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Polarity of Water

Water is a special molecule that has a unique set of properties which set it apart from all others. Life on Earth is made possible because of its unique nature. One interesting property is the polarity of water molecule. In this article, I talk about the reasons behind the molecular polarity of water.

What is Polarity?
What do we mean by ‘polarity’ of water molecule? Polarity is charge inequality on two ends of any molecule. Due to the unequal distribution of electrons in water molecule, one part of the molecule is more negatively charged than the other part. This is caused by the property of ‘electronegativity’, which can be defined as ability of an element to attract electrons. If the atoms that make up a molecule have different electronegativities, that differ substantially, they attract electrons unequally, causing polarity.

Water: Molecular Structure
To understand what is polarity of water, one needs to understand the molecular structure of water and the arrangement of electrons in it. The polarity of water molecule arises due to its peculiar molecular structure. The molecule is made up of one oxygen atom covalently bonded with two hydrogen atoms. A covalent bond is formed when two atoms share electron pairs with each other to achieve stability.

Oxygen has six electrons in its outermost orbit. Out of these six electrons, one electron each is shared with two hydrogen atoms, which have one electron in the outermost shell. Through covalent bonding, oxygen shares two pairs of electrons with hydrogen atoms to form the water molecule. Besides these two shared pairs of electrons, water has two pairs of unshared electrons that repulse the other two pairs, leading to distortion in its shape.

Why is Water Molecule Polar?
The polarity of water molecule arises due to its greater electronegativity or electron loving nature, than hydrogen. Even though the pairs of electrons are shared with the hydrogen atoms, they are not shared equally as oxygen pulls the electrons more towards itself. This gives rise to a greater negative charge on the side of oxygen atom and a positive charge on the hydrogen atoms. That means, water molecule becomes polar because of the greater ability of oxygen to attract electrons toward itself. This polar nature of water molecule gives rise to an electric dipole moment.

Importance of Polarity of Water
What are the consequences of molecular polarity of water on its nature and properties? This polarity has a large impact on the properties of water. Here are some of the properties of water that can be attributed to its polar nature.

Hydrogen Bonding
Due to the polar nature of water, a special kind of bonding occurs between every molecule of water and its surrounding molecules. The negatively charged oxygen atom attracts surrounding hydrogen atoms of other water molecules, which are positively charged, creating a weak bond called the ‘Hydrogen Bond’. This hydrogen bonding is responsible for most of the peculiar properties of water.

High Specific Heat & Heat of Vaporization
Specific heat is the amount of heat absorbed by one gram of a substance to raise its temperature by 1 degree Celsius. Heat of vaporization is the heat absorbed by a substance to transform from liquid to gaseous state. Water has a high specific heat and higher heat of vaporization than most substances due to hydrogen bonding. These two properties endow the water molecules with an ability to buffer large temperature fluctuations and regulate the climate on Earth.

Cohesion & Adhesion
The hydrogen bonding between molecules makes water more ‘sticky’ leading to a higher degree of cohesion and adhesion. Adhesion and cohesion are important properties of water which play a big role in its biological functions.

Higher Surface Tension
A higher surface tension of water endows it with the property of capillary action, which helps it rise against gravity through tubes and veins of plants to reach greater heights.

Ice Floats on Water
Due to the hydrogen bonding, which is a consequence of the polarity of water, ice is less dense than water causing it to float. This has important consequences for marine life as it gets protected in winter by the insulating sheet of ice that floats above water, while keeping underlying layers warm.

Water As Solvent of Life
The polarity of water molecule makes it a universal solvent. It can dissolve salts, acids, sugars, as well as alkalis and gases. Most cell components including proteins, polysaccharides and DNA dissolve in water making it the basis of life.

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