Science 7–10 · Year 7

Gas particles move: carbon dioxide diffusing between two test tubes

Solutions and mixtures

Practical, model not builtLow risk

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The idea

Gas particles are in constant random motion, so a gas spreads into the space available even without stirring or wind.

Safety card

Low riskAn adult supervises

Hazards

hydrochloric acid 0.5 mol/L is an irritant to eyes and skin
limewater is an irritant
carbon dioxide displaces air in a closed space if made in large amounts

Controls

eye protection throughout
work in a ventilated room
neutralise and dilute the acid residue 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

6 test tubes (one fitted with a bung and delivery tube as the gas generator), 5 corks
calcium carbonate (marble chips), 1 spatula per carbon dioxide tube
hydrochloric acid 0.5 mol/L, 10 mL per carbon dioxide tube
limewater (calcium hydroxide solution, about 0.02 mol/L), 2 mL per test
safety glasses to AS/NZS 1337 or EN166F, stop clock

How to do it

Put one spatula of calcium carbonate into the first test tube, add 10 mL of 0.5 mol/L hydrochloric acid and fit the bung and delivery tube at once.
Let the gas flow into a second (empty) test tube held mouth-up for about 1 minute, then cork it. This tube now holds carbon dioxide; a third corked tube holds air.
Trial A: hold the carbon dioxide tube upside down over the air tube, remove both corks and hold the tubes mouth to mouth for 5 minutes without shaking; cork both.
Trial B (the one that settles the question): make a fresh carbon dioxide tube and this time hold the air tube upside down over it, mouth to mouth, for 5 minutes; cork both.
Add about 2 mL of limewater to each of the four tubes and to a corked tube of room air (the control), cork and shake. Record which tubes turn cloudy.

What you should see

In both trials both tubes turn limewater cloudy. Trial A could be explained by the denser carbon dioxide falling, but in Trial B carbon dioxide has moved up into the air tube above it, against gravity, which only random particle motion explains. The learner knows it worked when the control tube of room air stays clear and the upper tube in Trial B turns cloudy.

What changes

What you change: arrangement of the tubes (carbon dioxide above or below the air)
What you measure: cloudiness of limewater in each tube (none, faint, cloudy)
What you keep the same: volume of limewater
acid volume and concentration used to make the gas
tube size
no shaking of the joined tubes

Common misconceptions

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

Gases only move if something pushes them (wind, a fan, shaking).
A dense gas such as carbon dioxide cannot rise into a tube of air above it.
Particles in a gas are touching each other, like a crowd.
Heating makes the particles themselves expand.

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 7–10 Syllabus (2023)SC4-SOL-01 SC4-WS-01
Australian Curriculum v9AC9S7U05AC9S7I03

Sources

The pages the author read to write this activity.

All Concept Studio activities