Demonstrating sodium's reaction with water
Equipment needed:
- The sodium metal will be stored in a glass or metal jar, under oil. This is to prevent reaction with water moisture found in air. It will appear dull due to an oxide layer formed on the outside of the metal (over time, oxygen can penetrate the glass/oil). Using tweezers, remove a chunk of sodium from the oil and place it on the paper towel/newspaper. Holding the sodium with the tweezers (it is inadvisable to touch it with your bare hands – it may react with moisture on your fingertips) use the scalpel to cut into the block of sodium. It is a soft metal, similar to hard butter out of the fridge, and very easily cut with a sharp implement. The inside will be shiny and metallic-looking.tweezers
- scalpel or cutting tool
- paper towel/newspaper
- phenolphthalein indicator
For a small-scale demonstration:
- Cut a piece about the size of a pea.
- In a large beaker, half filled with water, add some phenolphthalein indicator. This indicator is clear in acid or neutral solutions, and pink in basic solutions. As the reaction proceeds and produces sodium hydroxide, the solution will turn from colourless to pink.
- Ensure all observers are wearing safety goggles and are a minimum of 1 metre (3 feet) away, then add the sodium to the beaker. It will fizz around on the top of the water, leaving a pink trail wherever it goes. If enough hydrogen gas is produced, the intense heat of reaction will cause it to ignite.
- Once all the sodium has reacted, all that remains is sodium hydroxide solution, which can be washed down the sink.
Large-scale reaction - Explosion
For a large-scale demonstration you must go outdoors. Again, safety goggles must be worn. The container should be a large bucket (metal or plastic) half full of water. Do not use glassware – the resulting explosion will shatter it:
- Take a piece of sodium slightly smaller than a golf ball.
- Using the scalpel, cut into it many times while still keeping it as a single piece of metal. This exposes much more surface area of the sodium to water – ensuring an impressive result.
- Take note of the wind conditions, and have all observers standing upwind at least 8 metres (24 feet) away.
- Using the tweezers, drop the sodium into the container and move to a safe distance. It will take approximately 10-20 seconds before it explodes (consider asking observers to cover their ears – it is very loud). The explosion can sometimes result in small amounts of unreacted sodium being spread over the immediate area. This should be hosed down with water until there is no more left.
- The sodium hydroxide remaining in the bucket should again be washed down the sink.
Either as an introduction to basic chemistry, or as a special treat for more advanced classes, this demonstration can be safely performed, and is one that students will remember for a long time.
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