Australia's Science Forum

Hi
We have been doing the Heinemann 2.6 Investigation "Diffusion and Osmosis " and the part with the starch/ iodine exp has caused us some grief. I had the starch as required -5%- and loaded the dialysis tube as instructed then placed it a in a gas jar of dilute iodine/potassium sol. The starch changed colour within 5 mins but the level did not change in the thistle funnel. It is supposed to rise but it did not shift.
What are we doing wrong?
Glenpaul

How long have you left it? From memory it takes a few days for it to be noticable. Failing that, try it with a stronger starch solution and see if that works.

Thanks RosalieM
I have increased the concentration of the starch sol and it hasn't changed the level up the thistle funnel but i will keep 1 setup for a few days and see if there is any change.

It only changes by a few mm as well, so if the marker line is too thick that may make it difficult to see as well.

Hey Guys.

I use a 3mL disposable dropper and cut the end off the top of the bulb part. Then I insert it into the dialysis tubing and tie it off above the bulb part, so that the graduated 3 mL part extends above the tubing. That way the liquid has a much smaller diameter tube to go up, so the change is more obvious.

Great idea Smiley! Why didn't you tell me that before I bought new thistle funnels this year?

Sorry! My psychic ability is limited to my child.