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Matter Lab 6-8 / Week 08 / Bench Day
6/6
Week 08 · Nothing Lost: Conservation of Mass

Bench Day

Bench Day: The seesaw balance
// Atoms regroup, and the scale does not change
⏱ about 20 min

Bench Day: The Seesaw Balance

Saturday, the Bench holds a ruler balanced on a pencil. Rocket sets a sealed zip bag of vinegar and a baking soda twist on one end.

Raven sets a matching bag, untipped, on the other end. The ruler wobbles and settles level.

"Even," Rocket says. "Now the trick." He tips his bag. It fizzes and swells like a tiny pillow.

The ruler stays level.

"What do you notice?" Raven asks the room. A younger cousin leans in. "It got bigger but it did not get heavier."

Nova hovers over the pencil. "Bigger is volume," she says. "The seesaw measures something else. What?"

"Mass," Rocket says. "And nothing got out, so nothing changed."

Raven closes the Bench Log. "Nothing lost," she says. "Every time."

  1. Mass is the amount of matter in an object, measured with a balance. It does not change with location.
  2. In a chemical reaction, atoms are rearranged into new substances. None are created or destroyed.
  3. The total number of each type of atom is conserved, so the total mass is conserved.
  4. A sealed bag keeps its mass through a reaction. An open cup seems to lose mass because the gas escapes.
  5. Formulas are recipes for one molecule or unit. Counting the atoms before and after is the proof.
◇ FAMILY SEESAW BALANCE
Balance a ruler on a pencil. Put a sealed bag of vinegar with a baking soda twist on one end and a matching untipped bag on the other.
Slide the bags until the ruler sits level. Everyone predicts aloud: will it tip when one bag reacts?
Tip one bag, watch it puff, and watch the ruler. Then open both bags over the sink.
Talk it over: the bag got bigger. Why did the ruler not move? Where would the mass go if you opened the bag first?
For a grown-up
Bags hold one tablespoon of vinegar and one teaspoon of baking soda, sealed with most of the air pressed out. They puff, not burst.
Safety glasses on, bags on a tray, and the bags are opened over the sink right after. Nothing is stored sealed.
The law of conservation of mass and the formulas this week come from the CK-12 text at chem.libretexts.org and from PubChem.
Safety first
Wear safety glasses or sunglasses.
Keep the bags on a tray and point the zippers away from faces.
Nothing is tasted or smelled up close. Open the bags over the sink when done, and wash your hands.
THINK IT THROUGH
  • Read the question.
  • Tap your answer.
The bag puffed up but the ruler stayed level. What does that show?
If you opened one bag during the reaction, which way would the ruler tip?
The younger cousin said "bigger but not heavier." Which word is the bigger part?
What law says matter cannot be created or destroyed in a normal chemical process? Conservation of ____. Type one word.
WHY THIS EXERCISEThis law is the reason every count you did this week came out even.
Draw the seesaw balance with both bags, before and after the fizz. Show the ruler level both times.
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