From the record
Real cases, written from the published record. Predict first; the record and its sources come after.
1774 · Paris
Lavoisier's sealed jar
Heat tin in air and it turns into a dull powder, a calx, that weighs more than the tin did. Antoine Lavoisier sealed tin inside a glass vessel and weighed the whole thing.
He heated it until the tin dulled, then weighed it again, still sealed: no change. Then he broke the seal.
When the seal broke, what did the balance do?
What happened. Air rushed in and the weight rose by about as much as the tin had gained. The calx had taken something out of the air inside the jar. Nothing was created or destroyed; it had moved from the air into the metal. (The engravings of his apparatus in his 1789 textbook were drawn by Marie-Anne Paulze Lavoisier.)
Matter is neither created nor destroyed, but you have to seal the jar to see it.
1884 · Paris
Brown gas in ice
A sealed tube holds a brown gas, nitrogen dioxide, in balance with a colourless gas, dinitrogen tetroxide: 2 NO₂ ⇌ N₂O₄. Joining two NO₂ into one N₂O₄ gives off heat.
In 1884 Henry Le Chatelier described how a balanced reaction answers a change.
Put the tube in ice. What happens to the colour?
What happened. It pales. Cooling favours the reaction that gives off heat, so more brown NO₂ joins into colourless N₂O₄. In hot water the tube turns darker brown as the N₂O₄ breaks apart.
1664 · London
The cabbage rainbow
Robert Boyle noticed that syrup of violets changed colour with acids and alkalis. Red cabbage water does the same thing, and you can try it at a kitchen table.
Five glasses of purple cabbage water: one with vinegar, one with lemon juice, one with baking soda, one with soap, one plain.
Which glass turns pink-red?
What happened. Vinegar and lemon juice turn it pink-red (acid), baking soda and most soaps turn it blue to green (base), and plain water stays purple. The colour comes from anthocyanin pigments that change shape with pH.