Students were asked to watch the following videos before class:
and if they wanted more they could watch Crash Course and Bozeman Science:
Last week we soaked eggs in karo syrup and water and measured their change in mass due to osmosis after two hours. We put the eggs back in the solutions at the end of class and I had some students measure them one more time a few DAYS later and the mass change of the egg sitting in Karo syrup was dramatic. As you can see in the photo below, the egg in the lower left part of the photo has been reduced to basically just the yolk in an empty membrane, while the egg in the lower right of the photo looks and feels very full. The egg on the left had been soaking in karo syrup for over 2 days and the egg had lost half its mass as the water inside the egg diffused out into the karo syrup. The egg on the right was submerged in regular water and actually gained 7g in mass because water diffused into the egg, since it had a higher concentration of solutes than the plain water outside the egg.
I gave the students in this class the final set of masses and a copy of the above photo to put in their lab books.
After a presentation on energy and exergonic (releases energy) and endergonic (absorbs energy) reactions, we did a lab (6.2 Endergonic and Exergonic Reactions in Holt Biology Instructors Guide) demonstrating both types of reactions. Students put 75 ml of room temperature water in a beaker, measured its initial temperature and then added 15 g of calcium chloride (or 15 g of ammonium nitrate) to the water and observed the temperature change of the water. Calcium chloride can be bought at a hardware store as DampRid (among other names).
I have a Vernier Go Direct temperature sensor which can be used with an iPad to record the temperature throughout the reaction which we used for both reactions. The graph below shows the data for ammonium nitrate. The water was initially 24 C and dropped to 20.5 C. For this particular data set, only 9 g of ammonium nitrate was used because we didn’t have enough for both groups to use 15 g. The other group used 13 g and their temperature dropped to 14 C, which was a pretty dramatic difference to the touch (on the outside of the beaker). The graph below is not smooth because we would swirl or stir the solution once in a while to increase the reaction and then the temperature would drop again, like you see at 80 s.
The graph below shows the same experiment with calcium chloride. This graph ends a little early but students continued to observe the temperature and it went up to 40 C before plateauing.
Calcium chloride added to water is an exergonic reaction – it releases energy which heats up the water, while ammonium nitrate added to water is an endergonic reaction – it absorbs heat from the water, decreasing the temperature of the water.
I had hoped to play Cytosis, a cell biology game I got off kickstarter last year but we ran out of time, so we’l try that next week. In the time remaining I had the students do the same interactive notebook activity on cellular respiration and photosynthesis that the middle school kids did.