img_4618This lab is from Ian Guch’s 24 Lessons that Rocked the World and as you can see from the photo on the left the students had quite a bit of fun with this lab.   The purpose of the lab is to demonstrate the Law of Conservation of Mass, which states that the sum of the masses of the reactants will equal the mass of the products of a chemical reaction.  Unfortunately this can be difficult to demonstrate since its hard to capture gases created in a chemical reaction and we frequently lose some material in transfering chemicals from container to container, etc.  But we can try our best and see how close we can get.

For this lab we needed 2M concentration of acetic acid, which I made before class.  The lab called for sodium carbonate, not sodium bicarbonate so we had to bake the baking soda in the oven at 200F for 45 min which drives out carbon dioxide and water.  Students measurd  13 g of the sodium carbonate and placed it in the finge of a glove using a funnel.  The finger was then tied shut with rubber bands to keep the sodium carbonate contained.  The 2M of acetic acid was poured into a beaker (250ml) and the the glove was fitted over the beaker and secured with rubber bands. img_4602 The entire apparatus was then weighed on the scale so we would have the mass of the reactants before the reaction.  Then the rubber bands were removed from the glove and the sodium carbonate was dumped into the acetic acid.  Much fizzing and bubbling occurs as the glove fills with carbon dioxide.  The first group had a lot of gas escape from their glove and the liquid fizzed up so much it overflowed the beaker and escaped from the glove.  The second group had similar results but the third and fourth group having seen the leakage used bigger beakers and managed to contain their liquid.  Another thing we did after watching the first group’s reaction is weigh EVERYTHING, including the aluminum pan so if liquid did escape the beaker it would still be contained in the pan and the mass would not be lost.  The fourth group had their glove come off so they lost quite a bit of gas.  I would recommend using a larger beaker, 500ml and a larger latex glove and make sure you secure it with a couple of rubber bands.  The third group who had no visible leakage from the system lost only 3 grams of mass which is pretty dang good. You can watch their experiment in the video below.

The other groups lost about 6 grams.  Trying to prove conservation of mass in the lab is fairly difficult since no matter how many rubber bands we put on the glove we probably lose some gas thru the glove and the gas is buoyant so its not exactly putting its full mass on the scale.  But this lab was very exciting and the students enjoyed seeing the glove inflate and I have to say I’m pretty impressed with the strength of the latex gloves.

Here’s a nice Ted-Ed talk on the law of conservation of mass:

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