Phenol red question?

Question

Hi!

Recently at school we conducted an experiment, testing phenol red. We wanted to know what caused the phenol red to change color (yellow). At the end, our answer was that it was CO2 in our breath and in the dry ice. We had other test tubes that had Elodea plants in them. One of them (exposed to light) remained red. The other one (in the dark) changed to the color yellow. I was wondering why? I know plants INHALE CO2, but why would a plant, exposed to no light, make the phenol red change color? That's my question! Thank you in advance!

Answer 1

"A solution of phenol red is used as a pH indicator: its color exhibits a gradual transition from yellow to red over the pH range 6.6 to 8.0."

the CO2 generated by you, by dry ice  or by a plant  make the solution slightly acidic so that phenol red turns yellow

One of them (exposed to light) remained red......

because plants exposed to light consume CO2, & produce oxygen,...  they are not producing an acid , phenol red stays red

but at night time, plants do the reverse reaction, they consume oxygen & produce CO2 (acidic,... phenol red turns yellow)