# Proving Law of Conservation of Mass Doesn't Work

• February 3rd, 2013, 05:23 AM
Kandahar
Proving Law of Conservation of Mass Doesn't Work
I have often tried to use the baking soda and vinegar lab as a way for my students to prove the law of conservation of mass/matter. I have never been successful in having the before and after mass be the same. It has alway been less by at least a gram. This is despite sealing with tape, double bagging it etc. The only explanation I have read is that the buoyancy of air surrounding the inflated balloon or bag has more affect on the products due to the increase in surface area. Can anyone give me a definitive answer and a resource to support this conclusion.
Thanks for your help. A frustrated middle school teacher!
• February 3rd, 2013, 05:38 AM
Harold14370
If you try to measure the mass by measuring the weight, there will be an error due to buoyancy of the materials being weighed.

For example, as you know, helium has mass, but a helium balloon will rise in the air. If you weighed it, you would get a negative weight.

Buoyancy depends on volume, so if the volume of gasses is greater after the reaction, the products of the reaction will weigh less.
Buoyancy - Wikipedia, the free encyclopedia

According to Archimedes principle
Quote:

Buoyancy = weight of displaced fluid
The density of air is about 1.2 kilograms per cubic meter and a cubic meter is 1000 liters, so the buoyant force is equal to about 1.2 grams per liter of air displaced.

Density of air - Wikipedia, the free encyclopedia
• February 3rd, 2013, 07:35 AM
Kandahar
So what you are saying is that there will always be a loss of mass when trying to prove LCM when a gas is produced. I assume if you did it in a vacuum it would work??

Is there any lab that would be better to use, especially for middle schoolers as they are quite literal and what they remember from the vinegar and baking soda lab would be that the law is incorrect because of this observed change?
• February 3rd, 2013, 08:15 AM
Harold14370
No, there isn't a loss of mass. There is an error in measuring the mass. Mass is not the same thing as weight, and you should be teaching your students the difference.

I think if you had the students estimate the volume of gas produced, then make the correction for effect of buoyancy on weight, they should understand how mass is conserved.
• February 5th, 2013, 10:10 AM
Flick Montana
I definitely agree with teaching them how to compensate for state change. That will come in very handy later on.
• February 5th, 2013, 11:34 PM