By Adam Boxer

Here, Adam Boxer explains his teaching methods and thought processes as he teaches students the topic of Gas Tests in Chemistry.

Gas tests are not exactly the most exciting area of Chemistry to teach. For the non-specialist: there are lots of different gases. Generally, we can’t really see them or sense them directly, so we do special tests to figure out what they are. We normally teach the tests for four gases: carbon dioxide, hydrogen, oxygen and chlorine and even though it’s often thought of as an “easy” topic, students tend to make a lot of errors.

The errors normally fall into four camps:

  1. Clean forgetting them
  2. Muddling them up (i.e. doing the hydrogen test for oxygen)
  3. Getting the steps wrong
  4. Not using the correct framework of “test à result à conclusion”

As with anything, I always start by preparing Core Questions. This allows me to be hyper specific about what I am going to be teaching as well as provide students with a tool to furnish and bolster their long-term memory. In this article, we will only look at two tests; for hydrogen and oxygen:

What is the test for carbon dioxide?Bubble the gas through limewater
What is a positive result for carbon dioxide?Turns limewater cloudy
What is the test for hydrogen?Put a lit splint in the gas
What is a positive result for hydrogen?A squeaky pop

Eventually I will put all the gas tests in a table with headings gas/test/positive result. This helps me meet problems 1 and 2 above, as I will use both the Core Questions and the table as resources for retrieval practice, hopefully helping students to remember the tests as well as the distinctions between them.

Ordinarily, we show students the gas tests as a demonstration. This serves to render an abstract idea quite concrete: they can see and hear the results for themselves, aiding and adding to their understanding.

There is, however, a problem here which relates directly to point 3 above. In order to test for the gases, we first need to produce them. Taking carbon dioxide as a first case, we normally add some calcium carbonate to some acid in one test tube and bubble the gas evolved into limewater in another test tube. Frustratingly, students then describe the test in terms like:

Add the calcium carbonate to the limewater…

Add the carbon dioxide to the acid…

The acid turns cloudy…

I imagine the problem here is that when we describe (or demonstrate) the test, there are too many steps for a student to take in at once. We vocalise the steps, but that vocalisation is ephemeral, it vanishes as soon as it has been spoken, and students cannot hold all those steps in their minds at once. Something has to give, which results in fragmented or confused understanding.

To circumvent this problem, I use my trusty mini-whiteboards (MWBs). But instead of giving them to students to write on, I use them as temporary written supports by holding them in clamp stands. It’s a little difficult to describe in writing what I do, so I have included a diagram below as well to give you a better idea as you follow the script below:

Ok, so in this boiling tube I have some hydrochloric acid. I am going to add some calcium carbonate to it and there will be a chemical reaction [writes calcium carbonate + hydrochloric acid à                  “ on the MWB and holds it up]. Who can tell me what the products of this reaction are… Andy? Good! Calcium chloride, carbon dioxide and water. I’m going to add those to my MWB and attach it here so you can clearly see it.

When I do this reaction, a gas is produced [adds a small amount of calcium carbonate to the open boiling tube]. Looking at it, how can I tell a gas is produced?… Simone? Yep, its bubbling. That’s the gas. Here’s a difficult question though. Can I tell what gas it is by looking at it?… that’s right, I can’t because it’s just a see-through gas. It could be anything! Now we all know that it’s carbon dioxide, but we can’t tell that just by looking at it. Instead, we need to do a test on it.

What I’m going to do is put a bung on the boiling tube that lets the gas move out the boiling tube and into this orange tube [uses physical gesture to show how the gas moves through the tube]. I’m then going to dunk that orange tube into this second boiling tube which has another solution in it. This one is called limewater [writes limewater on the MWB and attaches it].

So again, the gas will be produced here, move through here, and then bubbled through here. It’s really important to note the different steps. Over here, we PRODUCE the gas, and here we TEST for it [adds to MWBs]. Let’s see what happens…

We then run the test, and I have a ready-made diagram like the one below (minus the clamp stands). To check for understanding, I remove all labels from it and ask students the names of the various substances and pieces of equipment, as well as what is happening at each point.

I follow a similar procedure for the production and testing of hydrogen: explain, show, use MWB supports, check for understanding.

Following this, students are ready for independent practice, and among more straightforward ones, I will ask questions like:

  • What are the differences and similarities between testing for carbon dioxide and testing for hydrogen?
  • A student says to test for carbon dioxide, you add calcium carbonate to limewater. Explain why the student is wrong.
  • A student wants to test for carbon dioxide and mixes calcium oxide with hydrochloric acid. Explain why this would not work.
  • In the limewater test we did, where is the calcium chloride at the end of the reaction?
  • Where is the water at the end of the reaction?
  • Where is the carbon dioxide at the end of the reaction?
  • A student is asked:
    Describe how you can test for hydrogen
    And answers:
    Use the squeaky pop test
    Explain why the student’s answer is not good enough.
  • Complete the table below:
Gas testResultConclusion
 Squeaky pop 
 Limewater does not changeCarbon dioxide is not present
Bubble through limewater  
  Hydrogen is not present

These aren’t necessarily “exam questions,” but they do promote deeper thought about the tests.

In sum, we are now much closer to our stated aim of having students not only able to recall the gas tests accurately, but to better understand the process that enable them, the similarities and differences between them, and the correct language to employ when describing them.

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