Testing for Gases

Have you ever heard about the gas known as 'the silent killer'? Its scientific name is carbon monoxide (CO). Of course, you'll never actually see carbon monoxide, or even smell it - this gas is colourless, odourless, and tasteless. These properties make carbon monoxide pretty hard for us to detect by ourselves. But that is an issue. You see, carbon monoxide is extremely toxic. It binds to haemoglobin in our blood and prevents oxygen from reaching our cells, where it is so desperately needed. CO concentrations of 100 ppm (0.01%) cause headaches and shortness of breath, whilst prolonged exposure to concentrations of just 400 ppm (0.04%) CO is likely to be fatal¹.

But although we may not be able to spot carbon monoxide ourselves, we have ways of detecting 'the silent killer'. Carbon monoxide alarms use chemical reactions to measure the concentration of CO in the air and alert us if its levels get too high. These life-saving devices reduce the risk of being accidentally poisoned in our homes and let us sleep a little more soundly. They're a prime example of how we can benefit from testing for gases.

• This article is about testing for gases in chemistry.
• We'll give you an overview of why we test for gases before looking more closely at how we identify specific gases, such as hydrogen, oxygen, carbon dioxide, chlorine, ammonia, and sulfur dioxide.
• We'll then provide a few tips to help you with gas test experiments.
• Finally, we'll finish with a handy testing for gases summary table to pull together your knowledge.

Testing for gases in chemistry

Testing for gases is an example of chemical analysis. We can use these tests to work out the gases produced in a chemical reaction. This is handy because it gives scientists an indication of the type of reaction taking place, as well as its mechanism. Gas tests can also be used in experiments to confirm a hypothesis. For example, what if you wanted to find out for yourself which gas plants release in photosynthesis? You could collect the gas given off and test it, using the methods you'll learn about today. Or perhaps you would like to know if a certain pollutant is present in the air? Once again, a gas test will help you out.

In this article, we'll look at testing for six different gases:

• Hydrogen (H₂).
• Oxygen (O₂).
• Carbon dioxide (CO₂).
• Chlorine (Cl₂).
• Ammonia (NH₃).
• Sulphur dioxide (SO₂).

We'll visit each test in turn, walking you through the process and showing you a positive result. For some gases, we'll also take a deep dive into the chemistry behind the reaction. We'll then look at how you can carry out gas test experiments in class and give you helpful tips on collecting samples of gases.

The first gas we'll learn how to test for is hydrogen (H₂). Although the element hydrogen is the most common element in the universe, hydrogen gas itself only makes up an infinitesimally small part of our atmosphere. This is because it is so light - it simply rises through the air and escapes into outer space. Hydrogen's relative mass meant that, until the mid-20th century, it was often used to help keep airships buoyant. However, hydrogen is also extremely flammable, and the 1937 Hindenburg disaster showed the world just how dangerous hydrogen airships can be². We exploit hydrogen's flammability when trying to identify it.

Here's how you test for hydrogen:

1. Hold a lit splint over the open end of a test tube full of gas.
2. If the gas is hydrogen, it will burn with a loud 'pop'. This is why testing for hydrogen is often known as the 'squeaky pop' test.

Fig. 1: Testing for hydrogen. Holding a lit splint over a test tube filled with hydrogen causes it to burn with a 'squeaky pop' sound.

Testing for gases: Oxygen

Almost all complex organisms on our planet need oxygen gas (O₂) for life. Plants and algae produce oxygen in photosynthesis by harnessing light energy from the sun; animals, bacteria, plants, and humans then use this oxygen in respiration to release their own energy. The average adult human takes in around 550 litres of oxygen per day - that's over 200,000 litres a year³! Simply put, without oxygen, we would die. After all, that's how carbon monoxide poisoning kills us!

Combustion reactions also require oxygen, too. We use this to identify oxygen gas. To test for oxygen:

1. Place a glowing splint inside a test tube full of gas.
2. If the gas is oxygen, the splint will relight and burst into flame.

Fig. 2: Testing for oxygen. Holding a glowing splint over a test tube filled with oxygen causes it to reignite.

Testing for gases: Carbon dioxide

Did you know that one stage of sugar production forms calcium carbonate, the same compound found in coral reefs, eggshells, limestone, and marble? This step is used to remove impurities from the sugar. Limewater and carbon dioxide (CO₂) are added to huge tanks full of raw sugar 'juice' and left for up to an hour. The carbon dioxide reacts with the limewater to form a precipitate of calcium carbonate, trapping any contaminants in the process. The precipitate and impurities are then removed.

We use the same reaction to test for carbon dioxide gas (CO₂) itself. The calcium carbonate formed turns the clear limewater cloudy, indicating a positive result. Here's the process:

1. Bubble a gas through a test tube of clear limewater.
2. If the gas is carbon dioxide, the limewater will turn cloudy.

Fig. 3: Testing for carbon dioxide. Bubbling carbon dioxide through clear limewater causes the solution to turn cloudy. We've coloured the background of the diagram to help you see the colour change more clearly.

