Expert: Dana Krempels, Ph.D. - 10/8/2009

Question
Your Question was:

If I fill three pieces of dialysis tubing with a 10% sugar solution, a 30% sugar solution and a 50% sugar solution, then place each in a beaker of distilled water for 45 minutes, will the % change in mass be linear or non-linear? If non-linear, could you please explain why.
Thank you

Expert: Dana Krempels, Ph.D. Replies "I do not answer homework problems."

It is not a homework assignment, I am the teacher. My data seems to indicate non-linear osmotic change, 10% tube gained 22%, 30% gained 61% and 50% gained 77%, If you graph these points theplot appears parabolic in nature, but I am not sure if that is the way it is supposed to be or rather an error on my part. It did the reverse experiment at the same time and got similar results but in the negative direction, mass losses of 21%, 56% and 63%. Again, the graph looks parabolic.

Answer
Dear Paul,

Sorry for the misunderstanding.  I get so many school kids asking me to do their homework that I assumed the same for your question.

The relationship should be linear, and looking at your data I'm not really sure how it appears parabolic to you.  When I plot the points, the data look linear, though there is not a perfect fit line that intersects all points.  This is not uncommon for data like these.

Since there are only three data points, I would suggest that perhaps you run the experiment again with a few more intermediate concentrations of sucrose, and run the experiment a few times to be sure there wasn't some sort of set-up error that gave you an unexpected result.

I'm betting that you will see the linear relationship emerge with more data points and more replications.  

If you have software such as Microsoft Excel available, you can graph the data and ask the program to draw you the best fit line for each experimental run.

Hope that helps.

Dana