Need Help With ACT Science

[Infamous]

So I'm studying for my upcoming ACT and I could really use some help on the science portion, especially the chemistry. On my pACT studying test, one of the questions asks:

"According to the information provided in the table, the concentration of which of the following ions and dissolved gases is constant for sediment depths of 10 cm or more?"

 

A. Sulfide (S^-)

B. Carbon dioxide (CO2)

C. Ferric iron (Fe(power of 3+))

D. Oxygen (O2)

 

(^ = squared)

 

 

my problem with this question is that I have no idea what sediment depth means, let alone what each of the answers sediment depths are. Can somebody please elaborate for me?

[Darkblade48]

I'm not quite sure how to explain sediment depth to you; it is exactly as it sounds. It is simply how deep the sediment (i.e. dirt, soil, other debris) at the bottom of a lake/pond/other body of water.

 

This question is not so much so a memorization question as a "thinking type" question. At a sediment depth of 10 cm, there will be an anaerobic environment (i.e. oxygen free environment). Let's keep this fact in mind while going through the options:

 

a) Sulfide. This will likely change in anaerobic environments, as there are many bacterial species that reduce sulfur (to hydrogen sulfide) to allow the electron transport chain to proceed.

 

b) Carbon dioxide. This is a byproduct of many anaerobic metabolic pathways, so this will not remain constant either.

 

c) Ferric ion. Again, this will most likely change in anaerobic environments. Ferric ions are terminal electron acceptors, and thus will be used up by bacterial microorganisms.

 

d) As mentioned above, at a sediment depth of 10 cm, there will be an anaerobic environment, and hence, the level of oxygen will be negligible, and will thus be constant.