We ran out of time last week with one activity left, so we started this week’s class labeling a periodic table with arrows showing trends in ionization energies, atomic radius, electronegativity, etc. This activity was from the Atoms and Chemistry Interactive Notebook Unit Bundle by Stephanie Elkowitz.
Then we jumped into another lab from 24 Lessons That Rocked the World by Ian Guch. In this lab I put 5 different substance in centrifuge tubes marked 1 through 5. The lab only calls for four unknowns but I included distilled water as a fifth unknown. Two of the substances were ionic (rock salt and sodium acetate) and the other two powders, coconut sugar and agar were the covalent samples. We talked a bit about the differences in ionic and covalent compounds and what tests we could perform to figure this out. Covalent compounds tend to have a lower melting point so students placed small samples of the unknowns in crucibles and placed them over the butane burners.
If the substance melted quickly then they knew it had a low melting point. Some never melted, even after several minutes, indicating a high melting point, and some just turned into a black powder.
Another test is to look at the material under a microscope (sodium acetate to the left) to determine if it has a crystalline structure, which is more commonly found in ionic compounds. And finally the students dissolved some of the substances in distilled water and checked its conductivity with a multimeter. Ionic compounds are more likely to dissolve in water and the resistance was measured with a multimeter. If the resistance dropped (increased conductivity) significantly then the substance might be ionic.
Any one of these tests alone is not really enough to conclude whether something is ionic or covalent so the students had to look at all their test results before making a conclusion.
At the end of class I showed the following two youtube videos, Dogs Teaching Chemistry – Chemical Bonds by snuggliepuppy and chemical party (this one just cracks me up).
I had asked the students to watch some of the videos below before coming to class.
or if they prefer Tyler DeWitt