Atoms Rearrange, Mass Stays the Same

By the end of this lesson, you will be able to show that the mass of the materials you start with in a chemical reaction equals the mass of the materials you end up with, and explain why using atoms.

Think about a backyard barbecue in Texas. When charcoal burns, it looks like the mass disappears, because all you find later is a small pile of ash. But nothing was destroyed. The carbon atoms in the charcoal combined with oxygen atoms from the air and floated away as an invisible gas. If you could trap every bit of that gas and weigh it along with the ash, the total mass would match the mass of the charcoal and oxygen you started with. That idea is called the law of conservation of mass, which means matter is never created or destroyed during a chemical change.

The reason is simple: a chemical reaction does not make new atoms or erase old ones. It only rearranges them into new combinations. The same atoms that went in must come out, just connected in different ways.

Scientists track those atoms with a chemical equation, a shorthand that lists the reactants, or starting substances, on the left and the products, or new substances, on the right. A balanced equation has the same number of each kind of atom on both sides.

Photosynthesis is a great example. A live oak tree takes in six molecules of carbon dioxide and six molecules of water and, using sunlight, makes one molecule of glucose sugar and six molecules of oxygen gas. Count the atoms: six carbons go in and six come out, twenty-four hydrogens and eighteen oxygens on each side. Nothing is lost, only rebuilt.

Try it: If twelve grams of carbon react completely with thirty-two grams of oxygen inside a sealed container, what is the mass of the carbon dioxide produced?