Video Transcript
In this video, we will learn about how to identify gases and water using chemical tests. Imagine you are performing a reaction in the lab, and your reaction mixture starts bubbling and giving off a colorless gas. How would you know what this gas is? Well, there are some easy tests to determine the identity of the common gases. The common gases include hydrogen, oxygen, ammonia, carbon dioxide, sulfur dioxide, and chlorine. Water is another common substance we can test for, although it is often found as a liquid in room temperature rather than as a gas. Most of these substances are colorless and odorless. And so it is not always possible to identify them through sight and smell. And so we use chemical tests to identify these gases.
The tests that we are about to discuss should not be done in isolation. It is always best to do other methods of analysis too to confirm the identity of a substance. Before we discuss how to identify each of these gases and water, let’s discuss how you would collect an unknown gas so that it can be tested. How a gas is collected depends mostly on whether it is more or less dense than air. The gases that are more dense than air, for example, carbon dioxide, chlorine gas, sulfur dioxide, and oxygen, can be collected by downward delivery. And those that are less dense than air, ammonia and hydrogen, are usually collected by upward delivery.
In downward delivery, the gas which is heavy or more dense than air moves into the gas jar and sinks to the bottom. At the same time, it displaces the air that was in the gas jar or pushes it out. In upward delivery, a gas which is lighter than air, in other words, less dense than air, will move out of the reaction vessel to the top of a gas jar. Simultaneously, the air that was in the gas jar will be pushed out or displaced by the collecting gas. Sometimes, gases which are not soluble in water are collected by a slightly different method to these two. For example, oxygen gas which has a low solubility in water can also be collected by downward displacement of water.
In that method, the gas is bubbled through and into a test tube full of water, and as the gas collects so the water will be displaced from the test tube. Now that we know how gases are collected, let’s jump in and have a look at the specific tests for these gases.
Test for hydrogen gas: If a gas collected in a gas jar or a gas given off from a reaction was suspected to be a hydrogen gas, we could confirm its presence using a lit or burning wooden splint. If a burning splint is placed near the mouth of the vessel and the gas is indeed hydrogen, a popping sound would be heard and the flame would be extinguished. Hydrogen gas is flammable in the presence of oxygen and a heat source. It undergoes a combustion reaction, reacting with oxygen to produce water vapor. This is a highly exothermic reaction. Much energy is released in the form of a mini explosion, and that is why we hear a popping sound.
Test for oxygen gas: This test also uses a wooden splint. However, here we use a glowing wooden splint instead of a lit or burning wooden splint. When the glowing splint is held near the mouth of a vessel containing a gas which we suspect is oxygen, if the gas is oxygen indeed, the glowing splint will burst into flame and ignite. This is because there is an increase in the concentration of oxygen gas, which will increase the vigor with which the wooden splint burns. It will no longer burn just with a glowing heat, but will burn with a flame.
Let’s have a look at the next test. Carbon dioxide gas does not support combustion. It would cause a lit splint to be extinguished. However, we saw that hydrogen gas also causes a lit splint to be extinguished. Ammonia gas does this too, so we need a unique test for carbon dioxide. If we have a gas which we suspect is carbon dioxide, we can bubble it through a solution of calcium hydroxide known as lime water. If the gas is indeed carbon dioxide, it will react with the calcium hydroxide. Calcium carbonate and water will be produced. Calcium carbonate is a white insoluble precipitate. These tiny white solid particles will be dispersed throughout the lime water solution, making the lime water appear cloudy or milky. This milky appearance tells us that the unknown gas was indeed carbon dioxide.
Test for ammonia gas: When an unknown gas either in a gas jar or being produced from a reaction is suspected to be ammonia gas, moist red litmus paper can be held near the mouth of the vessel. And a color change will be seen to occur. After some time, the red litmus paper will turn blue. Ammonia gas will react with water in the moist litmus paper to produce NH4OH, which is ammonium hydroxide. Ammonium hydroxide consists of the ammonium ion and the hydroxide ion. The hydroxide ion is alkaline, making the red litmus paper turn blue.
