Nothing is lost
Show on a big screenA fizzing flask can seem to lose weight. Does it?
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The rule
Not in a closed flask. Escaping gas lowers an open flask's reading.
Mass is how much matter something has. A closed system lets nothing in or out.
In a closed system, mass before a change equals mass after. A chemical change only rearranges matter. Matter is neither created nor destroyed in a chemical change.
This is the law of conservation of mass.
- A sealed flask sits on a balance. It reads 250 g.
- Inside, a gas forms. The cork keeps it in.
- The balance still reads 250 g. Nothing got out.
Closed, or open
| The change | Closed? | Reading before | Reading after |
|---|---|---|---|
| Ice melting to water | Yes | 250 g | 250 g |
| Lemon juice and baking soda, fizzing | Yes | 387 g | 387 g |
| A candle burning in open air | No | 40 g | 38 g |
In open air, hot gas and smoke float away. So the reading drops by 2 g. Nobody weighs the gas and smoke. Burn the same candle in a sealed jar. Then the reading stays the same.
Check the law
Two substances react in a closed flask. They weigh 6 g and 4 g. Three products form. They weigh 4.4 g, 1.8 g and 3.8 g.
Add the masses before. Add the masses after. Compare.
Before: 6 g + 4 g = 10 g.
After: 4.4 g + 1.8 g = 6.2 g. Then 6.2 g + 3.8 g = 10.0 g.
The two totals match. The law holds.
Try the lab to finish this lesson.