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)

Prior to Lecture  (prep time ~ 5 to 15 min.)

Assemble the following:

  1. Heat resistant mat
  2. Dropper bottle of phenolphthalein
  3. Vials of metals pieces the correct size to react in the vessels: Li, Na, K, Ca, Zn
  4. Forceps
  5. Spatula for cutting metals
  6. Paper towels for blotting oil from metal pieces
  7. Box of Kimwipes, also for blotting the oil from the metals
  8. One large (~190mm dia) crystallizing dish for the potassium
  9. One smaller (100mm) crystallizing dish for each of the other metal pieces 
  10. Goggles – for instructor and student volunteers, if any
  11. Gloves – for instructor and student volunteers, if any
  12. Sheet of transparency plastic to protect the overhead screen

Prepare

Fill small crystallizing dishes with ½” of distilled water.  Fill large crystallizing dish with 2” of distilled water.  Add phenolphthalein at this point if the instructor wishes or give the bottle to the instructor to add during the performance of the demo.  (Talk to the instructor about whether phenolphthalein has been added as demo is set up in the lecture hall).

Cut appropriate sized pieces of metal with spatula (metal is soft) and place in the labeled vials of oil provided.  NOTE:  Older, oxidized sodium requires more careful removal of oil and a larger sample.  Newer, fresher sodium requires only a small, pea-sized or smaller chunk.

To Conduct Demonstration

  1. If it hasn’t been done already, add phenolphthalein to the crystallizing dishes.
  2. Pick up a piece of sodium or other metal (not potassium) with the forceps and remove excess oil with a paper towel.
  3. 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.
  4. Repeat, if desired, with calcium (fizzes), lithium (slow bubbling), and magnesium (no reaction), or zinc(no reaction).
  5. 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.
  6. Student volunteers can be used to drop all but the potassium, and maybe the non-reactive metal, into the water.
  7. 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.