Where Does the Mass Go?
- Chemistry
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- Mass is conserved in a chemical reaction.
- Gases produced can escape, making mass appear to be lost.
- A closed system shows conservation directly.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
The disappearing mass
Weigh vinegar and baking soda together, then mix them. The reading on the scale drops. It looks as though mass has disappeared. It has not. The reaction made carbon dioxide gas, and the gas escaped into the room. It still has mass — it just is not on the scale any more.
Seal it and see
Now run the same reaction in a sealed lab flask, so the gas cannot escape. Mix the two again and watch the scale. It does not change at all. Nothing has been lost. The carbon dioxide is still in the flask, and it still has the mass it always had. This is the experiment that settles the question, and it is strong evidence that mass is conserved in every chemical reaction. The apparent loss in the open dish was about the container, not the chemistry.
Why equations have to balance
This is the reason chemical equations have to balance. Count each kind of atom on the left of the arrow, then count them on the right. The two counts must match exactly. If they do not, the equation is simply wrong, because atoms cannot appear from nowhere or disappear into nothing. Balancing is not an arbitrary rule that chemists invented to make the work harder — it is conservation of mass written in symbols.
Curriculum links: NGSS MS-PS1-5