Answer - the arrow. On the left side are the reactants - hydrogen and oxygen. We will ignore the two in front of the hydrogen for a moment. On the right side is the product - water. We will also ignore the two in front of the water, but we will soon return to it.
Coefficients are the numbers in front of the formulas.
O Note the presence of a two in front of the hydrogen and also the water. These are called the coefficients. These numbers give two very important pieces of information about the equation. You must understand both in order to read and to use chemical equations successfully. In the example reaction, two molecules of hydrogen react with one molecule of oxygen and produce two molecules of water.