11. Chromatography – Pure Substances and Mixtures

This Friday we had a “snow day”, which means the amount of snow, or in this case freezing rain, that fell overnight rendered the roads too dangerous for sensible people to risk their lives driving to the school buses are cancelled. In our school boards the school buses get canceled but the schools never close, so all the teachers must report for duty! In addition, because it is a “regular school day” we must all carry on with activities that are directly related to the curriculum, not watching movies or playing games. (Unless they are curriculum related, of course.)

In my case, I view these “snow days “ as opportunities to do some fun labs, that I might not otherwise do, so we get the best of both worlds -- a learning activity that is related to the curriculum but a fun activity. I want to make sure that the students who show up for class have an enjoyable learning experience but those who did not make it into school are not left behind in the dust.

Our grade nine class is deep into the chemistry unit at present. We have mastered the concept that matter is subdivided into pure substances and mixtures, etc. So we took the time of Friday to do a simple Smarter Science chromatography lab to explore mixtures and pure substances further.

1. Initiate and Plan = ENGAGE

I started the class by explaining chromatography to them in real simple terms. I modelled how I could put samples on a start line on filter paper and place the bottom of the paper into a beaker with a small amount of water in it. The water moved up the paper by capillary action dragging the components of the sample with it. Before class I had prepared several chromatography papers which I dried and showed to the students. In all cases the samples I used were dots of ink from a variety of markers that I had lying around my classroom. When we examined the finished chromatographs we could observe either one solid coloured line or a line that split into a prism of colours. I posed the questions “Which inks made solid lines and which inks split into a prism of colours?” the students were quick to assert that if more than one colour could be seen in the chromatograph the ink must have been a mixture whereas chromatographs with a solid colour must have been a pure substance.

I then distributed the Smarter Science starburst diagram so we could determine all the variables that could be controlled or tested for in a similar experiment that the students would run. We brainstormed six items.

1. The amount of time the chromatograph was run

2. The amount of liquid placed in the bottom of the beaker

3. The size of the ink dot

4. The colour of the markers

5. The brand of the markers

6. Size of paper

The students discussed the list of variables with their partner to decide which one they would test for. They moved the post it note with their dependent variable to the head of the fish diagram. The other post it notes became their independent variables and were moved to the “bones of the fish diagram.

2. Perform and Record = EXPLORE

The partners continued to design their experiments by planning the values to give each on the independent variable, ie. How long to run the experiment, how much water to use, what brand of markers to use, etc. Once they had determined their variables they began experimenting.

Yet again I had the pleasure of watching student’s experience a lab technique for the first time and react in wonder. When the inks actually began running up the paper they were amazed! This short video clip illustrates the joy experienced by one group as they produced a rainbow.

3. Analyze and Interpret = EXPLAIN

The students analyzed their chromatographs to determine if the various inks they used were pure substances or mixtures. They compared their results with each other while classifying their inks.

4. Communicate = EXTEND

Because it was a snow day and these students had actually made the effort to come to school I allowed them extra time to reflect on this lab and play with the technique. I had them explain their results to me orally and write a one sentence conclusion.

N.B. The students were pleased that although they had come to class on a snow day they “didn’t have to do any work” and were allowed to experiment instead! After the lunch break we blew up a gummi bear and a sour patch kid (two different types of candies), in two separate explosions, using potassium chlorate. We followed up with a discussion of which one caused the greater reaction therefore contained the most reactant (sugar). Then we played a Periodic Table game using the smart board wherein they had to identify elements by the number of orbitals and outer most electrons they contained. Lot’ s of fun! No work! And even the administration is happy!