Skip to 0 minutes and 10 seconds The reaction of carbonates and oxides with acids is a key chemistry practical. Here we’re going to react sulfuric acid with copper oxide to produce copper sulphate crystals. We’re going to use a classic setup now, which is often found in textbooks and schemes of learning, which is reacting copper oxide with heated sulfuric acid. We’re going to use 50 millilitres of 1 molar sulfuric acid and we’re going to put that into a 100 mil beaker. And then we’re going to heat this until it almost reaches boiling point.
Skip to 1 minute and 0 seconds OK, the sulfuric acid is almost starting to boil. So we’re going to take the heat away and put the Bunsen on a safety flame. And now we’re going to add some copper oxide, about 1/2 to a 1/4 a spatula full at a time. Beware that students often put in large amounts and that can lead to overflowing.
Skip to 1 minute and 25 seconds We will add 1/2 a spatula full and then gently stir. Most of that copper oxide will react.
Skip to 1 minute and 36 seconds When the reaction has subsided, we add another amount of copper oxide.
Skip to 1 minute and 44 seconds And we keep repeating this until there’s no more fizzing when we add, and we’re left with a residue of black copper oxide at the base of the beaker.
Skip to 1 minute and 56 seconds We need to make sure that we have added sufficient copper oxide to the sulfuric acid so that all the acid is neutralised. This may take 3 to 4 grammes of copper oxide to do this. Sometimes more.
Skip to 2 minutes and 24 seconds Now that’s stopped reacting, we’re going to heat this up very slightly to bring it close to boiling point again, whilst gently stirring.
Skip to 2 minutes and 37 seconds This will help make sure that all the copper oxide has reacted with this sulfuric acid.
Skip to 2 minutes and 52 seconds Now that’s almost boiling, I’m going to take the Bunsen away and turn it off.
Skip to 3 minutes and 1 second We now need to let this cool for two to three minutes before we can start filtering. Now we’ve let this cool for a while, we can safely pick up the beaker. And make sure students don’t pick up the beaker while it’s still hot. Sometimes they will try and wrap paper towels around it to pick it up, but we want to try to avoid them doing that. So I’ve fluted a philtre paper, or you can fold it conventionally, to sit into a philtre funnel. And I’m now going to pour my solution through so we get a clear solution.
Skip to 3 minutes and 41 seconds What I’m going to try and do now is get the last bit of my black solid into the philtre paper. Copper oxide is not good for the environment. So what we want to make sure happens is that any copper oxide that we use will end up going into the bin a not down the sink. So now our solution has gone through and we have a blue filtrate. Left behind in the paper is our copper oxide and it’s that now going to be disposed of through the bin. And ideally, anything left inside our beaker we will scrape inside that as well. So we’ll make sure that as much of that goes in there as possible.
Skip to 4 minutes and 18 seconds So now we’ve got our copper sulphate solution in our evaporating basin. We’re going to heat it with a roaring Bunsen flame to bring it to the boil and reduce the level of the liquid by at least half.
Skip to 4 minutes and 33 seconds So we’re now at the stage where students should have stopped heating because we were getting through to the stage of almost removing all the water that’s here. And one of the issues of this practical is that students will often keep heating. So even if I take the heat away now, what may happen is that we actually drive off too much water, and then we can get instances of spitting happening.
Skip to 4 minutes and 58 seconds We’re now going to take the heat away.
Skip to 5 minutes and 7 seconds And if students have gone this far then they will have probably ended up not with crystals, but with more of a fine powder. And as the last bits of water evaporate, we’re starting to see spitting coming out of our crucible.
Skip to 5 minutes and 23 seconds If students carry on heating that, they will drive out the water of crystallisation from the copper sulphate, which will leave anhydrous copper sulphate. And at that point, large amounts will actually be ejected from the evaporating basin. Here’s some copper sulphate crystals we made earlier. We allowed our solution to evaporate slowly over a period of a week. So our crystals are large compared to the very small powdery residue we got by heating very quickly our solution. As you can see, this practical uses quite a lot of reagents and it also has some in built problems. We start off by having to almost boil our sulphuric acid.
Skip to 6 minutes and 2 seconds And if students do allow it to boil, then we can end up with sulphur dioxide coming out of the solution, which again, is a toxic gas. Also, they’re having to stir a beaker on top of a Bunsen, burner whilst adding quite a lot of copper oxide. That means there’s more capacity for students to knock things over. After they’ve filtered their solution, they evaporate it. If they haven’t neutralised all the sulfuric acid and they heat it to dryness, again, we can have a decomposition go on and we can end up with more sulphur dioxide. So although this reaction can produce some lovely big crystals, there are safer ways of doing this practical.
Skip to 6 minutes and 39 seconds And so next time we’re going to show you a better way of doing this practical.
Opportunities for assessment: making a salt
Copper sulfate crystals are a mainstay of practical chemistry. Students may have grown big crystals much earlier in their education, from solutions and a seed crystal.
This practical looks at making a salt, by reacting an acid with a carbonate or oxide, to produce a well-known crystal.
The reaction here is quite large scale, and it uses quite a lot of different techniques, some of which are familiar to students, others may be less so.
This reaction also has some hazards, as discussed in the video. It also generates a lot of waste; copper compounds are not good for the environment, especially if they get into rivers and lakes.
Opportunities for learning
A good learning outcome from this practical may be to research the damage that copper compounds can do and to identify ways to reduce the waste from this activity.
More able students might be able to calculate, in advance, the mass of copper oxide required to react with the concentration and volume of the acid they are given. They can certainly practice their equation skills.
If students carry out the practical, their blue salt that starts to crystalise out will go almost white if they heat it too much and it will often start spitting out of the evaporating basin. This happens as they drive off water by excessive heating. This does introduce the idea of water of crystallization and it may be worth demonstrating the addition of water to anhydrous copper (II) sulfate, which is an exothermic reaction.
In the next step we’ll look at making copper sulfate crystals with a smaller setup, allowing for a better use of classroom time.
Mark has suggested a few alternative learning points when making a salt. One aspect that offers opportunity for context to engage students and provides an alternative learning activity of research, is the environmental impact of this particular experiment.
What other topics or experiments lend themselves to research-related tasks?
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