The sodium in water chemistry demonstration illustrates the reactivity of an alkali metal with water. This is a memorable demonstration that creates a spectacular reaction for students. Still, it can be performed safely.
What to Expect
A small piece of sodium metal will be placed in a bowl of water. If a phenolphthalein indicator has been added to the water, the sodium will leave a pink trail behind it as the metal sputters and reacts. The reaction is:
2 Na + 2 H2O → 2 Na+ + 2 OH- + H2(g)
The reaction is especially vigorous when warm water is used. The reaction may spray out molten sodium metal and the hydrogen gas may ignite, so use proper safety precautions when conducting this demonstration.
- Never use a piece of sodium larger than a pea or pencil eraser.
- Wear safety goggles.
- Perform the experiment behind a clear safety barrier or at a distance from the students.
- Sodium metal stored under mineral oil
- A 250 mL beaker, filled halfway with water
- Phenolphthalein (optional)
- Add a few drops of phenolphthalein indicator to the water in the beaker. (Optional)
- You may wish to place the beaker on an overhead projector or video screen, which will give you a way to show the reaction to students from a distance.
- While wearing gloves, use a dry spatula to remove a very small chunk (0.1 cm3) of sodium metal from the piece stored in the oil. Return the unused sodium to the oil and seal the container. You can use tongs or tweezers to dry the small piece of metal on a paper towel. You may wish to allow the students to examine the cut surface of the sodium. Instruct the students that they can look at the sample but must not touch the sodium metal.
- Drop the piece of sodium into the water. Immediately stand back. As water dissociates into H+ and OH-, hydrogen gas will be evolved. The increasing concentration of OH- ions in the solution will raise its pH and cause the liquid to turn pink.
- After the sodium has reacted completely, you can flush it with water and rinse it down the drain. Continue to wear eye protection when disposing of the reaction, just in case a bit of unreacted sodium remained.
Tips and Warnings
Sometimes this reaction is performed using a small piece of potassium metal instead of sodium. Potassium is even more reactive than sodium, so if you make the substitution, use a very small piece of potassium metal and expect a potentially explosive reaction between the potassium and water. Use extreme caution.