Class 10 Science (CBSE) — Chapter-wise

Chemical Reactions and Equations

When iron rusts or wood burns, atoms aren't being destroyed—they are performing a high-speed dance of musical chairs. No new atoms are created, and none vanish. They simply change partners to form entirely new substances.

This brings us to the golden rule of chemistry: the Law of Conservation of Mass. If you start with four hydrogen atoms, you must end with four hydrogen atoms. The scale must stay perfectly balanced.

Let's look at how we write this. When hydrogen gas reacts with oxygen to make water, it looks simple. But count the oxygen atoms: we start with two on the left, but end up with only one on the right. An atom has vanished!

To fix this, you might be tempted to just change the little subscript numbers and write H-two-O-two. But beware! Changing subscripts changes the chemical identity entirely. You just turned harmless water into bleach!

Instead, we can only add big numbers in front, called coefficients. These tell us how many entire molecules we have. Let's put a two in front of water to double the oxygen.

Now we have two oxygen atoms on both sides. But wait! Now we have four hydrogen atoms on the right, and only two on the left. To balance them, we place a two in front of the reactant hydrogen.

Let's do a final count. Four hydrogens and two oxygens on the left. Four hydrogens and two oxygens on the right. The chemical equation is now perfectly balanced, respecting the laws of the universe.

Remember these three simple rules: reactants are on the left, products are on the right, and only change coefficients, never subscripts. Master these, and you can balance any reaction in the universe.