Water is a polar molecule and also acts as a polar solvent. When a chemical species is said to be “polar,” this means that the positive and negative electrical charges are unevenly distributed. The positive charge comes from the atomic nucleus, while the electrons supply the negative charge. It’s the movement of electrons that determines polarity. Here’s how it works for water.
Polarity of a Water Molecule
Water (H2O) is polar because of the bent shape of the molecule. The shape means most of the negative charge from the oxygen on side of the molecule and the positive charge of the hydrogen atoms is on the other side of the molecule. This is an example of polar covalent chemical bonding. When solutes are added to water, they may be affected by the charge distribution.
The reason the shape of the molecule isn’t linear and nonpolar (e.g., like CO2) is because of the difference in electronegativity between hydrogen and oxygen. The electronegativity value of hydrogen is 2.1, while the electronegativity of oxygen is 3.5. The smaller the difference between electronegativity values, the more likely atoms will form a covalent bond. A large difference between electronegativity values is seen with ionic bonds. Hydrogen and oxygen are both acting as nonmetals under ordinary conditions, but oxygen is quite a bit more electronegative than hydrogen, so the two atoms form a covalent chemical bond, but it’s polar.
The highly electronegative oxygen atom attracts electrons or negative charge to it, making the region around the oxygen more negative than the areas around the two hydrogen atoms. The electrically positive portions of the molecule (the hydrogen atoms) are flexed away from the two filled orbitals of the oxygen. Basically, both hydrogen atoms are attracted to the same side of the oxygen atom, but they are as far apart from each other as they can be because the hydrogen atoms both carry a positive charge. The bent conformation is a balance between attraction and repulsion.
Remember that even though the covalent bond between each hydrogen and oxygen in water is polar, a water molecule is an electrically neutral molecule overall. Each water molecule has 10 protons and 10 electrons, for a net charge of 0.
Why Water Is a Polar Solvent
The shape of each water molecule influences the way it interacts with other water molecules and with other substances. Water acts as a polar solvent because it can be attracted to either the positive or negative electrical charge on a solute. The slight negative charge near the oxygen atom attracts nearby hydrogen atoms from water or positive-charged regions of other molecules. The slightly positive hydrogen side of each water molecule attracts other oxygen atoms and negatively-charged regions of other molecules. The hydrogen bond between the hydrogen of one water molecule and oxygen of another holds water together and gives it interesting properties, yet hydrogen bonds are not as strong as covalent bonds. While the water molecules are attracted to each other via hydrogen bonding, about 20% of them are free at any given time to interact with other chemical species. This interaction is called hydration or dissolving.