Procedure:
1. Place approximately 0.3 grams solid benzoic acid (structure shown above) in a 10-cm test tube. Add a single drop of phenolphthalein solution to the test tube.
2. Fill the test tube about half-full of distilled water and agitate to try to dissolve the solid. When you are confident that agitation will serve no further purpose, stop agitating. Use a piece of pH paper and forceps to test the pH of your benzoic acid “solution”.
3. Set up a bunsen burner, and with goggles firmly in place, heat your test tube to a gentle boil. Start by heating at the middle of the test tube at the water level and work your way down to the bottom of the test tube. Continue heating until you have a nice, clear solution. Be very careful here--it is quite easy to have the test tube boil over in a sudden rush--try to avoid this by heating gently, but also keep yourself in a position where you won't be burned if it does boil over.
4. Once you have the clear, hot solution, carefully test its pH. Then allow the tube to cool without disruption for 5 to 10 minutes in a test tube rack. Periodically look for evidence of crystallization. While you wait, ponder question #1 on the back of this page.
5. When you are confident that the crystallization is complete (and the temperature has returned to near room temp), obtain a single pellet of solid NaOH. Add this pellet to the tube and see what the NaOH does to your benzoic acid crystals. Allow the NaOH to act "on its own" for a couple minutes. While you wait, discuss questions #2 & #3 on the back of the page.
6. Agitate the tube to mix everything together. Then consult with your instructor to see whether you may proceed.
7. Your task is now to return your material to the nice, crystalline state that you had in the beginning of step 5. You may have access to anything in the chem stockroom. Discuss your chosen method with your group to make sure everyone is in agreement before actually carrying out the experiment. Make sure that you get crystals in the end, not some cheap imitation.
Write-up instructions: On a separate sheet(s) of
paper thoroughly answer the questions. Give
thoughtful explanations when requested. Your
explanations should include chemical
equations and/or molecular pictures.
Questions to be pondered as you do the experiments and answered for the
write-up: (Use this page for your notes,
not the final copy.)
1. Your pH test results should show that the solution becomes more acidic (lower pH number) when heated. This lower pH value means there are more free H+ ions in the solution.
