In the last column (which appeared in the April 2011 issue of Teacher Plus), we estimated the size of a molecule and realized that even very small quantities of substances are made up of huge numbers of particles. The particles interact with each other and take part in reactions. But we live in a macroscopic universe, we can see and measure only bulk properties, not count the particles.
C + O2 —> CO2
is an example of a chemical equation and in a very simplistic way we can say one atom of carbon reacts with one molecule of oxygen to give one molecule of carbon dioxide. (This description is simplistic because carbon does not exist as free atoms, maybe the reaction takes place in many steps, bonds have to be broken, formed – and so on). But atoms and molecules are so small that even the smallest speck of carbon, almost invisible, contains millions and millions of atoms. Elements exist and react as atoms and molecules, but we can only measure mass and volume, we cannot count the particles. There has to be a bridge between the atomic and macroscopic worlds, and this has been made possible by defining a quantity called the mole. Consider a fruit salad made of apples, oranges, bananas, grapes, and sunflower seeds. Buy a kilogram of each and the proportions are not right – too few apples and too many sunflower seeds. Buy a dozen each and we will have too many apples and too few sunflower seeds. This example highlights the problem of getting the right numbers of particles for a reaction when you go only by weight. So we have to be able to compare masses of different atoms.
The author works with Centre for Learning, Bangalore. She can be reached at yasmin.cfl @gmail.com.