samsowerby
Posts: 1
Joined: Sat Aug 18, 2007 11:20 am

### Independant Variables

I am studying AS level human biology and am investigating the effect of copper sulphate on catalase activity.

I will increase the concentration of copper sulphate until the rate of reaction reaches 0, when it does I will increase the volumes of catalase and hydrogen peroxide seperately to show that increasing the volume of catalase will increase the rate of reaction but increasing the the volume of hydrogen peroxide will not, so proving non-competitive inhibition,

I am writing my variables and was wandering if the copper sulphate concentration and and the volumes of catalase and hydrogen peroxide can both be independant variables.

Craig_Bridge
Former Expert
Posts: 1297
Joined: Mon Oct 16, 2006 11:47 am
I am writing my variables and was wandering if the copper sulphate concentration and and the volumes of catalase and hydrogen peroxide can both be independant variables.

If you are controlling them independently, then something about them are by definition independent varibles. Unfortunately, concentration and volumes are not independent of masses and other physical properties.

In thinking about your experiment in biochemistry terms, the "concentration of copper sulphate" and "volumes of catalase and hydrogen peroxide" are a bit troublesome to me. You probably need to be thinking in terms of mass units, concentrations, and reaction rates and be careful about what each change you make is actually changing. In other words, increasing a mass is changing a concentration. The chemical reaction itself is also changing concentrations.
-Craig

hhemken
Former Expert
Posts: 266
Joined: Mon Oct 03, 2005 3:16 pm
I agree with Craig. I suspect the independent variables should be expressed in concentration in the experimental solution if they are present from the beginning such as the copper sulphate appears to be. If you wait for reaction to proceed before adding additional substances, then these should be expressed as the volume of new reactant added to the reaction and the concentration of the new reactant in the stock solution you add.

You should also correctly calculate the final concentration of all the reactants you put into the reaction. If they all go in at the same time, it is the final concentration that should be reported. You would still need to report the volumes and concentrations of all reactants, including whatever solvent is used for the reaction (saline? buffer?).

Moral of the story: report all concentrations of everything as well as all volumes used.
Heinz Hemken
Mentor
Science Buddies Expert Forum