Mini-Lab 2-05: Why-odine?
Purpose:
Iodine – it’s a simple purple solid. Can you explain its behavior in various liquids?
Think about this:
When something dissolves in something else, it’s because the two substances have
similar intermolecular forces – “like dissolves like”.
Safety
Don’t eat or drink anything in the lab.
Always wear eye protection.
Wear protective clothing (lab coats, etc.).
Don’t play around – treat the lab with respect.
Avoid breathing iodine vapors and try not to get any organic solvents
on your skin.
6) Since some of these solvents are flammable, avoid any open flames
during this activity.
1)
2)
3)
4)
5)
Questions
Drop two or three small crystals of iodine into each of four different test tubes.
1) Pour about 10 mL of water into the first test tube. Mix well and record your
observations.
2) Repeat step 1) for the next two test tubes, using hexanes in one of them and ethyl
alcohol in the other one.
3) Record your observations concerning how iodine reacts with the three different
liquids. What’s different? What’s the same?
4) Pour about 10 mL of aqueous potassium iodide solution into the last test tube and
mix. What do you observe? How does this compare to the first test tube (water)?
Remember that the aqueous solution of potassium iodide is mostly water.
WAIT! Do not write down an answer to the Final question until your Instructor
tells you to.
5) FINAL: Can you explain the differences in the reaction of iodine with the first
three solvents? What do you think is happening in the last test tube? Why?
Instructor’s Page
2-05: Why-odine?
Source: Robert Brent, The Golden Book of Chemistry Experiments (New York, Golden
Press, 1960)
Concepts: molecular nature of matter, intermolecular forces
Materials: iodine crystals, test tubes and rack, water, 0.1 M sodium iodide, ethyl alcohol,
hexanes
Hints: If the students understand that water, ethyl alcohol and hexanes all have different
degrees of polarity, and that CCl4 is the most similar to non-polar iodine, then they
should have little trouble explaining why the iodine dissolves more completely in the
CCl4. Explaining the solvation of iodine in the sodium iodide solution will be the most
challenging, especially if they know anything about solubility rules. They should expect
that having iodine already dissolved in the solution (in the form of iodide) would inhibit
solubility of the iodine. It is due to the formation of an I3 species by this reaction: I2 + I
 I3 (Palmer, D; Ramette, R.; Mesmer R., Triiodide ion formation equilibrium and
activity coefficients in aqueous solution, Journal of Solution Chemistry; 13, 9, 1984) and is
not due to any redox reaction (as the students may think at first). Most students will have
no concept of this obscure complex ion formation, but they should at least think about it
hard enough that they realize that something out of the ordinary is going on.
Discussion ideas: talk about how we have to draw upon both past experience and
additional data when trying to explain something that we observe. A major point of this
activity is that you should be able to recognize when something unexpected happens.
MSDS for Iodine http://msds.chem.ox.ac.uk/IO/iodine.html