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Matter Lab 6-8 / Week 08 / Wednesday
3/6
Week 08 · Nothing Lost: Conservation of Mass

Wednesday

Bench Lab: The sealed bag
// Atoms regroup, and the scale does not change
⏱ about 20 min

Wednesday: Bench Lab: The Sealed Bag

Rocket holds a zip bag with vinegar pooled in one corner. A paper twist of baking soda sits dry in the other corner.

"Sealed," he says, pressing the zipper closed. "Now nothing can leave."

Raven sets the bag on the kitchen scale and reads the number aloud. "Write it down before anything happens. What do you notice?"

"It is just a number," Rocket says. "The boring part."

"The boring part is the evidence," Raven says.

Nova hovers at a safe distance with her light on the scale. "Tip the corner," she says, "and keep your eyes on the number, not the foam."

Rocket tips the bag. It fizzes, puffs and goes tight like a balloon. The number does not move.

Rocket stares. "Nothing lost."

Your mission

You will run the baking soda and vinegar reaction twice: once in a sealed bag and once in an open cup.

You will record the mass before and after each one. Then you will explain the difference using atoms.

No kitchen scale? Use a ruler balanced on a pencil as a seesaw, with a matching unopened bag on the other end.

  • Two zip bags that seal well, sandwich size
  • Vinegar and baking soda
  • A small square of paper to wrap the baking soda in a twist
  • A kitchen scale that reads in grams, or a ruler and a pencil
  • One clear cup
  • A tray to catch spills, a towel, and your Bench Log
Safety first
Wear safety glasses or sunglasses. The bag puffs up and the cup can splash.
Use only one tablespoon of vinegar and one teaspoon of baking soda per bag. The bag puffs but does not burst.
Press most of the air out before sealing. Keep the bag on the tray and point the zipper away from faces.
Nothing is tasted or smelled up close. Nothing goes in the mouth.
Open the bag over the sink after you finish. Never store a sealed bag after a reaction.
Pour leftovers down the sink with running water and wash your hands.
  1. Wrap one teaspoon of baking soda in the paper square and twist it closed.
  2. Pour one tablespoon of vinegar into a zip bag. Keep it pooled in one corner.
  3. Drop the paper twist into the other corner, press most of the air out, and seal the bag completely.
  4. Set the sealed bag on the scale. Record the mass before in the data table.
  5. Tip the bag so the vinegar soaks the twist. Watch the bag puff. Wait one minute.
  6. Record the mass after while the bag is still sealed.
  7. Now the open cup: set the cup with one tablespoon of vinegar on the scale and record the mass.
  8. Add one teaspoon of baking soda, wait one minute, and record the mass again.
SetupMass before (grams)Mass after (grams)ChangeCould gas leave?
Sealed bagno
Open cupyes
PREDICT BEFORE YOU TIP
  • Read the question.
  • Tap your answer.
The sealed bag reacts and puffs up. What do you predict the scale shows?
The open cup reacts. What do you predict the scale shows after one minute?
Which setup is a closed system?

Reading your results

In the sealed bag, the mass before and after should match. The gas was made, but it stayed inside, so its mass stayed on the scale.

In the open cup, the mass after is smaller. The missing grams left as carbon dioxide gas.

Both results agree with the law. Atoms regrouped and none were lost. The only difference was whether the gas could escape.

If your sealed bag changed by a gram, check whether the seal leaked. A small leak is the usual reason.

What the lab showsTrue or false?
The sealed bag is a closed system.?
The gas in the sealed bag has no mass.?
The open cup lost mass because gas left it.?
Atoms were destroyed in the open cup.?
WHY THIS EXERCISEYour two numbers are the evidence that mass is conserved when nothing can leave.
A setup where nothing can get in or out is a ____ system.
The gas that puffed up the bag was carbon ____.
If the sealed bag lost a gram, the usual reason is a ____. (leak or trick)
Draw the sealed bag before and after, with the scale reading under each. Then draw the open cup with its two readings.

Excellent evidence. Tomorrow you will read real chemical formulas and count the atoms in each one.

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