How to Test for CO₂

Carbon dioxide (CO₂) is colorless and odorless, so you can't detect it through direct observation. You'll need to collect an air sample (or a CO₂ sample), then run one of several simple tests to identify the presence of the gas. You can bubble the gas through limewater, or you can hold a lit splint into the sample to see if it is extinguished by the presence of CO₂.

Method 1

Method 1 of 3:

Preparing a Sample

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To begin your test, you will need a sealed test tube filled with collected gas. You can collect carbon dioxide in a gas jar, a boiling tube, or another airtight container. Collection is usually performed over water in a beaker. CO₂ gas is denser than air, so you can collect it using "downward delivery" or a gas syringe.
2
Mix calcium carbonate with hydrochloric acid (HCl). The simplest way to collect carbon dioxide is to react calcium carbonate (or limestone chips) with hydrochloric acid. First, pour 20 mL of HCl into a conical flask. Add a spoonful of calcium carbonate (or limestone chips) to the HCL. When the reaction starts, cover the conical flask with a bung and delivery tube: you will collect the gas through the delivery tube and into an upturned test tube (which is immersed in bowl of water). If the water in the test tube is displaced, then gas is being collected.^{[1]XResearch source}
- You can continue to collect the gas for as long as the reaction occurs.
- For classroom demonstrations, you only need a small amount of hydrochloric acid, which is diluted to 1 M; a 2 M concentration works best, but should be used with extra care because it's a strong acid. The equation is: CaCO₃(s) + 2HCl(aq) ==> CaCl₂(aq) + H₂O(l) + CO₂(g).
- Be very careful when working with hydrochloric acid – wear gloves, a lab coat, and protective goggles, and do not let the acid touch your skin! It's best to only perform this reaction if you have access to a lab environment.^{[2]XResearch source}
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Put the tube on a rack to keep it safe until you perform the test. The "bung" is essentially a small cork or cap that allows you to pipe the contents of the test tube elsewhere through a connected delivery tube. It's important to seal the CO₂ gas into the container. If you leave it open, the gas will mix with the air, and your test will be much less effective.

Method 2

Method 2 of 3:

Bubbling Through Limewater

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1
Bubble the gas through limewater. The most effective way to test for CO₂ is to bubble the gas through "limewater", a diluted solution of calcium hydroxide (slaked lime). When you bubble carbon dioxide through the solution, it forms a solid precipitate of calcium carbonate – chalk or limestone. Calcium carbonate is insoluble in water.^{[3]XResearch source} Thus, if there is CO₂ present in the sample, the limewater will turn milky, cloudy white.^{[4]XResearch source}
- Limewater is also called "white wash" or "milk of lime." If you see these terms, you'll know they mean limewater.^{[5]XResearch source}
2
Make a limewater solution. The process is simple: dilute calcium hydroxide with water. Calcium hydroxide (Ca(OH)₂) is a colorless white powder that you can buy from most chemistry suppliers. Pure limewater, once mixed, is clear and colorless, with a slight earthy smell and the bitter, alkaline taste of calcium hydroxide, as its pH is 12.3. Follow these steps to make your own limewater:
- Put 1 teaspoon (4.9 mL) of calcium hydroxide into a clean 1 gallon (3.8 L) or smaller glass jar. Limewater is a saturated solution, which means there will be some extra chemical that doesn't dissolve. A teaspoon will result in a fully saturated solution, whether you use a gallon jar or a smaller container.
- Fill the jar with distilled water. It's best to use distilled water for this experiment, as it will give you a purer solution.
- Put a lid on the jar. Shake the solution vigorously for 1-2 minutes, then let it stand for 24 hours.
- Pour the clearer solution off the top of the jar through a clean coffee filter or filter paper. Be very careful not to stir up the sediment. If necessary, repeat this filtering step until you obtain a clear limewater solution. Store in a clean jar or bottle.
3
Bubble the gas through the limewater. Half-fill a test tube with limewater – then boil it. Use a delivery tube to pipe the contents of the CO₂ sample test tube directly into the boiling limewater. You can use a flexible pipe or a (metal) straw as a delivery tube, if nothing better is around. Let the captured gas "bubble" through the liquid, and wait for the reaction to take place.
- If you don't want to boil anything, you can use a gas syringe to discharge the CO₂ gas directly into the half-filled limewater test tube. Stopper the test tube, then shake vigorously for 1-2 minutes. If there is carbon dioxide in the sample, then the solution should grow cloudy.^{[6]XResearch source}
If CO₂ is present, the limewater will turn milky white with calcium carbonate particulates, as the calcium carbonate solution is insoluble and will precipitate out of the solution. If the limewater is boiling, and the gas is piped directly into the limewater, then the reaction should begin immediately. If nothing happens after a minute or so, you can safely assume that there is no carbon dioxide in your sample.^{[7]XResearch source}
5
Know the chemical reaction. Understand what exactly is taking place to indicate the presence of CO₂. The equation for the test is: Ca(OH)₂ (aq) + CO₂ (g) ==> CaCO₃ (s) + H₂O (l). In non-chemistry language: liquid limewater + gas (which contains CO₂) react to solid lime (the particles) and liquid water.
- If you bubble the CO₂ through the limewater for an extended period of time, your water will eventually turn clear. That's because of a chemical reaction! Here's why:
  - CO₂ + H₂O ==> H₂CO₃ (carbon dioxide + water = carbonic acid)
  - H₂CO₃ + CaCO₃ ==> Ca(HCO₃)₂ (carbonic acid + calcium carbonate = calcium hydrogen carbonate)
  - Calcium hydrogen carbonate is soluble in water, making your solution clear!

