Modeling Chemistry Workshop – Day 2 Part 2
Mass & Volume Lab
Goal: To Graphically and Mathematically Model the Relationship between mass and volume of an unknown liquid
|What Can We Measure?||How Can We Measure It?|
|Volume||graduated cylinder (mL)|
Need 6-10 data points
Whiteboard summary to include: graph, slope of best fit line, “For every” statement
Groups in our workshop each tested one of two unknown liquids. Our group’s best fit line had a slope of 0.7905 g/mL, so our “For every” statement was: For every 1 mL of unknown B, there is 0.7905 g of B.
Mass & Volume Lab Discussion
In whiteboarding this one, our leader rolled dice to figure out who would present. 1st dice was the group number. For the 2nd dice, “odds” meant people in spots 1&3 presented, “evens” were spots 2&4. This is a great way to make sure everyone is ready to present!
What do you need to get out of this discussion:
- the physical meaning of the slope from the graphs – amount of mass for every unit of volume (density!)
- the physical meaning of the y-intercept – no volume = no mass, so the y-intercept is zero
- if we have samples with different densities it means we have samples that are of different substances
- Reinforce: independent & dependent variables, significant figures, “for every” statements
An addition to “The Story So Far”: Density is the relationship between mass and volume. In a graph of mass vs. volume, density is the slope of the best fit line. Samples with different densities mean samples that are of different substances.
Students should get the following graph and the general version of the line (mass = density*volume) into their notes:
This is listed as an optional activity in the Modeling Chemistry materials. I had tried it one year without much success and in subsequent years cut it out. I will be adding it back this year with a couple changes to make things a bit easier.
The activity calls for using a paint stirring stick as a glug and having students measure the length and width of a whiteboard with an unmarked glug to the nearest tenth. (You’ll want to make sure whatever you measure is not an integer multiple of your glug.) They then calculate the area. Point out differences in values between groups – if measurements were to the tenths place, should area be reported to the hundredths? why is there so much variation in area (usually about 10%)?
Next, have the students calibrate their glugs to the tenths place. (This is where some of my students struggled, because the process mentioned in the official materials is a bit difficult involving lining the stirrer up to span 22 lines over two sheets of notebook paper.) Once this is done, students repeat measurements, this time measuring to the hundredths place. Variation in areas should be smaller and you can again suggest that calculated area should not be reported with more decimal places than our measurements.
I love that this activity is such an organic way to introduce the value of significant figures.
What adaptations will I try with this?
- a contrived area at the end that is an integer value – so we can discuss an area like 4.0 square glugs
- substitute the paint stirrers with another material (possibly reusable) to eliminate the need for so many paint stirrers – ideas from our workshop: whiteboard strips, overhead strips, cardstock
- setting up another way to calibrate the glugs – maybe just have them be twenty notebook paper lines long and be able to line them up with one sheet of paper?
After this activity you can go through some standard notes on measurement and complete Unit 1 Worksheet 2.
The sequence mentioned here is slightly different from the one in the Modeling Chemistry materials.
Next Up: Unit 1 Worksheets 3-6, Density of a Gas Lab, Unit 1 Lab Practical