Day 114: When is Mass Conserved?


We've been doing several mini experiments in chemistry to see when mass remains constant.

1. Steel wool is pulled apart.
2. Steel wool is heated in burner flame.
3. Two chemicals are mixed together in which a precipitate forms.
4. Ice melts into liquid water.
5. Sugar dissolves in water.
6. Alka-Seltzer dissolves in water.

Mass remains the same for all excerpt for #2 and #6. The mass of the heated steel wool increases(!) several tenths of a gram. The Alka-Seltzer decreases.

But what if we could capture the gas released from the Alka-Seltzer? Thanks to a suggestion from a chemistry colleague, I did the demo in the photo. Both systems started out with 90 grams of mass. I used 4 tablets for each system. The baggie on the left had water in it and I squeezed add much air out as possible while keeping the tablets in the bag, but away from the water. Then I let the tablets go into the water and out the baggie back on the scale. As the bag inflated due to the gas produced, the mass stayed 90 grams. Simultaneously, I combined 4 tablets in water in a cup (water + tablets + cup = 90 grams). We lost 2 grams of escaping gas.?? Great way to show open and closed systems.

Also a good demo to show that gas is much less dense than solids or liquids, since the gas caught in the bag had a mass of just 2 grams, but a volume much larger than 2 mL of water or 2 paper clips.


About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

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