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Science 7–10 · Year 9

Conservation of mass in an open flask and a sealed bottle

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Practical, model not builtLow risk

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The idea

The total mass of reactants equals the total mass of products; an apparent loss in an open container is the mass of gas that left.

Safety card

Low riskAn adult supervises

Hazards

  • hydrochloric acid 2 mol/L is corrosive to eyes
  • gas pressure in the sealed bottle
  • vinegar spray when the cap is opened

Controls

  • eye protection
  • quantities as stated: 0.52 g of carbon dioxide in the roughly 540 mL of gas space raises the pressure by at most about 55 kPa (less as some dissolves), well below what a soft-drink bottle is made to hold; only bottles made for fizzy drinks are used
  • cap loosened slowly, pointing away from faces
  • acid neutralised before disposal

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

  • balance reading to 0.01 g
  • conical flask 100 mL with 20 mL of hydrochloric acid 2 mol/L and 2 to 3 marble chips (about 3 g) in a weighing boat, cotton-wool plug
  • clean 600 mL plastic soft-drink bottle with its screw cap, 50 mL of white vinegar (4 percent acetic acid), 1.00 g sodium hydrogencarbonate in a small test tube that stands upright inside the bottle

How to do it

  1. Open system: place the flask, the acid, the boat of marble chips and the plug on the balance together and record the total mass.
  2. Tip the chips in, plug the neck loosely with cotton wool and record the mass every 30 seconds for 5 minutes.
  3. Closed system: pour the vinegar into the bottle, lower the small tube of sodium hydrogencarbonate in upright with tongs, screw the cap on tightly and weigh.
  4. Tip the bottle so the vinegar reaches the powder, swirl until the fizzing stops, and reweigh with the cap still on.
  5. Loosen the cap slowly (it hisses), leave it off for 5 minutes with an occasional swirl, and weigh the bottle, cap and contents again.
  6. Account for every change in mass with the equations CaCO3 + 2HCl gives CaCl2 + H2O + CO2 and NaHCO3 + CH3COOH gives CH3COONa + H2O + CO2.

What you should see

Open flask: the reading falls steadily and levels off. The acid can release 0.88 g of carbon dioxide with excess chips, but the loss levels off a little lower, at about 0.8 g, because carbon dioxide is denser than air and fills the flask above the liquid, and a little stays dissolved. Sealed bottle: the readings before and after the reaction agree to within the balance resolution even though 0.52 g of carbon dioxide has formed (the 50 mL of vinegar holds nearly three times the acetic acid needed to react with all 1.00 g of sodium hydrogencarbonate). Opening the bottle lowers the reading as gas escapes, but at first by only about 0.4 g: the carbon dioxide left in the bottle's air space weighs more than the air it replaced, and some stays dissolved, so the loss creeps up only slowly as that gas mixes out of the neck. A flexible zip-seal bag is avoided on purpose: as it inflates by about 291 mL it displaces that much air and the balance reads about 0.34 g low (air at 25 °C is 1.184 g/L), a buoyancy effect that looks like lost mass. The learner knows it worked when the two sealed-bottle readings agree and the open-flask loss comes close to, but a little below, the 0.88 g calculated.

What changes

What you change
open or closed container
What you measure
change in balance reading
What you keep the same
  • masses of reactants
  • time allowed
  • same balance

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Mass is lost in a reaction that fizzes.
  • Gases weigh nothing.
  • A sealed bag that reads lighter after reacting proves mass was destroyed.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. edu.rsc.org/conservation-of-mass/conservation-of-mass-practical-videos-14-16-years/4012966.article
  3. edu.rsc.org/cpd/teaching-conservation-of-mass-at-14-16/4020546.article
  4. education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf

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