Question

After 12 hours
Tube 1:Water + BTB + Light (No plant) --->

Tube 3:Water + BTB + Light + Elodea --->

Tube 4:Water + BTB + Dark + Elodea --->

Answer 1

Assuming the BTB started at its neutral/blue color (pH ≈ 7–8), you would expect:

- Tube 1 (water + BTB + light, no plant): little or no color change — stays blue/green.
Reason: no photosynthesizing/respiring plant to change CO2 levels. (Small shifts possible if microbes or CO2 from air dissolve, but generally no big change.)

- Tube 3 (water + BTB + light + Elodea): turns more blue (more basic).
Reason: in light Elodea photosynthesizes, consuming CO2 from the water. Less CO2 → less carbonic acid → pH rises → BTB shifts toward blue. You may also see O2 bubbles on the plant.

- Tube 4 (water + BTB + dark + Elodea): turns yellow (more acidic).
Reason: in the dark Elodea cannot photosynthesize but continues to respire, producing CO2. More dissolved CO2 → more carbonic acid → pH drops → BTB shifts toward yellow.

If you want, you can confirm by measuring pH or dissolved O2, or include sterile controls to rule out microbial respiration.