Where did the log go?

About this video

A family has a campfire in the evening, and in the morning a pile of heavy logs has become a small heap of grey ash. Put one log on a kitchen scale and it might read 1,500 grams; its ash might read only 30. It looks as if most of the wood was destroyed. It wasn't. Burning is a chemical reaction: the wood reacts with oxygen from the air and turns into new products, mostly carbon dioxide and water vapour. Those are gases you can't see, and they rose up and spread out into the sky.

To check, you have to catch every bit of gas and weigh it too. In the 1770s the French chemist Antoine Lavoisier made reactions happen inside closed glass containers, so no gas could get in or out, and he weighed them very carefully before and after. Again and again the total mass did not change, even though substances turned into other substances. In his book of 1789 he wrote it as a rule: in a chemical reaction, nothing is made from nothing, and nothing is lost. Today we call it the conservation of mass.

You can see it at home with vinegar in a bottle and baking soda in a balloon on its neck. The gas the reaction makes is trapped in the balloon, so the scale reads almost the same. Let the gas out and the reading goes down. A rusting nail seems to break the rule the other way: it gets heavier, because rust is made from iron, oxygen and water. Whenever mass seems to appear or disappear, a gas has slipped in or out without being weighed. This video goes with the lesson Nothing Disappears in Chemistry Level 1 on Knaph.

Transcript

1. 0:00A log weighs fifteen hundred grams. After the fire, its ash weighs only thirty. Where did the rest go?
2. 0:08Most of the wood turned into carbon dioxide and water vapour. Gases you can't see, rising into the sky.
3. 0:17In the 1770s, Lavoisier sealed reactions inside glass and weighed them. The mass never changed.
4. 0:26Try a balloon on a bottle. The gas is trapped, so the reading stays. Let the gas out, and it drops.
5. 0:37A rusting nail gets heavier. The extra mass came from the air and water.
6. 0:42Nothing is lost. A gas slipped in or out.