To identify a testable hypothesis using a bromothymol blue solution, we must consider how the indicator responds to the presence or absence of [tex]$CO_2$[/tex].
Key Facts:
1. Bromothymol blue turns yellow in the presence of [tex]$CO_2$[/tex] (acidic conditions).
2. Bromothymol blue turns blue when [tex]$CO_2$[/tex] is removed (basic or neutral conditions).
Given these facts, let's analyze each statement:
A. If water plants take in [tex]$CO_2$[/tex], then the solution will turn from yellow to blue.
- Water plants (like all plants) typically take in [tex]$CO_2$[/tex] during photosynthesis. If they are effectively taking in [tex]$CO_2$[/tex], the concentration of [tex]$CO_2$[/tex] in the solution would decrease, causing the solution to become more basic and the bromothymol blue to turn from yellow to blue. This hypothesis is testable using the properties of bromothymol blue.
B. If algae give off [tex]$O_2$[/tex], then the solution will turn from blue to yellow.
- Algae, like most plants, give off [tex]$O_2$[/tex] during photosynthesis. However, the release of [tex]$O_2$[/tex] itself does not directly impact the [tex]$CO_2$[/tex] levels in a solution. Hence, the [tex]$O_2$[/tex] release should not change the color of bromothymol blue based solely on [tex]$O_2$[/tex] output. Therefore, this hypothesis does not directly involve a testable reaction with [tex]$CO_2$[/tex].
C. If yeast cells give off [tex]$O_2$[/tex], then the solution will turn from yellow to blue.
- Yeast cells, during aerobic respiration, do not typically give off [tex]$O_2$[/tex] as they use glucose to produce energy, [tex]$CO_2$[/tex], and water. Therefore, the statement is incorrect on the biological basis and does not leverage the bromothymol blue indicator's reaction to [tex]$CO_2$[/tex].
D. If fish give off heat, then the solution will turn from blue to yellow.
- Fish respiration results in the release of [tex]$CO_2$[/tex] into the water. However, the key element in the statement—giving off heat—does not correlate with the bromothymol blue reaction. Fish giving off heat would not result in a [tex]$CO_2$[/tex] change to turn the solution yellow. It inaccurately addresses what fish contribute to the water in the context of [tex]$CO_2$[/tex].
Conclusion:
The hypothesis that is directly testable using the properties of bromothymol blue, which indicates changes in [tex]$CO_2$[/tex] levels, is:
A. If water plants take in [tex]$CO_2$[/tex], then the solution will turn from yellow to blue.
