Friday, December 18, 2009

Mass Lab: Conservation and Chemical Reactions


Not all the labs we did using the teacher resources from Einstein's Big Idea worked the way we hoped. One problem was my kitchen scale -- although it had lots of little numbers, it really wasn't sensitive enough to measure anything within the accuracy of its scale. (I'll post about the outcome of those labs another time.)


This lab, which comes from the Messing with Mass activity, also required a scale. So instead, I tried a technique from The Joy of Chemistry. We made up two identical bags of materials (see below) and hung them from a wire coat hanger set up as a balance. Then we mixed the contents of one bag while leaving the other untouched. The balance did not tip, theoretically showing that the mass remained the same even as the materials underwent a change of state from solid and liquid to gas. However -- like our insensitive scale -- it could just have been that the balance we set up wasn't very accurate. But basically the lab illustrated, if not demonstrated, what conservation of mass looks like. Here's what we did:



  • citric acid
  • baking soda
  • quart freezer bag
  • film canister, filled with water
  • wire coat hanger
  • rod for hanging
  • clips for hanging bag
  • measuring spoons
  1. Examine the two chemicals involved. (Ours came in packets left over from a root beer making kit. Although the original instructions warns students not to taste, if they're from your kitchen they're perfectly safe.)
  2. Measure out 1 teaspoon of citric acid into each bag. (We found the original 1/4 teaspoon too little to see much reaction.)
  3. Add 1 teaspoon of baking soda to the bags.
  4. Fill the film canisters with water and close the lids. Dry off the outside if needed and place 1 canister in each bag. Seal the bags tightly, squeezing out as much air as possible.
  5. Set up the rod so that the hanger can be hung from it. (We laid it across two tables.)
  6. Use the clips to attach the bags to the hanger as shown.
  7. Place the hanger on the rod, positioning the bags so that they are balanced. Use tape to hold them in place. (We didn't, and the bags did slide around.)
  8. Keeping the bag sealed, carefully open the film canister in one of the bags and pour the water out. You might want to leave the lid loose to make it easier to open.
  9. The chemicals and the water will react and produce a gas (carbon dioxide). The two bags should stay in balance.
Explanation:

Mass is the amount of matter an object contains -- as opposed to weight, which is a measurement of the force of gravity acting on it. As we saw in the documentary, Antoine-Laurent Lavoisier was the first to demonstrate that mass is conserved in a chemical reaction. Lavoisier made careful measurements of changes including water to steam in the late 1700s, aided by his wife, Marie Anne. Mass is always conserved in a chemical reaction in a closed system (except for an extremely small amount which is lost or gained in the form of light and/or heat energy).

We know a chemical reaction has taken place in the bag where the water was opened because the matter changed state, and because there was a temperature change. As the baking soda and citric acid combined, energy was absorbed producing an endothermic reaction. That means the bag got colder.

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