Mass stays the same
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Why
When wood burns, only a little ash is left. Has the rest vanished? Or can you just no longer see it? With a balance you can find out.
Rule
- Conservation of mass
In a chemical reaction the total mass of the substances afterwards is just as large as before.
This holds only if you weigh all the substances involved. If the reaction takes place in a closed vessel, no gas can get in or out, and the balance shows the same before and after. In an open vessel the mass can increase (a gas from the air is taken up) or decrease (a gas escapes).
The explanation lies in the particle model: in a reaction the atoms rearrange. No atoms are added and none are lost. Every atom keeps its mass, so the total mass stays the same too.
Example
The balance experiment
- You put a bottle with water and an effervescent tablet on the balance, closed with a balloon. The balance shows a number.
- You let the tablet fall into the water. It dissolves with fizzing; a gas forms and inflates the balloon. That is a reaction.
- The balance shows the same number as before. The gas stayed in the balloon and is weighed too.
- If you open the bottle, the gas escapes and the balance shows less. The mass has not vanished; the gas is just no longer on the balance.
Practice
Exercise
Solution
The same mass. right answer
The gas cannot escape. All substances stay on the balance.
Less mass afterwards, because gas is lighter than a solid.
A gas has mass too. As long as it stays in the vessel, it is weighed.
More mass afterwards, because a new substance was formed.
A new substance does not mean more mass. The old substances are only put together anew.
In a closed vessel the total mass stays the same.
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Exercise
Solution
The iron has taken up oxygen from the air. right answer
The oxygen is now in the new substance and is weighed. The air was not on the balance before.
New atoms have appeared out of nothing.
Atoms are not newly created in a reaction. That would contradict conservation of mass.
The heat has a large mass.
Heat does not measurably raise the mass. The increase comes from the oxygen.
The total mass stays the same when you count the oxygen from the air. Only the iron was on the balance.
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Exercise
Solution
Right answer: 5 g
The total mass stays the same. Before, 3 g + 2 g = 5 g, so 5 g of magnesium oxide are formed.
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Exercise
Solution
Right answer: The statement is false.
The atoms are kept. After the reaction they are just arranged differently. That is why new substances are formed, but no new atoms.
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