Density of soap

Tuesday saw lecture/lab day one. Units of measurement and density in a single go. Forgot to buy sugar  water bottle, picked those up right before class.

Accidental plus this term: the stores were out of 500 ml bottles, so I purchased a liter bottle of water. 1022 grams. Fortunately a student also had a 1000 ml water bottle, finished the water, and this allowed determination of the bottle to be 29 grams.

Began with space, time, matter, and energy. Worked my way to density over the course of an hour. 

Wrapped up with the density of water, floating versus sinking densities. Then the class took the 11:00 break. 

At 11:30 the lab started with the rearrangement of the morning equation and a hand waving argument that if the density were a constant, a linear relationship exists between the volume and the mass. The students expressed the opinion that the density varied with the size of the soap, which is a logical conclusion if one does not grok how ratios actually work.

In the laboratory session the exploratory questions were framed as:

• Does the density change with the size of the soap?
• If the density does not change and is a constant value, then what is the density of the soap?
• And... does that density predict whether the soap will float or sink in water?

The demonstration was done using Silka soap.

Which proved more problematic than expected. Squaring this off was lengthy and inaccurate.

An attempt to measure the volume of the whole bar by displacement was a failure. 

The bar displaced 90 cubic centimeters, massed 86 grams, and sank like a stone  in water. That was a fail right there. 

The listed mass on the box was 90 grams which suggested an almost neutral buoyancy.

The density measurements also suggested neutral buoyancy. As a demonstration of the lab this was not a desired result. The rate at which the soap sank argued against near neutral buoyancy. Silka was a multilevel fail. 

Full attendance today.

The laboratory went no better. The Ivory soap all sank. Yet two groups were pushed to use more maximal readings out of the expectation that the Ivory would float. One group was switched to calipers for more accurate measurements, but their inaccurate underestimates of length, width, and height would have yielded a self-consistent result. Their volume would have been inaccurately low, but their prediction of sinking would have come true. 

The group with the Dial made either a measurement or calculation error that led to a density estimate of 0.5 grams per cubic centimeter. Their Dial, however, sank.

Only the Jergens group had floating soap and predicted floating. One Ivory group predicted sinking. I expected Ivory to float, but the group was right. The Ivory sank. Older Ivory sinks.

All-in-all the results were confusing at best for the students. The Dyson quote was not borne out. Summer session is off to a sinking start. 

The session wrapped with how to complete a report. The class slid past 14:00 and final clean-up was done at 14.22

The next day would include a return to the laboratory report to clarify the structure and contents.