Skip to 0 minutes and 10 secondsMaking a salt doesn't just have to be about a technique. We can also use the practical to test students' understanding of some of the math that supports chemistry. In this case, working out theoretical fields using molar calculations. This means that the learning objectives for this practical are about the maths and calculations rather than focusing on the procedure for producing salt crystals. One of the simplest reactions is to use sodium hydroxide and hydrochloric acid to produce sodium chloride and water. Here we're going to react quantities of known concentration in a very simple way. Using 0.1 molar solution of sodium hydroxide and 0.1 molar solution of hydrochloric acid reduces the risk, as well as the volume of chemicals required.
Skip to 0 minutes and 53 seconds25 centimetres cubed of 0.1 sodium hydroxide is added to 25 centimetres cubed 0.1 molar hydrochloric acid in an evaporating dish. Using a Bunsen burner, gently heat the solution to evaporate around three quarters of the liquid, then allow the solution to cool. Salt will precipitate out over a period of time. If the evaporating base is left in a warm place for a few days, the remainder of the solution will evaporate. Normally, when solutions are prepared it's almost impossible for sodium hydroxide and hydrochloric acid to be made up accurately. This does mean often the sodium hydroxide is slightly less concentrated and calculated, which is an advantage here.
Skip to 1 minute and 34 secondsHaving a very slight excess of hydrochloric acid can help prevent any sodium hydroxide being present in the final salt. If you make up the sodium hydroxide solution, consider diluting the sodium hydroxide by 5% with distilled water to ensure that all the sodium hydroxide has been neutralised. Once the students have made their solutions, they can then work out what they should have produced, the theoretical yield. To do this, they will need to show understanding of molar masses, concentrations, and volumes. Then they can weight their dry salt and calculate the percentage yield
Not assessing practical skills
In step 3.1 we asked you why you include practical work in your lessons. In this step we explore the way practical work doesn’t necessarily need to involve an assessment of practical skills. Students in the 14-16 age range are expected to become familiar with molar calculations, and being able to apply them to reaction equations.
In this example, we are looking to react an acid with a base to produce a salt (and water). There are a number of ways of carrying this out, here we are using measuring cylinders for the liquids, but there are other alternatives.
Students might carry out an acid-base titration (a whole skill-set in itself) to determine the concentration of the base being used, using an acid of known concentration and a suitable pH indicator.
Once they have determined the quantities of acid and alkali to reach the end point (neutralisation), students can repeat the experiment to make a salt solution and then evaporate the water to extract the salt.
In our case, we are assuming that the solutions are as indicated, although as suggested, we can make the alkali a little more dilute, so we have neutralised all the sodium hydroxide in our example.
Students could apply their scientific knowledge to their results, and work out the mass of salt they should produce (theoretical yield). Once the water has evaporated, they can look at the actual yield for the practical, and work out their percentage yield.
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