It is the classic high school chemistry experiment. A small amount of sodium metal dropped into a beaker of water, fascinating a group of students as it fizzes and skips over the surface of the water, perhaps even bursting into flame. Exciting to perform, and an informative demonstration of a vigorous chemical reaction.
This article will provide you with the know-how to safely perform this (and larger scale) demonstrations to classes of school students. Sodium is readily available to high school science teachers, and can also be purchased over the internet.
Sodium (atomic number 11) is an extremely reactive alkali metal (meaning it forms alkaline solutions when mixed with water), with an atomic mass of 22.99 g/mol. It has a number of uses in industry, as well as in everyday life. For example, table salt is also known as sodium chloride – a sodium containing compound. It should be noted however, that sodium in this form is very stable, unlike pure elemental sodium. Other uses include soaps, sodium streetlights and fireworks.
Reaction: Sodium with water
Sodium reacts violently with water in an exothermic (heat producing) fashion, producing sodium hydroxide and hydrogen gas. The complete equation is:
2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
As shown, the reaction produces hydrogen gas (extremely flammable) and sodium hydroxide (lye, or caustic soda), so safety is of great importance. The immense heat produced by this reaction can be enough to ignite the hydrogen gas, causing the sodium to burst into flame as a result.
Demonstrating sodium’s reaction with water
- scalpel or cutting tool
- paper towel/newspaper
- phenolphthalein indicator
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.
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.
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.
“Sodium: The essentials” WebElements: the periodic table on the web. webelements.com