Safe ways to 'Wow!' with salt

2nd July 2004 at 01:00
Students always find this demonstration exciting - obtaining sodium chloride, common salt. It uses many basic practical techniques and gives the teacher opportunities to cover several areas of the curriculum at key stages 3 and 4. We combine a teacher demonstration that has real "Wow!"

factor with two other activities that students can safely do themselves.

We do the reaction of sodium and chlorine as a demonstration behind safety screens. The gas chlorine is prepared beforehand by heating concentrated Hydrochloric acid (HCl) with Manganese dioxide (MnO2) in a flask. We don't add any water to the flask as a small amount of water collects at the bottom of the flask as the gas collects above it. We find that when we add a small piece of sodium (Na) to the flask the reaction of the sodium with this water is enough to set it on fire and initiate the reaction with the chlorine. Sodium chloride is produced as a white cloud, which is quite spectacular and has been known to blow the lid off the gas jar!

Students can make salt themselves using the reaction of hydrochloric acid and sodium hydroxide (NaOH). Check the Hazcards for the appropriate concentrations. 10ml of acid is added to a beaker plus a few drops of universal indicator. 10ml of like concentration sodium hydroxide (NaOH) is added until neutrality is achieved. Students enjoy the challenge - a reward is given to the first group to achieve the required green colour. After adding a spatula of charcoal students filter the solution and evaporation leaves salt.

Students can also separate salt from impure rock salt. After grinding and dissolving, the solution is filtered and again evaporated. The procedure allows key vocabulary to be revisited and pupils can also be encouraged to use their numeracy skills to determine the percentage of salt in their sample of rock salt.

It is also a good opportunity to discuss some of the traditional methods of filtration: these include leaves, woven cloth and soils. Pupils can explore the extraction of salt in many cultures, and often students can contribute to this discussion with examples provided by family and friends from around the world.

Mandip Bahra, second in science, and Stuart Batt, head of science The Cressex Community School, Bucks

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