Investigating Science 11–12 · Year 11

Collecting primary data for the law of conservation of mass

Module 4: Theories and Laws

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Mass is conserved in every reaction; an apparent gain or loss means matter crossed the boundary of the system, and even a sealed balloon can mislead a balance through buoyancy.

Safety card

Low riskAn adult supervises

Hazards

calcium chloride and sodium carbonate irritate eyes
pressure in the sealed bottle
vinegar splashes

Controls

safety glasses
no more than 0.50 g of hydrogencarbonate in the sealed bottle
point bottle caps away from faces when opening

Note

Chemicals are used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1 (general information for all staff, including the CSIS 1.7 risk assessment requirement), read the Safety Data Sheet for each chemical and complete a RiskAssess risk assessment before the lesson. The precipitation pair used here replaces lead(II) nitrate and potassium iodide, avoiding a toxic heavy-metal salt.

What you need

electronic balance reading to 0.01 g
1.11 g anhydrous calcium chloride and 1.06 g anhydrous sodium carbonate, each dissolved in 25 mL of water in separate beakers
2.00 g and 0.50 g portions of sodium hydrogencarbonate; white vinegar labelled 4 % acidity (about 0.67 mol/L ethanoic acid)
250 mL conical flask, a balloon, a 1.25 L PET bottle with screw cap and a small cup that fits inside it

How to do it

Precipitation: weigh the two beakers of solution together, pour one into the other, then weigh both beakers again.
Open flask: weigh 50 mL of vinegar in the flask together with 2.00 g of sodium hydrogencarbonate in a weighing boat; tip the solid in, wait until the fizzing stops and reweigh everything.
Balloon: repeat with the balloon stretched over the flask neck so the gas inflates it; reweigh.
Rigid bottle: put 100 mL of vinegar in the bottle and 0.50 g of sodium hydrogencarbonate in the cup standing inside it; cap, weigh, tip to react and reweigh.
Repeat each three times and tabulate mass before, mass after and change.
Explain each change in terms of what crossed the system boundary.

What you should see

Precipitation: 0.0100 mol of each reactant gives 1.00 g of calcium carbonate and the total mass is unchanged within ± 0.02 g. Open flask: 2.00 g of sodium hydrogencarbonate (0.0238 mol, with 0.0333 mol of acid in excess) releases up to 1.05 g of carbon dioxide, and the measured loss is slightly smaller because some gas stays dissolved. Balloon: about 0.58 L of gas inflates it and the balance reads about 0.69 g less, because the air it displaces (1.18 g/L at 25 °C) buoys it up; the mass is conserved though the reading drops. Rigid bottle: no change within ± 0.02 g while the bottle firms up, about 0.146 L of gas raising the pressure by about 13 kPa in 1.15 L of headspace.

What changes

What you change: type of system (open, balloon, rigid sealed) and reaction
What you measure: change in balance reading
What you keep the same: same balance, zeroed before each weighing
same masses and volumes
reaction complete before reweighing
temperature near 25 °C

Common misconceptions

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

Gases have no mass.
Mass is lost when something burns or fizzes.
A balance reading always equals the mass of what is on it.

Curriculum references

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

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-11 INS11/12-5 INS11/12-6 INS11/12-7
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities