How to Test for CO2

Three Methods:Preparing a SampleBubbling Through LimewaterTesting with a Lit Splint

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
Preparing a Sample

  1. Image titled Test for CO2 Step 1
    Collect a CO² sample. 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. Image titled Test for CO2 Step 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 20ml 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]
    • 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 1M; a 2M concentration works best, but should be used with extra care. 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]
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    Cover the test tube with a bung. 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
Bubbling Through Limewater

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    Bubble the gas through limewater. The most effective way to test for CO² is to bubble the gas through "lime water", 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] Thus, if there is CO² present in the sample, the limewater will turn milky, cloudy white.[4]
  2. Image titled Test for CO2 Step 5
    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. Follow these steps to make your own limewater:
    • Put 1 teaspoon of calcium hydroxide into a clean one-gallon (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.[5]
    • Fill the jar with distilled water or tap water. Distilled water will always give you a purer solution, but the minerals present in tap water should not hinder your test.
    • 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. Image titled Test for CO2 Step 6
    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]
  4. Image titled Test for CO2 Step 7
    Look for cloudy water. If CO² is present, the lime water will turn milky white with calcium carbonate particulates. 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]
  5. Image titled Test for CO2 Step 8
    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.[8]

Method 3
Testing with a Lit Splint

  1. Image titled Test for CO2 Step 9
    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.[9]
  2. Image titled Test for CO2 Step 10
    Collect the gas in an upside-down test tube. 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.
  3. Image titled Test for CO2 Step 11
    Place a small flame inside the test tube. 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.
  4. Image titled Test for CO2 Step 12
    Alternately, try using a gas syringe to put out a candle. 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.

Things You'll Need

  • Limewater
  • Heat-able test tube
  • Gas syringe
  • Delivery tube
  • CO² sample (or just water)

Article Info

Categories: Science for Kids