Count the atoms

About this video

Two ideas about chemical reactions seem to pull against each other. A reaction makes new substances that weren't there before. Yet in a sealed container the mass after a reaction is exactly the same as before. How can something new appear while nothing is added and nothing is lost? Atoms answer it: in a chemical reaction, the atoms change partners.

Watch the blue flame of a gas cooker. The gas is methane, CH₄: one carbon atom and four hydrogen atoms. When it burns, each methane molecule reacts with two oxygen molecules, O₂, from the air. The molecules come apart, and their atoms join up again in new groups: one carbon dioxide molecule, CO₂, and two water molecules, H₂O.

Now count the atoms, one element at a time. Carbon: one before, one after. Hydrogen: four before, four after, two in each water molecule. Oxygen: four before, two in each oxygen molecule, and four after, two in the carbon dioxide and one in each water molecule. No atom is made, no atom is destroyed, and no atom turns into a different kind of atom. Every atom has its own mass, so if all the same atoms are still there, the total mass must be the same too. That is the reason behind the conservation of mass.

It also explains a burnt log, whose ash weighs far less than the log: its carbon and hydrogen atoms joined up with oxygen from the air and floated away as carbon dioxide and water vapour. This video goes with the lesson Atoms Change Partners in Chemistry Level 1 on Knaph.

Transcript

1. 0:00A reaction makes something new. But nothing is added, and nothing is lost. How?
2. 0:06Methane, the gas in a cooker, meets two oxygen molecules from the air.
3. 0:14The atoms let go of their partners and join up again. One carbon dioxide, two water.
4. 0:21Count them. One carbon, four hydrogen, four oxygen. Before and after.
5. 0:31Every atom has its own mass. Same atoms, so the same mass.
6. 0:39No atom is made or lost. Atoms only change partners.