Visible reactions
Yasmin Jayathirtha
In the last column, we looked at experiments that suggest that matter is made up of particles and described one that can estimate their sizes. I would like to share some sample data from one class:
Number of drops in one cm3 of oil: 44
Number of pointfuls in one drop: 35
Volume of one pointful (V): 0.0007 cm3 or 7×10-4 cm3
Diameter of the pointful on spreading: 11 cm
Area of the pointful (A): 95 cm2
Thickness of the oil layer (V/A): 7.4 x 10-6 cm
Size of one molecule: 4×10-16 cm3 (assuming that the molecule is a cube)
Number of molecules in one pointful: 1.75×1012
1012 is a huge number but I realized that while the 12-13 year old students could use the scientific notation, they couldn’t really feel it. I asked them to write it out and the class erupted in oohs and aahs. The discussion that followed was interesting; they asked ‘can you see a molecule under the microscope?’ ‘No, not with our eyes, but it was visualized by the STEM probe’. I showed them the picture of the IBM logo made by moving xenon atoms on a metal surface and told them it was just about 35 years old. I pointed out that they themselves had got the size and the number of oil particles in a drop from measurements that had been made in the macroscopic world and with a few inferences and assumptions (all reasonable from our view of the world). The importance of the STEM picture is not just that it visualized something only inferred until now, but how well the inferences agreed with the picture. This is an important learning point because then the students (and the teachers) can do the experiments and make the connection between the atomic and the macroscopic world.
To echo a well-worn phrase; chemistry is ‘all about’ reactions. We see the reactions at the test tube scale and try to make sense of it at the scale at which the reactions are actually occurring – that of atoms, ions and molecules. This leap is the hardest to make because it involves a lot of assumptions and visualizations. This is the skill we need to give students, to see a reaction, write down the observations and then visualize it at the level of the particles. We can use the story of the periodic table as the link; many scientists realized that there is a pattern in reactions, and that it was a bold step to say that the pattern can be used to predict the arrangement. Later on, when the structure of the atom was elucidated, the table did not change; a huge vindication of the observations and reasoning power. It can be pointed out that direct observation is limited by our senses, and science extends these by instruments and experiments.
The periodic table can and should be used in all classes. It should not be memorized, but used like a dictionary; in the younger classes to just make a connection between the names and the symbols. Students enjoy this hugely if they are not compelled to remember information.
An activity to link the reactions to information from the periodic table can be as follows:
Heat some substances and observe. A good selection of chemicals are:
iodine, copper sulfate, wax, tin, sand, salt, sugar, sulfur, copper carbonate, cobalt chloride, lead nitrate, lead monoxide, lead dioxide, zinc carbonate
On heating, first gently and then strongly and cooling;
some appear to remain unchanged – sand, salt,
some change state and return to the original – iodine, wax, tin, sulfur
some change colour – lead monoxide
some rather obviously lose water and return to the original on adding water; copper sulfate, cobalt chloride
some react, i.e., look different at the end of the process; copper carbonate, lead nitrate, sugar, lead dioxide.
We can test for carbon dioxide and oxygen as we heat, bubbling a dropper full of gas from the tube into lime water and putting a glowing splint (agarbatti works well) into the tube.
This data can be used in many ways, but we will look at it from the idea of particles. Get the students to look up the formulae of the compounds they have heated. Does copper carbonate have the right atoms to form carbon dioxide? What is left over? Similarly for the other compounds. Can you get carbon dioxide from lead dioxide? From zinc carbonate? Did the formula of copper sulfate have water?
Thinking over these questions root the students into leaping between the macro and atomic descriptions. Don’t underestimate the difficulty all of us have with this visualization: After a discussion on the elements in the universe making all matter, a student asked ‘you mean all the chemicals here are made up only of these elements?’ and I had to point out that everything including her was made up of the elements and she was very surprised. You will find most adults also have the same difficulty; there are elements and chemicals and then there is stuff that makes up the natural world.
Next time, we will use these reactions and other experiments to understand the quantitative nature of chemistry and how to convert the data to equations.
References
• Richard Dawkins – Queerer than we can suppose: the strangeness of science
• An interesting talk on the ‘middle world’ – our view of the universe. TED talk 2006
The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.
