Hydrogencarbonate indicator in light and dark sim

What the sim shows

Hydrogencarbonate indicator in light and dark. Four boiling tubes of hydrogencarbonate indicator stand in a rack, each closed with a bung; tubes 1 to 3 each hold a sprig of pondweed, and tube 4 holds none (the control). Every tube starts red: the indicator is in balance with the CO₂ in air. Drag the lamp to its taped mark, 20 cm behind the tubes with the heat shield between, then slide the foil sleeve over one pondweed tube (dark) and the gauze sleeve over another (dim light); the third stays in bright light, and the control stays in the light too. Press Wait 1 hour, read each tube's colour against the printed colour chart and record it. The indicator turns orange then yellow when the solution holds more CO₂ than air does, and magenta then purple when it holds less. A plant respires all the time, in light and in dark, which adds CO₂; in light it also photosynthesises, which takes CO₂ in. In bright light photosynthesis is faster than respiration, so the CO₂ falls and the indicator turns magenta then purple. As the CO₂ runs low, photosynthesis slows: CO₂ is now the limiting factor, and the tube nears its own balance point (about 0.09 × the air level), where photosynthesis matches respiration. In dim light the two rates are equal (the compensation point), so the CO₂ stays as in air and the indicator stays red. In the dark there is no photosynthesis but respiration goes on, so the CO₂ rises and the indicator turns yellow. The control stays red, which shows that the changes are caused by the pondweed. Simplified on purpose: the times and rates are illustrative (≈): a real tube changes at a rate set by the size of the sprig, the light and the temperature; the colour scale is a model: the indicator really follows the pH, which dissolved CO₂ lowers, and here each tube is drawn in the chart colour it is nearest to, while a real tube changes smoothly between them; the CO₂ level is given as a multiple of the level in balance with air (≈), and the rates as CO₂ per hour on the same scale; the temperature is held steady at 25 °C in every tube, so the lamp does not warm them; one sprig of the same size in each tube; the gauze is chosen to let through exactly half the light, which puts the dim tube at the compensation point; with real gauze you would have to find the right number of layers; with its bung in, no CO₂ passes between a tube and the air; the lamp lights all four tubes equally before the sleeves cut down the light; to read a wrapped tube you slide its sleeve off for a moment; the close-up shows every tube's colour as you would then see it; the bench apparatus is drawn to scale, and the close-up is not to scale.

• Lit: the lamp's index tab, and the line on the tape that marks its place
• Close-up: the printed colour chart, and each tube magnified
• Dashed ring: where a carried sleeve will go

Use it in your lessons

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