Priestley’s restored air made measurable: algae and hydrogencarbonate… · Concept Studio

The idea

Priestley inferred that plants restore air that burning or breathing had spoiled; hydrogencarbonate indicator lets a learner measure the same effect as carbon dioxide removed in light and added in darkness.

What you need

algal balls made from Scenedesmus algae immobilised in sodium alginate set in calcium chloride solution (SAPS method)
hydrogencarbonate indicator, clear screw-top bottles of equal size, aluminium foil
an LED lamp and a ruler to set distances, colour standards from pH 7.6 to 9.2 in 0.2 steps (or a colorimeter at 550 nm)

How to do it

Put the same number of algal balls (for example 15) and the same volume of indicator in each bottle.
Set bottles at 5, 10, 20 and 30 cm from the lamp; wrap one bottle in foil; keep one bottle of indicator with no algae in the light as the control.
Leave until some bottles show a clear colour change (1 to 2 hours).
Match each bottle to the nearest colour standard and record the pH.
State the hypothesis Priestley tested and compare his methodology with this one.

What you should see

The indicator is orange-red when in equilibrium with air, turns yellow when carbon dioxide is added and passes through red to deep purple when carbon dioxide is removed (SAPS). Bottles close to the lamp turn purple, toward pH 9.2, as photosynthesis removes carbon dioxide; the foil-wrapped bottle turns yellow-orange as respiration adds it; the control without algae stays orange-red. The colour change falls off with distance from the lamp, and visible change takes 1 to 2 hours, as the SAPS protocol states.

What changes

What you change: light (distance from the lamp, or darkness)
What you measure: indicator colour matched to pH
What you keep the same: number of algal balls
indicator volume
bottle size
time
temperature (LED lamp)

Common misconceptions

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

Plants respire only in the dark.
Plants take in oxygen and give out carbon dioxide all the time.
Priestley knew about oxygen and carbon dioxide when he did his work.

Safety card

Low riskLearners carry it out

Hazards

calcium chloride solution irritates eyes
indicator solution stains
lamp and water nearby

Controls

safety glasses when making algal balls
keep the lamp and its cable away from water

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.

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; currentINS12-12 INS11/12-1 INS11/12-2 INS11/12-3
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.