Chem Riddles. Number 1

Reprint from Chem 13 News, September 2000, page 17

Yehoshua Sivan  
Menachem Begin High School
Safed, Israel

Yehoshua has agreed to coordinate this new column.  He has four riddles ready to go—after that it depends on you!   

Chem Riddle #1.  What causes the black stain?

For this you need a piece of marble, limestone, dolomite or other light coloured carbonate stone, and a couple of centimetres of magnesium ribbon.  If you can’t find the stone lying around in your area, an ornamental mason will have plenty of scraps.

Holding a piece of magnesium ribbon with tweezers, ignite it and let it burn in air, and collect the white oxide ash.  (Incidentally, if you hold an inverted 400 mL beaker over the burning magnesium, you can collect the smoke on the inner surface and show that it is a solid—useful when teaching about colloids.)  Now take an identical piece, ignite it, and place it on the stone while it is burning.  A black stain is formed, in addition to the white ash.

The riddle posed is, “What causes the black stain?”

Didactic points

I let my students do this by themselves, rather than as a demonstration.

Obviously the students are going to have to know that they are dealing with a carbonate.  They could be told, or they might want to make a couple of simple tests (acid and limewater to identify the CO2).

I used the riddle in the context of learning about the effect of heat on calcium carbonate, but it wasn’t that obvious to most of the Grade 12 class that this was a miniature version of the major industrial process.

They also have to apply their knowledge of the order of affinity for oxygen, between magnesium and carbon;  alternatively, this exercise could be a launching point for discussing the subject.

This is also a good example of endo- and exothermic reactions, and of how the heat from one reaction is used to drive another.

Finally, and perhaps most importantly, I introduce the riddle with the story of how it came to my notice this year.  A Grade 10 student, Yaakov Weinstock, was “playing” with a little piece of magnesium ribbon that I had given him, and while it was burning he accidentally let it fall on the tiled floor.  He bent down to look at the product and noticed a black stain on the floor.  He came to me to ask what was the cause of the stain.  This is a classic example, I feel, of the importance of allowing students to "play", and of encouraging them to relate to every single observation, no matter how trivial it seems.  It is also an example of how a “discovery” is not only dependent on these factors, but also on the propitious circumstances—in this case that the tiles are made from limestone scraps cemented together, the ubiquitous flooring in Israel.

The solution

        2 Mg(s)  +  O2(g)  -->  2 MgO(s)                      delta H < 0

        CaCO3(s)  -->  CaO(s) + CO2(g)                       delta H > 0

        2 Mg(s)  +  CO2(g)  -->  2 MgO(s)  +  C(s)       delta H < 0  

Want another of Yehoshua Sivan's riddles? Riddle 6