Method 3

Method 3 of 3:

Testing with a Lit Splint

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1
Try using the sample to extinguish fire. Carbon dioxide extinguishes fires, in high concentrations. You'll simply need to hold a small lit flame inside a test tube that you suspect contains CO₂. If the gas is present, the flame should go out immediately. Combustion (creation of a flame) is the reaction of oxygen with another substance; it is a rapid oxidation of the organic compound and a reduction of oxygen. The fire goes out because the oxygen is replaced by the CO₂, which is not a combustible gas.
- Bear in mind that any oxygen-free gas will also extinguish a flame in this manner. Thus, this is an unreliable test for carbon dioxide, and it may lead you to misidentify the gas.^{[8]XResearch source}
Make sure that the sample has been properly stored and stoppered before you try to test for CO₂. Be reasonably certain that the test tube does not contain any flammable or explosive gases; in this case, the introduction of fire could be dangerous, or at least very frightening.
Use a splint or any long, thin strip of wood. In a pinch, a match or a lighter will do – but the further your hands are from the opening of the test tube, the safer your experiment will be. If the flame immediately goes out, there's likely a high concentration of CO₂ in the test tube.
Fill a syringe with carbon dioxide. Then, use a drop of molten wax to affix a short candle to the surface of a coin. Next, place the candle and coin into a wide-mouthed cup – and light the candle. Equip the syringe with the tubing, and push the syringe to transfer the CO₂ to the bottom of the cup. If you push out the entire contents of the syringe within a second or two, the flame should go out.

Expert Q&A

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Question
Why should the test for carbon dioxide be performed immediately after collection of water sample?
Meredith Juncker, PhD
Scientific Researcher
Meredith Juncker is a PhD candidate in Biochemistry and Molecular Biology at Louisiana State University Health Sciences Center. Her studies are focused on proteins and neurodegenerative diseases.
Meredith Juncker, PhD
Scientific Researcher
Expert Answer
You want to perform the test immediately because the carbon dioxide that's present in the water can escape into the atmosphere if you wait.
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Is carbon dioxide harmful to humans?
Meredith Juncker, PhD
Scientific Researcher
Meredith Juncker is a PhD candidate in Biochemistry and Molecular Biology at Louisiana State University Health Sciences Center. Her studies are focused on proteins and neurodegenerative diseases.
Meredith Juncker, PhD
Scientific Researcher
Expert Answer
It depends on how much carbon dioxide is present. In small amounts, it's not harmful to humans. However, if a large amount of carbon dioxide is present and displaces the oxygen in the air, it can cause respiratory issues for humans. The lack of oxygen can cause a person to breath rapidly and experience rapid heartbeat. If the lack of oxygen happens for a long time, the person might have damage to the brain, heart, and other organs.
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What happens to the calcium hydroxide solutions when carbon dioxide bubbles through it?
Community Answer
Calcium hydroxide, or limewater, will turn milky/cloudy when carbon dioxide is bubbled through it.
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