Note that any gas which is soluble in water which forms hydroxide ions will also turn red litmus paper blue. So this test is not unique to ammonia. However, of the common gases that we learn about and that we listed at the beginning of this video, ammonia is the only one which will produce hydroxide ions on the moist litmus paper, turning it from red to blue.
The test for chlorine gas is very similar to that for ammonia gas, except that it does not produce alkaline hydroxide ions on the moist litmus paper. Let’s have a look. To test for a gas which we suspect is chlorine gas, we would need to hold moist blue litmus paper near the mouth of the vessel containing the unknown gas. The blue litmus paper would turn red if the gas was indeed chlorine. This is because when chlorine gas reacts with the water in the moist litmus paper, it produces two types of acid, hydrochloric acid and hypochlorous acid. And acid turns blue litmus paper red.
It is the H+ ions from the acids which can react with water in the moist litmus paper producing acidic hydronium ions. The hydronium ion is H3O+. If the litmus paper is held in the presence of the chlorine gas for some length of time, the red color will eventually be bleached to white. This is due to the oxygen in the hypochlorous acid. So the litmus color change from blue to red to white will indicate the presence of chlorine gas.
Test for sulfur dioxide gas: When a gas is suspected to be sulfur dioxide, we can test for the spot bubbling through a solution of potassium permanganate. Potassium permanganate, or KMnO4, contains the permanganate ion, MnO4−. This is also known as the manganate(VII) ion. And this is what gives the solution its purple color. When the solution is slightly acidified, sulfur dioxide and the manganate(VII) ion will react with each other. The reaction is an oxidation–reduction reaction and produces some complex products.
Only two of the important products have been shown here, the Mn2+ ion and the SO4 2− ion. The Mn2+ ion is colorless. And this change from purple to colorless indicates that an oxidation–reduction reaction occurred. And knowing that MnO4− is an oxidizing agent and that SO2 is a reducing agent, we can conclude that sulfur dioxide was the gas present causing this color change.
We’ve looked at tests for common gases, but what about a very common liquid? And that is water. Test for water: The following test is commonly used for water in the liquid phase. However, it can be used to test for water vapor. The colorless, odorless liquid which is suspected to be water can be added drop by drop onto anhydrous white copper(II) sulphate. Anhydrous means without water, in other words, without waters of crystallization. If the colorless liquid is indeed water, the white anhydrous copper(II) sulphate will turn blue. This blue form of copper(II) sulphate is hydrated copper(II) sulphate, which means it contains waters of crystallization.
The equation for this hydration process is shown at the bottom. It is the presence of the waters of crystallization which turns the copper in copper sulfate from white to blue. A similar test for water uses blue anhydrous cobalt(II) chloride. And this turns pink when it gains waters of crystallization because of the presence of water. Remember, there are many gases and liquids which cannot be identified just by sight or smell alone. We have only looked at tests for some gases and for water.
Let’s summarize everything we have learned. We have learned how to test for water and several common gases. However, we know that these tests are not foolproof. They should be done in conjunction with other identification methods to confirm what substance is present. However, they do give us a good indication of which gas is probably present.
In the test for hydrogen gas, a burning splint is extinguished with a pop. When oxygen is present, a glowing splint reignites and bursts back into flame because of the excess amount of oxygen present. Carbon dioxide gas will extinguish a lit splint, but this is not confirmation enough because other gases do this, too. A better test is to bubble it through lime water. And the lime water will turn milky in the presence of carbon dioxide as the insoluble white precipitate calcium carbonate forms. And this will confirm the presence of carbon dioxide.
Ammonia, though quite a pungent and choking gas, needs to be tested for with a chemical test. If ammonia is present, it will turn moist red litmus paper blue. And this is because of the formation of hydroxide ions which are alkaline when the ammonia reacts with water in the litmus paper. Chlorine gas will turn moist blue litmus paper red and then, after some time, will bleach it white. The change from blue to red in the litmus is because chlorine gas reacts with the water in the moist paper, forming acids.
The last gas test we looked at was for sulfur dioxide. SO2 will turn or reduce aqueous solutions of purple permanganate ions or manganate(VII) ions to colorless Mn2+ ions. And lastly, we learnt that liquid water, or water vapor in some cases, turns white anhydrous copper sulfate powder crystals to blue hydrated copper sulphate which contains waters of crystallization.
