Introduction
When a small piece of sodium is placed in a petri dish with water and phenolphthalein, the sodium becomes a molten ball and darts around the dish, leaving a pink trail. The reactions of potassium, lithium, magnesium, and calcium with water can also be shown.
Equations
Oxidation half-reaction: Na Na+ + e-
Reduction half-reaction: 2 H2O + 2e- H2 + 2 OH-
Overall reaction: 2 Na (s) + 2 H2O (l) 2 H2 (g) + 2 NaOH (aq)
To Conduct Demonstration
- If it hasn’t been done already, add phenolphthalein to the crystallizing dishes.
- Pick up a piece of sodium or other metal (not potassium) with the forceps and remove excess oil with a paper towel.
- Place the sodium a small petri dish that is sitting on the lab bench. Very quickly thereafter, to ensure that the metal is not spitting on the students nearby, transfer the dish to the overhead projector. If the sodium sticks to the side, push it with the forceps.
- Repeat, if desired, with calcium (fizzes), lithium (slow bubbling), and magnesium (no reaction), or zinc(no reaction).
- For potassium, perform steps as above, but place large crystallizing dish in the middle of the bench. Leave it there as the potassium reacts. The potassium will burst into flames if an adequately large piece is used and has, historically, jumped out of the dish (hence the placement far from students. Be prepared to repeat the potassium, as this is a very popular reaction with the students.
- Student volunteers can be used to drop all but the potassium, and maybe the non-reactive metal, into the water.
- There is a YouTube video of Rb and Cs
Demo Time: ~10 – 15 minutes
Safety and Disposal
Explosions can occur when large pieces of potassium are placed in water. The dish may be covered with a large watch glass to prevent spattering on the projector. The water left after the reactions can be sink disposed.
Acknowlegment:
L.R. Summerlin and J.L. Ealy Jr., Chemical Demonstrations: A Sourcebook for Teachers, 1985