Testing for gases: Chlorine

All the gases we've met so far have been colourless and odourless. This makes them tricky to identify. Chlorine gas (Cl₂), on the other hand, is a little easier to spot. It is a yellow-green gas with a pungent, disinfectant-like smell that might remind you of swimming pools. However, to be sure that we actually have chlorine on our hands, we can carry out a simple identification test.

To test for chlorine:

1. Insert damp blue litmus paper into a test tube filled with gas.
2. If the gas is chlorine, the litmus paper will bleach white.

Fig. 4: Testing for chlorine. This gas bleaches damp blue litmus paper white.

Testing for gases: Ammonia

We'll briefly look at two more gas tests, starting with ammonia (NH₃).

Like chlorine, ammonia has a distinct smell - you might compare it to window cleaner. Also like chlorine, we test for ammonia using litmus paper, but we look for a different result:

1. Insert damp red litmus paper into a test tube filled with gas.
2. If the gas is ammonia, the litmus paper will turn blue.

Fig. 5: Testing for ammonia. This gas turns damp red litmus paper blue.

Testing for gases: Sulfur dioxide

Finally, let's learn how you test for sulfur dioxide (SO₂). Sulfur dioxide is another pungent gas that smells a bit like burning matches. As we mentioned earlier, it also bleaches damp blue litmus paper white, so it is easy to get SO₂ mixed up with Cl₂. However, we have a further test we can use to tell the two gases apart.

To identify sulfur dioxide:

1. Bubble a gas through acidified potassium dichromate solution.
2. If the gas is sulfur dioxide, the solution will turn from orange to green.

Fig. 6: Testing for sulfur dioxide. This gas turns acidified potassium dichromate green.

Flame tests for gases

The first two gas tests we learned about (testing for hydrogen and oxygen) were both types of flame tests.

• Hydrogen gas (H₂) causes a lit splint to burn with a characteristic' squeaky pop'.
• Oxygen gas (O₂) relights a glowing splint.

You might read in other resources that you can also use a flame test to identify carbon dioxide (CO₂). This is partly true. If you insert a lit splint into a test tube full of CO₂, the splint will go out. However, the same thing happens if you use other gases, too. For example, a lit splint will also be extinguished if you put it in a test tube full of nitrogen gas! For this reason, we use limewater to test for CO₂. At GCSE level, you should only use flame tests to identify H₂ and O₂.

Testing for gases experiment

That's the theory out of the way - how about testing for some of these gases in class? We'll now give you some tips for carrying out gas test experiments. In particular, we'll focus on producing a known gas so that you can check that your gas tests really work. We'll cover:

• Reactions that produce each of the gases we've tested for so far.
• Different ways of collecting the gas.
• Any hazards that you need to be aware of.

We'll then finish with a summary table that pulls together all that we have learned.

Producing the gas

First up, you have to produce a known gas. This allows you to see for yourself that the gas tests actually give a positive result for the right species! To produce all the gases we've considered today, you simply mix specific reactants together in a flask or test tube. You should see bubbles as gas is given off. You can then carry out the gas test directly over the mouth of the container, or collect the gas and test it separately.

Collecting the gas

Your teacher or lab technician might get you to both produce and test for the gas in the same container. For example, you might mix the reagents needed to make oxygen in a small flask and hold a glowing splint over the flask's mouth. The oxygen produced will relight the glowing splint.

However, you might instead collect the gas and test for it separately. There are different ways of doing this, depending on the properties of the gas itself.

• If the gas is insoluble or only slightly soluble in water, you can collect it in an inverted test tube over water.
  • This method is suitable for collecting O₂ and H₂.
• If the gas is lighter than air, you can collect it in an inverted test tube using upward delivery.
  • This method is suitable for collecting NH₃ and H₂.
• On the other hand, if the gas is heavier than air, you can collect it in a test tube using downward delivery.
  • This method is suitable for collecting Cl₂, SO₂, and CO₂.
• You can also collect any gas using a gas syringe. However, you then might have to transfer it to a test tube before use.

Fig. 7: Different methods of collecting gases. The top diagram shows a flask containing reactants, which react together to give off a gas. The gas leaves the flask via tubing and flows into a test tube, using one of the collection methods shown in the bottom three diagrams.

Hazards

Like with all practicals, you need to be aware of any potential hazards.

Some gases are toxic. Their tests must be carried out in a fume cupboard. The reagents used to make the gases themselves could also be toxic or corrosive, so must be handled with care; wearing eye protection is always essential:

• Some gases or reagents are flammable. Keep them away from open flames - apart from if you are testing for hydrogen, of course!
• Be careful when handling glassware and lit splints.
• Take measures to avoid spillages.
• Report any breakages, spillages, or other injuries to your teacher or lab technician.

Testing for gases: Summary

To wrap up this article, here's a handy table that brings together all that we have learned today about testing for gases. It covers not only the tests and their positive results, but also tells you the reactants used to produce each gas and any further notes about hazards.