Unraveling the Mystery of "Floating Rule" in an Experiment

[MaggiesScienceConnec]

Today I was leading my students in an experiment that would reveal "the rule for what floats." We had previously floated (or sank) various objects in regular, room temperature tap water. I then weighed out 100 grams each of sugar and water, mixed them together and weighed them again. To my surprise, instead of having 200 grams of sugar water solution, I had 125 grams.

At first I thought we had not tared our scale properly, so I poured the mixture into a newly tared beaker. We still came up 75 grams short. We then weighed out another 100 grams each of sugar and water. This time I tared the beaker containing the water back to 0.0 grams and then poured the newly weighed 100 grams of sugar directly into the water as it was on the scale. It only increased the weight by 25 grams.

Then I thought there was something wrong with how the scale tared the beaker and weighed the beaker separately. It weighed about 115 grams, so this could not account for the loss.

Given that you cannot destroy matter, can someone help me understand why adding the water to the sugar in equal weight resulted in a mixture that weighed less than 200 grams?? I'm flummoxed and I can't seem to find an explanation on the internet. I promised my students I would try to figure out why this happened.

 

[Doc Al]

Given that you cannot destroy matter, can someone help me understand why adding the water to the sugar in equal weight resulted in a mixture that weighed less than 200 grams??

It's got to be some sort of measurement error. Try this: put the sugar in a zip lock bag and dunk it in the water and see if the same issue arises. (That would eliminate issues with dissolving.)

 

[Merlin3189]

I've just tried myself and within the accuracy of my kitchen force meter (2g) the results add up.
As Doc says, has to be faulty method or equipment.
Perhaps you could record actual readings for us to look at? Like
Empty sugar container = 20g
S.Container & sugar = 119g
Empty water container = 75g
W.Container and water = 177g
W.Container and water and sugar = 276g or whatever it came out to.

Have you tried a different force meter or have you tried calibrating the one you are using?

 

[MaggiesScienceConnec]

I FINALLY figured it out! The OHaus scale I was using had a 200 gram limit and when I tared the 115 gram beaker back down to 0 grams, I exceeded the limits of the scale! I used a 5000 gram scale, and it added up to 200 g. Phew! I was SO perplexed. I was even asking my science colleagues how this could possibly be. It was such a reproducible result. No one could answer, but I thought I'd try another scale, and when it worked, it finally dawned on me what the problem was. My students will have a good laugh over this! Great lesson! So much for having a Ph.D.!

 

[MaggiesScienceConnec]

By the way, thanks so much for your thoughts. Much appreciated!

 

[Merlin3189]

Finally! Exactly! You knew something was wrong and kept going until you sorted it out. 10/10

 

[Doc Al]

Good work!

 

[NTL2009]

I think your students learned something more important than the planned lesson! Now you just need to convince them that was the plan all along. ;)

Hah, easy to say in hindsight, but always check your instruments over the full range you expect to measure.