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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.
Like other chemical analysis techniques (such as testing for ions), the best gas tests give a clear, definite, unique result for their target gas, and that gas only. You want a gas test to tell you the identity of your gas, with no ifs, buts, or ambiguity! However, the simple gas tests that we'll learn today aren't that useful for mixtures of gases - they're best for pure species. Bear this in mind if you decide to run any experiments yourself.
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.
Check with your exam board to find out which gas tests you need to know about. For example, your examiners might not expect you to be able to identify sulfur dioxide (SO2). However, that doesn't mean you can't read about these tests - learning more is never a bad thing!
Testing for gases: Hydrogen
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:
- Hold a lit splint over the open end of a test tube full of gas.
- 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.
Explaining testing for hydrogen
Why does hydrogen 'pop' when it burns, whilst other flammable substances don't? It is because hydrogen releases a tremendous amount of energy when it combusts - even more than petrol or coal. This energy causes the particles to move faster than the speed of sound, which creates a sonic boom - although when in class, you use such small quantities of hydrogen that it is hard to call the 'squeaky pop' produced a 'boom'!
Testing for hydrogen equation
Here's the equation for the test for hydrogen:
$$2H_2+O_2\rightarrow 2H_2O$$
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:
- Place a glowing splint inside a test tube full of gas.
- 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:
- Bubble a gas through a test tube of clear limewater.
- 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:
- Insert damp blue litmus paper into a test tube filled with gas.
- If the gas is chlorine, the litmus paper will bleach white.
Fig. 4: Testing for chlorine. This gas bleaches damp blue litmus paper white.
Explaining testing for chlorine
You might notice that the damp blue litmus paper turns red before it bleaches white. This is because litmus paper tells you about the pH of a solution. Here's what happens:
- The chlorine gas reacts with the water on the damp paper to produce hydrochloric acid (HCl). This turns the paper red.
- The reaction between chlorine gas and water also produces the hypochlorite ion (ClO-). ClO- is a bleaching agent that can kill microorganisms, which is why we add chlorine to swimming pools (hence the smell). ClO- also bleaches litmus paper, removing its colour and turning the paper white.
- You should be aware that the gas sulfur dioxide also bleaches damp blue litmus paper white. That's OK - we can use acidified potassium dichromate to distinguish between chlorine and sulfur dioxide, as you'll find out later on in this article.
- You'll probably come across litmus paper again when learning about acids and alkali. Acids have a low pH and so turn litmus paper red, whilst alkalis have a high pH and turn litmus paper blue.
Testing for gases: Ammonia
We'll briefly look at two more gas tests, starting with ammonia (NH3).
We mentioned this earlier, but we'll repeat it again - check with your exam board to find out which gas tests you need to know about.
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:
- Insert damp red litmus paper into a test tube filled with gas.
- If the gas is ammonia, the litmus paper will turn blue.
Fig. 5: Testing for ammonia. This gas turns damp red litmus paper blue.
Explaining testing for ammonia
Do you remember what we said about litmus paper? Alkalis turn it blue. Ammonia becomes an alkali when it meets water, which is why the damp red litmus paper changes colour.
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:
- Bubble a gas through acidified potassium dichromate solution.
- 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.
Explaining testing for sulphur dioxide
Have you come across acidified potassium dichromate before? You might have met it when learning about alcohols. Acidified potassium dichromate is an oxidising agent, meaning it gives oxygen to another species. We use it to oxidise alcohols into carboxylic acids. Check out the article Alcohols for more information.
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.
Check out the summary table at the bottom of this article to find out the reactants you use to produce each gas.
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.
| Gas | Reactants | Test and positive result | Notes |
| H2 | Zinc + dilute acid | Lit split burns with a 'squeaky pop' | Flammable - keep away from open flames when not testing |
| O2 | Hydrogen peroxide + manganese(IV) oxide | Relights a glowing splint | |
| CO2 | Marble chips + dilute hydrochloric acid | Turns limewater cloudy | |
| Cl2 | Concentrated hydrochloric acid + potassium manganate (VII) crystals | Bleaches damp blue litmus paper white | Toxic - carry out the experiment in a fume cupboard |
| NH3 | Ammonium chloride + calcium hydroxide | Turns damp red litmus paper blue | Toxic - carry out the experiment in a fume cupboard |
| SO2 | Sodium sulfite + dilute hydrochloric acid | Turns orange acidified potassium dichromate green | Toxic - carry out the experiment in a fume cupboard |
Testing for Gases - Key takeaways
- Chemical tests help us identify the products of a reaction and confirm a hypothesis.
- We can identify gases using the following tests:
- Hydrogen (H2) causes a lit splint to burn with a 'squeaky pop'.
- Oxygen (O2) relights a glowing splint.
- Carbon dioxide (CO2) turns limewater cloudy.
- Chlorine (Cl2) bleaches damp blue litmus paper white.
- Ammonia (NH3) turns damp red litmus paper blue.
- Sulfur dioxide (SO2) turns orange acidified potassium dichromate green.
- When carrying out gas test experiments, you need to consider ways of both producing and collecting the gas, as well as any hazards.
References
- 'Effects of Carbon Monoxide Poisoning'. Healthy Hearty
- 'Hindenburg disaster'. Wikipedia
- 'How much oxygen does a person consume in a day?' Sharecare
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Frequently Asked Questions about Testing for Gases
How do you test for gases in electrolysis?
When conducting the electrolysis experiment, the gases can be collected in the test tubes and tested separately later to identify them.
How do you test an unknown gas?
To identify an unknown gas you might have to go through several procedures, as each gas has its own tell-tale sign. If you have an idea of which gas it might be, you can start with the test that was designed to identify that gas. For example, to see if your gas is hydrogen (H2), you set a lit split on the border of the beaker where the gas is. If you hear a squeaky pop, that's hydrogen.
How do you test for hydrogen and carbon gases?
To test for hydrogen (H2) you set a lit split on the border of the beaker where the gas is. If you hear a squeaky pop, that's hydrogen.
To test for CO2, you bubble the gas through limewater. If the solution turns cloudy, that's CO2.
How do you test for oxygen, hydrogen and carbon dioxide?
To test for oxygen (O2), you set a glowing split on the border of the beaker where the gas is. If it relights, that's oxygen.
To test for hydrogen (H2) you set a lit split on the border of the beaker where the gas is. If you hear a squeaky pop, that's hydrogen.
To test for carbon dioxide (CO2), you bubble the gas through limewater. If the solution turns cloudy, that's carbon dioxide.
What are the 3 gas tests?
The 3 gas tests are:
- Hydrogen: set a lit split on the border of the beaker where the gas is. If you hear a squeaky pop, that's hydrogen.
- Oxygen: set a glowing split on the border of the beaker where the gas is. If it relights, that's oxygen.
- Carbon dioxide: bubble the gas through limewater. If the solution turns cloudy, that's carbon dioxide.
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