Science and Technology K–6 · Year 6

Fizz in a sealed bottle: a new substance, and no mass lost

Science understanding: Chemical sciences

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

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

Bicarbonate of soda and vinegar react to make new substances, including carbon dioxide gas, an irreversible change; in a sealed bottle the total mass stays the same, and it falls only when the gas is let out.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

pressure in the sealed bottle can push the cap off
vinegar spray in the eyes
a glass or thin bottle could burst

Controls

never more than 2.0 g of bicarbonate in a 1.25 L soft-drink bottle
safety glasses for everyone near the bottle
the teacher opens the cap slowly, pointing away from people
no glass containers and no heating

Note

Household chemicals only. Risk assessment before the lesson using the NSW Department of Education Chemical Safety in Schools package (CSIS 1.7 Risk Assessment) or Primary RiskAssess.

What you need

a clean 1.25 L PET soft-drink bottle and its cap (made for fizzy drinks; never glass or a thin water bottle)
100 mL white vinegar labelled 4 percent acidity (40 g of acetic acid per litre)
2.0 g bicarbonate of soda (sodium hydrogen carbonate), weighed, wrapped in a 10 cm square of tissue paper
a kitchen balance reading to 0.1 g
a plastic cup with a second 100 mL of vinegar and 2.0 g of bicarbonate for the open comparison
safety glasses

How to do it

Pour 100 mL of vinegar into the bottle. Stand the bottle, its cap and the tissue parcel of bicarbonate together on the balance and record the total mass.
Drop the parcel into the bottle and screw the cap on tightly at once. Put the bottle and cap back on the balance and record the mass every 30 seconds for 3 minutes. Squeeze the bottle gently to feel it firm up.
The teacher, wearing safety glasses, points the bottle away from everyone and unscrews the cap slowly. Put the bottle and cap back on the balance and record the mass after 1 minute and after 5 minutes.
Open comparison: stand the cup of vinegar and the bicarbonate side by side on the balance, record the mass, tip the bicarbonate in and record the mass every 30 seconds for 3 minutes.
Graph mass against time for the sealed bottle and the open cup.
Decide whether the change is reversible, and explain why the two graphs differ.

What you should see

2.0 g of bicarbonate (0.0238 mol) reacts with acid in excess (100 mL at 40 g/L holds 0.0666 mol; only 36 mL was needed) to make 0.0238 mol of carbon dioxide, 1.05 g, which would fill 0.57 L at 20 degrees Celsius and normal pressure. Sealed, the balance reading does not change beyond its 0.1 g resolution while the bottle firms up; the pressure inside rises by up to about 50 kPa (half an atmosphere, well below the pressure in an unopened bottle of soft drink), a little less because some gas stays dissolved. When the cap is opened the gas hisses out and the reading falls by close to 1.0 g; the open cup's reading falls as it fizzes, by close to 1 g in all. Mass is conserved in the closed bottle; it only seems to vanish when a gas escapes. The fizz cannot be turned back into bicarbonate and vinegar: new substances formed. A rigid bottle is used because an expanding bag or balloon pushes aside extra air and reads lighter on the balance (about 0.7 g for 0.57 L of air at 1.2 g per litre) even though no mass has gone.

What changes

What you change: sealed bottle or open cup
What you measure: total mass over time
What you keep the same: bicarbonate mass
vinegar volume and strength
same balance
temperature

Common misconceptions

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

Gases have no mass, so letting the gas out should not change the reading.
Mass is destroyed in a chemical reaction.
The bubbles are air that was trapped in the powder.

Curriculum references

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

Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST3-6MW-S ST3-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST3-PQU-01 ST3-DAT-01
Australian Curriculum v9AC9S6U04AC9S6I02AC9S6I03AC9S6I05

Sources

The pages the author read to write this activity.

All Concept Studio activities