Mass is conserved

Think of making khichdi. Mummy puts 100 g of rice and 20 g of dal in the pot. How much khichdi should there be? 120 g. If it came out as 110 g, you would ask where the other 10 g went. Chemistry asks the same question about reactions.

When elements join into a compound, the compound behaves very differently: hydrogen burns and oxygen helps things burn, yet water puts out fires. But the mass of the water still equals the masses of the hydrogen and oxygen that made it.

In a physical change, mass stays the same. Dissolve salt in water on a balance: the solution weighs exactly as much as the water plus the salt.

In a chemical change too, if nothing escapes. Mix vinegar and baking soda: they fizz, making carbon dioxide gas and other substances.

• In an open flask, the balance reading drops, because the gas escapes into the air.
• Put a balloon over the flask's mouth: the gas is trapped, and the reading stays the same.

So the mass was never lost; the gas just went somewhere you couldn't see.

Another test: mix sodium sulfate and barium chloride solutions. A white solid (barium sulfate) forms, along with sodium chloride. No gas is made, and the mass stays the same.

This is the law of conservation of mass, stated by Antoine Lavoisier in 1789 (he is called the Father of Modern Chemistry): matter can be neither created nor destroyed in a chemical reaction. Total mass before = total mass after.

One practical note: the last digit on a digital balance can jump by ±1. Differences that small are experimental error, not a broken law.