Physical Science

Experiment 2-3

Mass During a Chemical Reaction


Background Information

By now you probably have a hunch about the reliability of mass as a property to measure the quantity of substances.  Scientists as well as non-scientists get such hunches from time to time.  But the success of science is due to scientists continuing to check to see if their hunches are correct.  The more tests that confirm the hunch, the more confident scientists are that the hunch is correct.  Eventually, after many confirmations, after scientists become very confident their hunch is right, they begin to call it a fact.  But note that science never PROVES an idea is correct;  it just compiles evidence of support.  And if someday an experiment seems to contradict the hunch, then the hunch may be restricted to the conditions where it is correct, or perhaps revised.

Now it should be noted that there is a false view spread by a few people that if some evidence ever contradicts an idea in science, then that idea is immediately found false and must be rejected by scientists.  That view may be consistent with some religious beliefs, but it is a false portrait of how science works.  Almost NEVER is an idea in science rejected because of a few contradictions.  First of all, the contradictions must be checked because maybe those observations were flawed or the measurements tainted by experimental errors.  And even if the contradictions are confirmed, if the idea still works MOST of the time, then it will continue to be used until it can be improved or a more successful replacement found.

It may seem strange, but scientists are eager to find ideas that are flawed!  If contradictions are found and confirmed, the ideas are IMPROVED.  That is how science normally progresses.  It is extremely rare that old ideas need to be totally replaced.  It is this process of checking out hunches, rechecking, and making improvements so that our understanding of the world gets better and better that has resulted in our highly developed civilization.


We are searching for a reliable method to measure materials.  We know that volume often changes as a result of a chemical reaction.  This experiment investigates if mass also changes during chemical reaction.

Considering the sensitivity and limits of your balance, find a small container and a balloon that can be stretched over the container's neck so as to contain the contents.  You will also need small amounts of milk and vinegar.  Wear eye protective goggles for this experiment.  Vinegar, while a weak acid, is corrosive.   If you get any vinegar in an eye, flush the eye with water for 15 minutes.apparatus

  1. Put a little bit of milk in the balloon.  If you get too much, discard the excess rather than return it to the original container.  (This is for the same reason it is bad manners to double dip in the fondu, salsa, or chip dip after taking a bite.)
  2. Fill the container less than half full of vinegar.  (Again, it is bad practice to return excess to the original container.)
  3. With the top of the balloon drooping to keep the milk inside, place the mouth of the balloon over the neck of the container.
  4. Before allowing any mixing, find the total mass the apparatus and contents.
  5. Lift the balloon to allow the milk to flow into the container and curdle with the vinegar (forming curds and whey).
  6. After the curdling chemical reaction is complete, find the mass of the apparatus and its contents.
Does the mass change significantly during the chemical reaction?

Since no measurement is perfect, do the same experiment several times then average the changes in mass.  To reach a conclusion, use the results that seem consistent and where no errors are apparent.  Of course if a leak or spill occurs, those results should not be included.

Make a histogram of your results to visualize any pattern in your measurements.

Record in your science journal the mass measurements, first in table format, then by constructing a histogram.  Write a Formal Report if you need to earn credit.

Optional Experiment

There are a great many more chemical reactions.  Some are so slow that many days would be required.  Others are so fast that it would be difficult to contain with the balloon.  Many reactions such as occur in living beings are too difficult to easily isolate and study.  With your current understanding of chemistry it is probably prudent to avoid attempting chemical reactions which you incompletely understand.  However, if you have a balance capable of measuring larger masses and a container capable of withstanding the heat, you might try burning a small candle inside a sealed container, looking for mass changes.  Caution:  Always try unknown reactions first with very small amounts of materials.  Increase to larger amounts in small increments because unforeseen dangers sometimes arise when larger amounts are used..


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created 11/3/2002
revised 12/9/2003
by D Trapp
Mac made