In this lesson students will learn about what boiling point is and why different liquids have different boiling points by taking notes from a lecture. They will then use the concept to try to identify different unknown liquids.
This lesson demonstrates the NGSS cross-cutting concept of Structure and function. The way in which an object or living thing is shaped and its substructure determine many of its properties and functions. How the different liquids are structured in terms of their polarity and molecule size determines their boiling point, but we do not wrestle with this idea at this time. Later on in the semester, when students are studying intermolecular attractions, I will remind them of this example of different boiling points, and we can look at the nanoscale reason for it.
This lesson corresponds with the NGSS Science Practice Planning and carrying out investigations because students will use the concept of boiling point to investigate the identity of unknown liquids. It corresponds to the NGSS Science Practice of Analyzing and interpreting data because students must compare the data they have collected to known values in order to figure out the identity of the liquids they are investigating.
Students do not need any prior knowledge to conduct this lAnalyzing and interpreting data esson.
To do this lab you will need:
A hot plate
4 test tubes
A 250 ml beaker
10 ml of acetone, ethyl alcohol, and isopropyl alcohol labeled Liquid A, Liquid B, and Liquid C
Please note that I have been conservative in my choices. While there are many more liquids with low boiling points than the ones I have chosen, I have chosen not to use liquids such as methanol because it is so so flammable.
During this time I take attendance and interact with students who need additional assistance in getting started with class.
Students have come to appreciate the opportunity to draw as an activator, and the process of reading and writing about the procedure helps us to review it more quickly and in a more fun way than if I simply went over the procedure.
I show Boiling Point slides and explain the idea of boiling point at the molecular level. I remind students that what we see at the macro level often has an underlying reason at the nanoscale. Because each substance has a unique composition at the nanoscale, each substance has a unique boiling point. This property can be used to identify unknown liquids. Students use the Boiling Point Notes Organizer to record these ideas. After the lecture is done, I post the Boiling Point Notes Organizer answers on the projector and students check to make sure they understand their notes.
Then the class views a a student's boiling point cartoon and we discuss it in terms of each step in the procedure, matching the cartoon square to steps in the procedure. This helps the whole class engage in the procedure in a fun way. The safety concerns for today are using flammable liquids near a hot plate, using high heat, boiling water and glass. Students are asked how to be safe today, and they respond with many responses, including watch where you are going, do not spill chemicals, use tongs to move hot glass, inspect glass before using for cracks or chips, do not leave glassware near the edge of the bench, do not put hot glass on a cold surface, and wear safety goggles.
Application: Students conduct the lab. This picture of the boiling point lab shows what the set-up looks like. I ask students to be careful to record data every 60 seconds and watch for when the materials in the test tube began to boil. I ask students to record data every minute so that they are careful to watch the temperatures and look for evidence of boiling constantly. The alcohols with lower boiling points only boil for a little while; if left to boil over a several-degree increase in temperature they will eventually stop boiling (perhaps as the concentration of the alcohol diminishes due to boiling) so it is important to catch the boiling as soon as possible.
After students have seen the boiling points for the different liquids I ask them to compare their data and try to identify which liquids they had using the third slide from the boiling point slides.
Catch and Release Opportunities: Lab safety is a very important component of the chemistry classroom. By this point in the year students may test the idea that they have to wear safety goggles. I am vigilant about this. I also watch to make sure that every member of every group is recording all of their data. Throughout the year this is a cultural piece; I do not want students to claim that they cannot process the learning that was intended for a lab because they did not have data.
We compare student data, first looking at student choices for A, B, and C, then looking more closely at the individual boiling points that students chose for each liquid. In our lab A was acetone, B was ethyl alcohol, and C was isopropyl alcohol. Most students are able to make a reasonable guess as to which substance they have. The close proximity of the two alcohols makes it unlikely that the students will get exact answers, but these chemicals in these amounts are safer to use than other chemicals that might have more divergent boiling points.
Because there are discrepancies in the Student Boiling Point data, I ask students to explain why that might be, in light of the fact that each substance only has 1 boiling point. Student responses include the difficulty of the procedure and the proximity of the boiling points.
The main objective of this lesson was achieved. Students came away from the lesson understanding that different liquids have different boiling points, and this fact can be used to identify unknown liquids.