Water: A Weird Wonder

 

“If there is magic on planet earth, it is contained in water.” – Loren Eiseley                          

Water is a magical substance, quite literally. Oceans, rivers, lakes and waterfalls invoke a sense of wonder and beauty. Our planet is special because of the presence of water. Water has shaped our planet in unique ways, and life only exists because water exists.  We are so accustomed to the role of water in our everyday life, that we regularly make the mistake of presuming its behaviour is normal. We also feel all other liquids exhibit properties similar to water. Water, however, is unlike any other liquid. It behaves in such an anomalous way, that scientists have a difficult time explaining its chemistry even today. In fact, life only exists on earth because water behaves in a weird way. Why is the chemical nature of water so different? Let us have a look.

Universal solvent and the basis for life

A water molecule is composed of one oxygen and two hydrogen atoms. That’s why the chemical formula of water is H₂O. The oxygen atom in a water molecule carries a slightly negative charge, while the two hydrogen atoms carry a slightly positive charge. This makes water a polar substance. As a polar molecule, water interacts effectively with other polar molecules, including itself. Since opposite poles attract, the positive hydrogen atoms of one water molecule are attracted to a negative oxygen atom of an adjacent water molecule. This attraction makes it possible to have hydrogen bonds between water molecules. Due to hydrogen bonding, water molecules are highly cohesive and stick together. It is due to this feature that water can dissolve almost everything, and is known as the universal solvent. When life began on Earth, it began in water because all chemicals and nutrients necessary for life were present in water. Molecules like DNA and proteins, which are essential for life, can only perform their functions in water. Salts, sugars, vitamins, minerals and many other chemicals can easily dissolve in water. That’s why water is the medium for transporting nutrients and oxygen in and out of living cells. Water also maintains the shape and structure of cells. Water constitutes 50 – 80% of all living organisms, be it bacteria, plants, animals or humans. In fact, some organisms like the jellyfish are composed of 99% water. 

                      

 Ice floats on water – How strange!

It is common knowledge that ice floats on water. However, this is a very weird phenomenon! Usually, when a liquid is condensed into a solid form, the density increases. For any other substance, the solid form will almost always sink to the bottom when immersed in its liquid form. Solid butter sinks when immersed in liquid clarified butter; solid wax too sinks when immersed in liquid wax. However, water exhibits the opposite trend. Ice floats on water because ice is less dense than water. This is due to anomalous expansion of water.

When water is cooled, say, from room temperature, it contracts, just like any other liquid. However, when the temperature reaches 4°C, water starts expanding! Water expands when the temperature drops below 4°C. The maximum density of water is at 4°C when it is still in liquid phase. Ice at 0°C is less dense, and therefore it floats.

This a very important feature of water. It is because of this property of water that aquatic life still thrives in frigid winters, when surfaces of lakes and rivers freeze and turn into a sheet of ice. In these water bodies, as the temperature drops and reaches 4°C, the density of water becomes maximum. This dense layer of water at 4°C sinks to the bottom as it is heavier, and only the top portion of the water freezes. There is thus a temperature gradient with solid ice at the surface and liquid water at 4°C below. Further, as ice is a bad conductor of heat, it insulates the water underneath from getting colder. Aquatic life is thus protected as water continues to be in liquid state below the top surface of ice. Had water behaved like any other liquid, the whole lake or river would have frozen and aquatic life would have been destroyed completely.

 

Other unique features of water

Due to hydrogen bonding, water has a high surface tension compared to other liquids. What does surface tension mean? Surface tension is a feature of a liquid that allows it to resist an external force. Since there is hydrogen bonding between water molecules, there is a lot of cohesion between them. Imagine a glass of water. Water molecules within the glass cling to each other due to hydrogen bonding. At the surface however, the water is in contact with air. Water molecules on the surface thus have fewer neighbours to cling to. Therefore, they form stronger bonds with each other so that an external object does not disrupt them. If you carefully place a paper clip or a needle on the surface of water, it will float due to surface tension. The water strider is a stick-like insect that can walk on water due to this feature. One of the clinical tests for jaundice involves the principle of surface tension. In this test, powdered sulphur is sprinkled on urine. In normal urine, it floats. In a jaundiced person, the presence of bile reduces surface tension and the sulphur sinks. 

 

Water also has a high latent heat. Latent heat of water is the energy required to change its state from solid to liquid or from liquid to gas at a particular temperature. Since water has a high latent heat, a lot of energy is required to convert it from its liquid state to gaseous state. This is one of the reasons why water predominantly continues to be in liquid state on earth.

A unique feature associated with water is the lowering of freezing point when chemicals are added to it. Normally, water freezes at 0°C. However, if chemicals like salt, sugar or alcohol are added to water, water freezes at a temperature below 0°C. This lowering of the freezing point is called as freezing point depression. An application of this principle is the spraying of road salt in cold countries by de-icing trucks to prevent ice formation on roads.

Another very weird feature of water is that hot water freezes faster than cold water! This phenomenon is called as the Mpemba effect. Erasto Mpemba, a Tanzanian high school student discovered this property of water while making ice cream. Many different theories have been proposed to explain this, but they haven’t been verified yet.

In conclusion, water is unique and special. Unravelling the mystery of this unique liquid can lead to many important applications.  No other liquid can match water when it comes to supporting life. Responsible usage of water to ensure its conservation and preventing water pollution are the ways forward to sustain life.