Science 7–10 · Year 8

Algal balls and hydrogencarbonate indicator: photosynthesis and respiration in one tube

Science understanding: Biological sciences (the only ACARA v9 content description that names photosynthesis and respiration is AC9S9U03, a Year 9 Earth and space sciences description of the carbon cycle, and AC9S8U01 is about cell structure and the plant and animal cell comparison, so the ACARA side is coded to Year 8 science inquiry only; the NSW side carries the concept in Stage 4 Cells and classification, whose content point asks learners to identify cellular respiration via mitochondria and photosynthesis via chloroplasts)

PracticalLow risk

The idea

Photosynthesis removes carbon dioxide from water and respiration adds it, and an indicator that tracks carbon dioxide shows which process dominates under each light condition.

Safety card

Low riskAn adult supervises

Hazards

Calcium chloride granules are an irritant (the dilute solution is low hazard)
Lamp near water

Controls

Eye protection while making balls; wash spills
Lamp on a stand; dry hands on the switch

Note

Chemicals: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package (section 1, general information for all staff) and complete a RiskAssess risk assessment before the lesson. Confirm the CSIS category of hydrogencarbonate indicator in CSIS Appendix A or D before learners use it: under CSIS Section 1 (Figure 1.7-3) a chemical not listed in Appendix A or D is treated as category N, for demonstration by approved teachers only, until an approved (Code 1) user completes a site-specific risk assessment.

What you need

A culture of the green alga Scenedesmus quadricauda from a biological supplier, grown up for two to three weeks as the SAPS algal balls resource describes (about 30 mL serves a class)
Sodium alginate solution (2 to 3 %) and calcium chloride solution made up as in the SAPS technical notes, a syringe to drip the algal alginate into the calcium chloride, a tea strainer
Hydrogencarbonate indicator solution, bubbled with air until bright red (pH 8.4, atmospheric carbon dioxide), and the SAPS colour chart
Small screw-cap tubes (bijou bottles), foil, a lamp of 1200 lumens or more as SAPS recommends, a metre rule

How to do it

Mix the concentrated algal culture with sodium alginate and drip it from the syringe into calcium chloride; leave the balls in the calcium chloride for 5 to 10 minutes to set, then rinse them in water.
Put 20 balls into each tube and fill with indicator; note the starting colour against the chart.
Set tubes at 10, 20 and 40 cm from the lamp, one wrapped in foil (dark), and one with indicator but no balls (control).
Record the colour against the chart every 15 minutes for the lesson, or photograph the tubes beside the chart.
Rank the tubes by the direction and speed of colour change.

What you should see

Tubes in bright light shift from red toward magenta and purple as carbon dioxide is removed, fastest in the closest tube; the foil-wrapped tube shifts toward orange and yellow as respiration adds carbon dioxide; the tube without balls stays red. The SAPS teaching notes report that beside a suitable lamp the indicator changes to purple within 30 minutes, and the SAPS technical notes recommend a lamp rated over 1200 lumens for photosynthesis practicals. The learner knows it worked when the control holds its colour and the lit and dark tubes move in opposite directions.

What changes

What you change: Light intensity (lamp distance, or dark)
What you measure: Colour of hydrogencarbonate indicator against the chart over time
What you keep the same: Number of balls (20)
Indicator volume
Temperature
Tube size
Starting colour

Common misconceptions

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

Plants photosynthesise by day and respire only at night; they respire all the time.
The indicator measures oxygen; it tracks carbon dioxide through pH.
Algae are not plants so they do not photosynthesise.

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-CLS-01 SC4-WS-03 SC4-WS-06
Australian Curriculum v9AC9S8I02AC9S8I05

Sources

The pages the author read to write this activity.

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