The law of conservation of mass

Does matter vanish?

A candle burns, and the wax gets smaller. A piece of paper burns, and only a little ash is left. It looks as if matter has gone. Is it true?

About 250 years ago, the French scientist Antoine Lavoisier asked the same question. He did something new: he weighed the substances before and after a reaction, in a closed vessel, so that nothing could escape.

The law

He found that the total mass did not change. This is the law of conservation of mass.

In a chemical reaction, mass can neither be created nor destroyed. The total mass of the reactants (the substances that react) is equal to the total mass of the products (the substances that are made).

Some examples

• $2\ \text{g}$ of hydrogen reacts with $16\ \text{g}$ of oxygen to make $18\ \text{g}$ of water. The mass before is $2\ \text{g} + 16\ \text{g} = 18\ \text{g}$, and the mass after is $18\ \text{g}$.
• $100\ \text{g}$ of calcium carbonate (limestone), when it is heated strongly, gives $56\ \text{g}$ of calcium oxide and $44\ \text{g}$ of carbon dioxide gas. The mass after is $56\ \text{g} + 44\ \text{g} = 100\ \text{g}$.

You should find that the sealed flask's reading stays put, while the open flask loses about half the mass of the baking soda as gas.

What about the burning candle?

The wax does not vanish. It burns with oxygen from the air, and it turns into carbon dioxide and water vapour, which are gases. They go into the air, so we do not see them. Suppose we could collect all the gases and weigh them, together with the unburnt wax. The mass would be the same as the mass of the wax and the oxygen that were used.

Atoms are only rearranged

The law has a simple reason. A chemical reaction does not make new atoms and does not destroy any atom. The atoms of the reactants are rearranged into new groups, and these new groups are the products. The number of atoms of each element is the same before and after.

Practise with new numbers

Try it in Python

This program checks the law for three reactions.

Remember these

• Law of conservation of mass: the total mass of the reactants equals the total mass of the products.
• To check it, weigh in a closed vessel, so that no gas escapes.
• Atoms are not made or destroyed in a reaction. They are rearranged.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In a sealed plastic bag, $6\ \text{g}$ of baking soda is mixed with $50\ \text{g}$ of vinegar. It fizzes, and the bag puffs up. What does the bag and everything in it weigh afterwards, if the bag weighs $4\ \text{g}$?

2. A piece of magnesium ribbon is burnt in air, and the white ash weighs more than the ribbon. Is the law of conservation of mass broken?

3. Why must the law be tested in a closed vessel?

4. Burning $1\ \text{kg}$ of petrol makes about $3\ \text{kg}$ of carbon dioxide. How can the product weigh more than the fuel?