expansion
What is an anomalous behavior?
An anomalous behavior means a behavior with uniqueness. It does not correlate with its family members or references.
What are the effects of the anomalous expansion of water?
The effects of the anomalous expansion of water are:
- Floating of ice over liquid water.
- Low density in solid-state, than in the liquid state.
What are the 4 basic properties of water?
The four basic properties of water are:
- High specific heat
- Intermolecular forces (Adhesion/Cohesion)
- Low-density solid-state and high-density liquid state
Do any other chemicals exhibit anomalous behavior like water?
Besides water, bismuth, silicon, gallium, antimony, and germanium also show anomalous behavior. However, the extent of anomalies in water is not present in any other element or compound.
What are the applications and uses of anomalous expansion of water?
One of the primary applications of the expansion of water is the density of ice being less than liquid water. Due to this, ice floats over water.
Why does water start solidifying from the top?
When water molecules start losing heat to the nearby environment, the molecules also go downward like other materials. But, upon further cooling (below 4 °C), its density starts to decrease and the molecules come to the top.
At what temperature does water have a maximum density?
The temperature of maximum density (TMD) of water is 4 °C (3.98 °C to be precise). On the Fahrenheit scale, it becomes 39 °F.
Why does water expand on solidification?
Upon cooling, the water molecules, due to an extensive network of hydrogen bonding start to arrange themselves in an orderly manner. This leads to solidification and also leaves spaces between molecules, decreasing overall density.
- Water in Biological and Chemical Processes: From Structure and Dynamics to Function By Biman Bagchi (Indian Institute of Science, Bangalore)
- Water: A Very Short Introduction By John Finney (Central Connecticut State University, US)