We’re almost done with this class and actually finished all the labs that I had previously pulled aside so I spent some time last week browsing the labs that come with the Modern Chemistry homeschool curriculum and found a few more to try. Today, we did “How much calcium carbonate is in an eggshell?” from Chapter 15, Modern Chemistry. We didn’t get the expected result but its a really good lab and uses a number of concepts including stochiometry and titration.
To start you need a clean eggshell with membranes removed. Bake in an oven at 110F for 15 minutes – my oven can’t be set this low so I set it to 160F then turned it off and put the eggshells in the oven for 10 minutes. Once the eggshells are cool, grind them up to increase surface area. Put 0.1 grams of powdered eggshell in a small beaker (50ml) and then add 6.0 ml of 1.0M HCl. The lab handout actually has students figure out the volume of one drop from a pipette and put in 150 drops of HCl into the beaker but since we had pipettes marked in 1/2 ml increments I decided to do it the less monotonous way and just put in 6.0ml. Students swirled the beaker and watch the reaction. After 4 minutes, two drops of phenolphthalein indicator were put in the beaker. Phenolphthalein remains clear for acids and neutral solutions, but turns bright pink in basic solutions.
Students then added 1.0M NaOH solution in 0.5 ml increments into the beaker to neutralize the remaining HCl that did not react with the eggshell. Both groups ended up putting in 6 ml of the NaOH solution which meant that none of the HCl reacted with the egg shell, which couldn’t be right since we saw a reaction take place (bubbles!). So we tried again but this time we used red cabbage indicator, which has a range of colors depending on the pH. The students also used 10ml graduated cylinders to measure the HCl they put in initially and started with 6.0ml of 1.0 M NaOH in a graduated cylinder and slowly moved it to the beaker with a pipette. This way we could just measure the remaining NaOH and know how much we had put in the beaker. Unfortuanately we still ended up putting in all the NaOH, only with the last few drops did the indicator record a significant shift in pH.
So then we thought about the titration process and how it was only going to work if the the solutions were both 1.0 M and perhaps we had done that wrong, so we started from scratch and remade the 1.0M solutions from the 6.0M HCl and 6.0M NaOH (These chemicals come in the Home Scientist Chemistry kit) This time we used the same graduate cylinder to make both solutions. We used the cabbage indicator and very carefully swirled between drops for the last bit NaOH and found that yet again we had to use almost all the NaOH, 5.6 ml, which means only 0.4ml of HCl reacted. We went ahead with the calculations and found that our eggshell was only about 20 percent calcium carbonate and the expected number is closer to 80 percent. I’m not sure what went wrong – did I over heat the eggshell, breaking down the calcium carbonate? Don’t think so since it really didn’t get very hot. The eggshells were sitting out on a counter for a few days before we did the experiment (and before heating) so next week I’l have the students try fresh eggshells. Perhaps our 6.0M solutions have been contaminated or aren’t quite 6.0M? Were the eggshells not crushed finely enough? I’m not sure what went wrong but since this lab didn’t take very long to perform we were able to do it three times and tried to improve the procedure each time. Blair Lee from SEA Homeschoolers just posted last week about ‘When Experiments Don’t Work, That’s When the Science Really Gets Fun!’, which is exactly what happened today.