A science teacher explains: Is water a good conductor of electricity?

The water molecule is composed of two atoms of hydrogen and one oxygen atom (H2O) bound by sharing of electrons and forming a covalent bond. The electrons are not uniformly shared by oxygen and hydrogen atoms, but are attracted to oxygen atoms more strongly than to hydrogen. This gives water a disproportionate distribution of charge — the hydrogen side of the molecule carrying a slight positive charge and oxygen side a slight negative charge, making it a polar molecule.

It is this polarity that enables the water molecules to strongly attract other polar and ionic molecules. The relatively-smaller water molecules surround the molecules of ionic or polar compounds, disrupt the attractive forces that hold the solute molecule and thus dissolve it. Most water we encounter in daily life contains some amount of dissolved substances, ions that can move freely.

Water mixed with electricity is considered to be a bad idea. It can be lethal to come in contact with this dangerous pair. If we investigate the complexities of chemistry, we find that pure water is actually not a good conductor of electricity. Water that we use in our day-to-day life contains a substantial quantity of dissolved substances, chemicals and minerals. In order for electricity to flow through a liquid, movement of charges must take place in the liquid. Whether tap water, rain water or sea water, these contain innumerable impurities such as sodium, calcium and magnesium ions, atoms that have a net charge due to loss or gain of electrons.

Since water has free charges due to the presence of these impurities, it becomes a conductor of electricity. When water is made a part of an electric circuit, the positive ions are attracted by the negative terminal and the negative ions are drawn in by the positive terminal of the battery creating a closed path, thus enabling the flow of current and making water a conductor of electricity.

Actually the presence of a small number of ions is all that is required to make water a good conductor. However, the more the number of ions, the more efficient it is as a conductor. That is why the likelihood of getting an electric shock in sea water is much slimmer than at home as current would choose the low resistance path of sea water with such high salt content and spare us. This also elucidates why it is taught in schools that one should not touch electric appliances with wet hands as water can easily slip into cracks and crevices, closing the path between us and the electric circuit giving a rude shock.

Pure water and distilled water have no impurities and, therefore, no ions, but only neutral molecules. So such water cannot conduct electricity. In fact, distilled water is one of the best insulators known to science. It is about 80 times less conductive than air. This means we can operate hair dryers and toasters inside water, with no fatal consequences, provided the water is pure or distilled.

Water is amphoteric in nature, it can act as both a base and an acid by accepting or releasing protons depending on the circumstances. Water also undergoes a process called auto ionization or self-ionization. If we have pure water with pH of 7, and hence neutral, it might so happen that two of the water molecules interact, while one may donate a proton, the other will accept it. This results in the formation of the negative hydroxide ion and the positive hydronium ion. Here, water is acting as an acid and a base. Thus, self-ionization may seem to be sabotaging the insulating properties of water, but very few water molecules dissociate into the hydroxide and hydronium ions, forming the same strength of positive and negative ions, thus cancelling out the net effect. Self-ionization does not result in making water conductive and still maintains the pH of 7.  It is the presence of impurities that is conclusively responsible for the conduction of electricity by water.

Electrical conductivity of water has many environmental and industrial applications as this can tell us how much dissolved substances, chemicals and minerals are present in water. In a water treatment plant, a significant increase in conductivity can indicate an increase in pollutants or a sewage leakage and thus corrective measures can be taken. Hydrologists also keep a close tab on electric conductivity of water as fish can survive up to a certain conductivity range.

Salt or no salt, water has been a vital resource throughout history. It touches every aspect of our lives from health to safety and serves as the foundation of our economy. We put water to work in a million different ways every day. It is our collective job to safeguard water, promote environmental stewardship and deliver sustainable solutions for this essential element.

(The writer is PGT- Physics at Shiv Nadar School, Noida.)

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