What is Polarity?

In simple chemical terms, polarity refers to the separation of charges in a chemical species leading into formation of two polar ends which are positively charged end and negatively charged end. Polarity in any molecule occurs due to the differences in the electronegativities of the bonded atoms. Water, as we all know has two hydrogen atoms bonded to an oxygen atom. As oxygen is more electronegative than hydrogen thus, there exists polarity in the bonds which is why water is known as a polar solvent. 

Why is Water called the Universal Solvent? 

Water is an essential lifeline required by all living organisms. Water is called as the universal solvent popularly, because of its ability to dissolve many substances and hence acting as a solvent most times. This ability to dissolve numerous substances is attributed to the high polarity that water possesses. Many everyday activities like cooking, action of soaps and detergents, melting of ice etc. are various results of the polarity of water.  

Why is Water Polar?

Water is polar mainly due to two main concepts that are electronegativity difference and Molecular Shape.  

Electronegativity  

Electronegativity is the ability of a substance to attract a pair of electrons towards itself. It increases along a period and decreases along a group in the periodic table. The electronegativity of hydrogen is $2.1$ and oxygen has an electronegativity of$3.5$. As oxygen is clearly electronegative than hydrogen, it pulls the electrons of hydrogen atoms towards itself, causing a separation of charges in both the $O-H$bonds, hence there exists polarity in the molecule. Electronegativity difference between oxygen and hydrogen are not as high as in the case of ionic bonds, so the bonds in water are covalent, but they are polar too, hence they are called as polar covalent bond.

Molecular Shape        

Water is not a linear structure and has a bent geometry with $s{p}^3$hybridization. Here, oxygen has two filled orbital bonding with hydrogen atoms and two lone pairs. As lone pair-lone pair repulsions are higher by nature, the lone pairs get occupied in maximum distance possible. But as there is an attraction between oxygen and hydrogen due to electronegativity difference, both the $O-H$ bonds align in such a way as to balance the repulsion and attraction resulting in a Bent geometry or V-shape. This is an important reason why water is polar, because if water had a linear structure the charges would cancel out each other on either side of oxygen. This might have resulted in a non-polar molecule like in the case of carbon dioxide, which has a linear geometry.

Dipole Moment

Dipole moment is a vector quantity that serves as a measure of polarity in bonds. Mathematically Dipole moment is the product of charges and distance of separation between the charges. It is denoted by ‘$\mu$’.

Where, $q$is magnitude of charges, measured in Coulombs (C) and $d$is distance of separation between the positive and negative charges in the system, measured in meters (m). Dipole moment is measured in Debye units, denoted by ‘D’.

$$1D=3.33564\times {10}^{-30} C.m$$

As mentioned earlier, dipole moment is a vector quantity with both magnitude and direction, which is why water being bent in shape, makes it a polar molecule as the vectors do not cancel out each other being in V-shape. But in case of Carbon dioxide ($C{O}_2$) the geometry is linear making the vectors align in equal and opposite manner cancelling out each other. This is why though there is good electronegativity difference between oxygen and carbon,$C{O}_2$ it is not polar like in the case of water.

The net dipole moment of water is known to be 1.85 Debye units. This is slightly high value for a covalent molecule, hence the bonds in water are known as polar covalent bonds according to Fajan’s rule.  

Dipole moment serves as a basic property to distinguish bond nature among atoms in a molecule. Very high dipole moments are seen in Ionic bonds, moderate values are obtained for polar covalent bonds and least values are obtained for non-polar bonds and covalent bonds like in organic molecules. On the other hand, metallic bonds and bonds in homonuclear diatomic molecules possess no dipole moment permanently. But temporary weak forces may come into the picture if they are in contact with other molecules.  

How does Polarity Affect Solubility in Water? 

