Conservation of mass
The total mass of the substances after a reaction equals the total mass before. Atoms are not made or destroyed, only rearranged.
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Analogy
Like moving furniture between rooms. The house contains the same furniture afterwards.
Example
Magnesium 24 g + oxygen 16 g= magnesium oxide 40 gMass before = mass after
In practice
If a metal seems to gain mass when it burns, it has joined with oxygen from the air.
Practice
Flashcards
- What is the law of conservation of mass?The total mass stays the same during a reaction because atoms are not made or destroyed.
Interview questions
- Balance the equation Mg + O2 -> MgO and explain what balancing shows.
- 2Mg + O2 -> 2MgO
- Same number of each atom on both sides
- Atoms are conserved
- Mass is conserved
Questions people ask
Why does a reaction in an open flask seem to lose mass?
If a gas is made, it escapes into the air and takes its mass with it, so the flask reads lighter. In a sealed container the mass stays the same. Mass is always conserved. It is just that some of it left the flask.
Who discovered the conservation of mass?
Antoine Lavoisier, a French chemist in the 1700s, is usually credited. He weighed everything carefully before and after reactions in sealed vessels and found the total mass did not change. This idea became a foundation of modern chemistry. Later work on atoms explained why: atoms are only rearranged in reactions.