A grown-up sweeps the cold ash out of the fireplace while Rocket watches. "A whole log," he says. "Now a handful of ash."
"Where did the rest go?" he asks.
Raven points at the chimney. "What do you notice about the smoke? It goes up and out. Burning makes gases."
"So the log is an open cup," Rocket says. "The gas took the mass with it."
Nova dims her light near the hearth. "And the rusty nail on the porch?" she asks. "It got heavier over the summer. Where did that mass come from?"
Rocket thinks. "Iron plus oxygen makes rust. The oxygen came from the air, and it joined on."
Raven writes two lines in the log: open systems lose gas, and open systems can gain it too.
Burning is a chemical change. In the combustion example chemists use, a fuel plus oxygen becomes carbon dioxide and water.
A log in a fireplace is an open system. The gases leave through the chimney, so the ash left behind has much less mass than the log.
The mass did not vanish. Trap every gas and weigh it with the ash, and the total matches the log plus the oxygen it used.
Rust forms when iron combines with oxygen in the presence of water. The rust contains the iron atoms plus oxygen atoms from the air.
That is why a rusting nail can gain mass in an open system. Atoms joined it from outside.
| Everyday change | Open or closed? | What the scale shows | Where the matter went or came from |
|---|---|---|---|
| Log burning in a fireplace | Open | Ash has less mass than the log | Gases left through the chimney |
| Nail rusting outdoors | Open | Rust has more mass than the iron | Oxygen from the air joined the iron |
| Baking soda and vinegar in a sealed bag | Closed | Same mass | Nothing left or entered |
| Baking soda and vinegar in an open cup | Open | Less mass | Carbon dioxide left the cup |
| Week review | True or false? |
|---|---|
| Mass is a measure of the amount of matter in an object. | ? |
| In a chemical reaction, atoms are rearranged, not created or destroyed. | ? |
| A sealed bag reaction loses mass. | ? |
| An open cup reaction seems to lose mass because gas leaves. | ? |
| A rusting nail loses mass to the air. | ? |
Vinegar and lemon juice both fizzed with baking soda. What do vinegar and lemon juice have in common?
Next week you will sort kitchen liquids into acids and bases, and a red cabbage will do the sorting for you.
Strong week, Bench scientist. Tomorrow is Bench Day: a family seesaw balance that never lies.