When it comes to solubility the one ground rule to follow always is “Like dissolves like”. This means that polar solutes can only get dissolved in polar solvents and non-polar solutes can only get dissolved in non-polar solvents. This rule applies to the universal solvent water as well, which is a polar solvent. Being a polar solvent, water can dissolve only polar substances and non-polar substances are insoluble in water.

A very popular example of this is the dissolution of common salt in water. Salt is an ionic compound with $N{a}^{+}C{l}^{-}$ ions that possesses polarity. Therefore it gets dissolved easily in water. But oil which is an organic compound is insoluble in water as it is a non-polar substance with a long fatty acid chain. Fatty acids do not have polarity in their bonds.

Properties Affected by Polarity    

Surface tension and cohesion in water      

As we know, water has hydrogen bonding in it. In addition to that, the polarity of water also plays a role in making water molecule stick together, called as the cohesive property of water. Surface tension is the property by which water pulls the substance on its surface to the rest of the liquid down, it is resultant effect of water’s polarity. Due to separation of charges in water molecule, there are electrostatic forces acting on molecules on surface of water which immediately pulls it to the inside or bulk. Both negative and positively charged substances can be pulled due to the presence of dipolar nature of water. This is why water is known to have high surface tension and known to be reducing surface area. High surface tension in water makes water form droplets than to spread out as a layer like in oil. 

Liquid state of water 

We know there are three phases in which molecules can exist-Solid, Liquid, and Gas. But water at room temperature is known to exist in a liquid state, this is because of the strong intermolecular forces in water such as hydrogen bonding. The hydrogen bond formation is due to the polarity in water. Electrically negative oxygen tends to attract the electrically positive hydrogen of another water molecule, resulting in a hydrogen bond. Four such hydrogen bonds can be formed per water molecule, strengthening its intermolecular forces of attraction. When there is more such attraction among the molecules, the molecule tends to stick together and exist in a liquid state.  

Other physical properties 

As mentioned above polarity of water is the reason for hydrogen bonding in them. Hydrogen bonding in turn results in higher specific heat, higher vapor pressure, higher adhesion, higher melting and boiling point, and higher density in the water. The density of water is of unique interest as water when it gets frozen into ice, has a lesser density than liquid water. This is due to the fact that when water freezes, it expands leading to higher volume with the same number of molecules in it, resulting in reduced density. This is the very reason why ice floats on water and aquatic life exist in cold.  

Common Mistakes    

• Though we say water is polar and has charge separation in it, the water molecule as a whole is a neutral molecule, it is not charged. Separation of charges in bonds does not necessarily mean that the molecules must exist as ions, these charges that are separated are not fully positive or fully negative in nature, which is why they cannot exist as stable ions. Moreover, being in a covalently bonded system they cannot exist as ions and hence water is always a neutral molecule.  

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry, Biochemistry, Chemical engineering. 

Practice problem 

Q1. Given that dipole moment of water is $1.85$D. Find out the dipole moment of water in Cm?

Q2.  Mention the factors that affect polarity of water?  

Q3.  Explain why water has high boiling point? 

Solution 

Ans1. We know that $1D=3.33564\times {10}^{-30} C.m$.

So if polarity of water is $1.85$ D, then $1.85\times 3.33564\times {10}^{-30}$ will be its value in Cm.

The value obtained is $\text{6}\text{.1709 }\times {10}^{-30} C.m$.

Ans2.  Polarity of water depends on the electronegativity difference between the atoms and the molecular shape of water.  

Ans3.  Water is polar in nature. Polar compound tend to arrange them in a fashion with separation of positive and negative charge. Because of their charge separation, weak forces such as dispersion forces come into picture and strengthen the intermolecular forces of attraction compared to that of a non-polar compound. Water in addition to this has hydrogen bonding as a result of its polarity, thereby possessing really strong intermolecular forces. This makes the molecule to be held together and exist in the liquid state. Hence, in order to break these intermolecular forces and become gaseous, high energy is required. Higher temperatures only can provide such high energy in the form of heat to break these intermolecular forces and result in vaporization of water.