Chemistry 11–12 · Year 11

Conservation of mass: vinegar and sodium hydrogen carbonate in an open cup and a capped bottle, and a precipitation on the balance

Module 2: Introduction to Quantitative Chemistry

Practical, model not builtLow risk

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The idea

Mass is conserved in every chemical reaction; an apparent loss in an open system is the mass of gas that left, which the balanced equation predicts.

What you need

Electronic balance reading to 0.01 g (0.001 g preferred)
600 mL PET soft-drink bottle with its screw cap, clean and dry, and a small test tube (about 10 mm by 75 mm) that passes through the bottle neck
Sodium hydrogen carbonate, 0.50 g portions, two
White vinegar (about 4 percent ethanoic acid), 20 mL portions, two
Plastic cup, 200 mL, and a weighing boat
0.5 mol/L calcium chloride solution, 25 mL
0.5 mol/L sodium carbonate solution, 25 mL
100 mL conical flask with a rubber stopper and a small test tube that fits inside

How to do it

Open system: weigh the plastic cup holding 20 mL of vinegar together with the weighing boat holding 0.50 g of sodium hydrogen carbonate; record the total. Tip the powder in, wait until fizzing stops (about two minutes) and reweigh the cup and empty boat together.
Closed system: pour 20 mL of vinegar into the bottle, lower the small test tube holding 0.50 g of sodium hydrogen carbonate upright into it without spilling, and screw the cap on firmly. Weigh, then tilt the bottle so the powder falls into the vinegar, let the reaction finish and reweigh with the cap still on. Finally point the cap away, unscrew it slowly to let the gas out, and reweigh.
Precipitation: stand the test tube of 10 mL calcium chloride solution inside the conical flask holding 10 mL sodium carbonate solution, stopper and weigh; tip to mix, observe, reweigh.
Record all masses in a table with the change for each system; repeat each trial once more.
Run the simulation with the masses used and compare the predicted gas mass with the measured loss in the open cup and in the bottle after opening.

What you should see

Open cup: the mass falls by close to 0.26 g, the mass of carbon dioxide from 0.50 g of NaHCO3 (NaHCO3 + CH3COOH gives CH3COONa + H2O + CO2), about 148 mL of gas at 25 degrees Celsius and 100 kPa; the measured loss falls a little short because some carbon dioxide stays dissolved in the liquid. Capped bottle: the bottle firms up as the gas builds pressure (no more than about 26 kPa above atmospheric pressure if all the gas collects in the 0.575 L of space left in the bottle) and the balance reading does not change within 0.01 g; once the cap is loosened the gas escapes and the reading falls by up to 0.26 g. A rigid bottle is used rather than a flexible bag because a bag that swells by 148 mL displaces about 0.17 g more air (air is 1.17 g/L at 25 degrees Celsius and 100 kPa) and reads lighter although no mass has left it. Precipitation: a white precipitate of calcium carbonate forms and the mass is unchanged within 0.01 g.

What changes

What you change: whether the system is open or closed
What you measure: change in measured mass (g)
What you keep the same: mass of sodium hydrogen carbonate (0.50 g)
volume and brand of vinegar
time allowed for reaction
same balance

Common misconceptions

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

Mass disappears when a gas forms (the gas has mass; the capped bottle shows it).
A precipitate adds mass because a solid appears (the atoms were already in the solutions).
Bubbles mean the reaction is making energy, not matter (the bubbles are carbon dioxide, a product with its own mass).

Safety card

Low riskLearners carry it out

Hazards

a capped bottle holds the gas under pressure and can spray when opened
vinegar irritant to eyes

Controls

PET soft-drink bottle only, never glass
use 0.50 g of powder and no more
unscrew the cap slowly, pointing it away from faces
eye protection

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

Curriculum references

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

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH11-9 CH11/12-2 CH11/12-4 CH11/12-6
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-02 CH-11WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities