Egg Diffusion Lab
So I meant to blog about this weeks ago, but between the great Snowpocalypse closing school for 3 days, parent-teacher conferences, and the general mayhem of life, I never got around to it.
Some of you may not realize this, but teachers are all filthy thieves. We beg, steal, borrow, and mash up anything and everything to get a decent lesson out because, in all honesty, it is simply too much work to write a new lesson every single solitary day. Plus there are a plethora of amazing ideas and resources, and a good teacher will pick and choose what best fits the needs of her students without having to reinvent the wheel every time. I have a "take a penny, leave a penny" philosophy when it comes to sharing lessons, so while I have no idea who reads this thing, I am at least doing my part to share.
To introduce the concept of diffusion, I did a clicker case called The Agony and the Ecstasy from National Center for Case Study Teaching in Science, which is one of my favorite resources ever. The case deals with the physiological effects of using ecstasy, so I primed the kids with Learn.Genetics Mouse Party animation, and they always get a big kick out of the drugged up mice. After that we did the classic egg diffusion lab, which you can find many variations of online. Here's the gist of it, though...
- Place a raw egg in vinegar and leave for 2-3 days so that the shell dissolves. This is a good lab to start on Friday, let it sit over the weekend, and pick up again on Monday. Be careful there are no cracks in the shell though, or you will end up with Humpty Dumpty!
- Once the shell is gone, the egg will be held together by the outer and inner membranes. It's very fragile at this point, so tell your kids to be really careful! (Every semester I always have at least one group break theirs at this point, so it's not a bad idea to make a few extra.) Mass the egg and record.
- You can do the next part several ways. In order to move more quickly, I have each group put their egg in a different solvent and predict what will happen. You can also put the egg first in distilled water, then in salt water or some other hypertonic solvent. In either case, mass the egg after letting it sit overnight in the solvent and compare it to the pre-solvent weight.
- Discuss the movement of water based on concentration. I've found that this is a really tricky concept for kids to grasp and explain articulately, and I myself had a ton of trouble understanding osmotic pressure until I thought of it in terms of water concentration. I always tell them that diffusion is the movement from HIGH concentration to low concentration, and then I say that higher concentrated solutions are "HIGH"-pertonic and lower concentrated solutions are "hypo for low"-tonic. (I've also found that I almost never speak in a normal voice during class, but who's keeping track?)
We put this egg in Gatorade and it came out a lovely orange color and did not change much in mass, which suggests that Gatorade is in fact isotonic with our cells.
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