Rust that weighs the same

In the picture

1. 1Sealed flask with shiny steel wool: iron and the oxygen in the air before the reaction
2. 2Sealed flask with rusted wool: iron oxide after the reaction, with the same atoms inside
3. 3Identical brass weights: the same mass sits on both balances
4. 4Level beam: the total mass has not changed

Examples

• A candle burning under a sealed jar: the wax seems to disappear, but the jar and its gases weigh the same as before
• Fizzy tablet dropped in water in a closed bottle: the gas stays trapped and the bottle’s mass stays the same

Quiz: Rust that weighs the same (Age 13)

1. 1

   Steel wool is sealed in a flask with air and slowly turns to rust. How does the total mass of the sealed flask after the rusting compare with before?

   • It is heavier, because rust has been added
   • It is lighter, because some of the iron has been eaten away
   • It is exactly the same
2. 2

   Why does the total mass stay the same during a chemical transformation?

   • Atoms are rearranged into new substances, and none are created or destroyed
   • Oxygen atoms change into iron atoms, so the numbers balance out
   • Some atoms disappear, but new atoms form to replace them
3. 3

   When steel wool rusts inside a sealed flask, where does the oxygen in the rust come from?

   • It is created during the reaction
   • It was already in the air inside the flask
   • It comes in from the room outside the flask

The answers, each with its explanation, are in the app. This quiz is free there while the lesson is in the free week.