Eleanor Finch
There are several methods for identifying gases, including observing their physical and chemical properties, performing experiments, and using special instruments. The best gas tests give a clear, definite, and unique result for their target gas. For example, if hydrogen is present in a test tube, a lighted splint will ignite with a squeaky pop. Oxygen, on the other hand, will relight a glowing splint held inside a test tube. Carbon dioxide turns limewater cloudy or milky, while chlorine bleaches damp blue litmus paper white. Ammonia turns damp red litmus paper blue, and sulfur dioxide turns orange acidified potassium dichromate green.
| Characteristics | Values |
|---|---|
| Hydrogen | A lighted splint ignites with a squeaky pop |
| Oxygen | A glowing splint relights |
| Carbon dioxide | Turns limewater cloudy |
| Chlorine | Bleaches damp blue litmus paper white |
| Ammonia | Turns damp red litmus paper blue |
| Sulfur dioxide | Turns orange acidified potassium dichromate green |
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What You'll Learn
A glowing splint will relight in the presence of oxygen
The presence of oxygen in a gas sample can be tested by introducing a glowing splint to the sample. If the splint relights, this is a positive test for the presence of oxygen. This is because oxygen supports combustion, which is a chemical reaction that produces heat and light.
To understand why a glowing splint relights in the presence of oxygen, it is important to first understand combustion. Combustion is a chemical reaction that involves a substance reacting with oxygen and producing heat and light. For combustion to occur, three things are required: heat, fuel, and an oxidising agent (usually oxygen). In the context of a glowing splint, the splint itself provides the heat and the fuel. When the glowing splint is introduced to an oxygen-rich environment, the oxygen acts as the oxidising agent, allowing the combustion reaction to continue and causing the splint to reignite.
This test is commonly used to demonstrate the presence of oxygen in gas samples. It is a simple yet effective method that can be performed in a classroom setting. The process involves lighting a splint (a small piece of wood, like a matchstick) and then blowing it out. The splint will continue to glow due to residual heat, but it is no longer actively burning because it requires a constant supply of oxygen to sustain combustion. The glowing splint is then cautiously reintroduced into an environment rich in oxygen, such as a container filled with pure oxygen gas. If the splint relights, this confirms the presence of oxygen in the sample.
It is important to note that this simple gas test is best suited for pure species rather than mixtures of gases. Additionally, the test for any gas should ideally provide a clear, definite, and unique result to ensure accurate identification of the gas in question.
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Hydrogen ignites in air with a 'squeaky pop'
The combustion of hydrogen is an exothermic reaction, meaning it releases a lot of energy in the form of heat. This heat imparts kinetic energy to the molecules of the gaseous mixture, causing them to expand rapidly. The air surrounding the gases also expands very quickly, leading to a sudden and swift expansion of gas molecules. This rapid expansion causes a break in the sound barrier, resulting in a distinctive "squeaky pop" sound.
The "squeaky pop" sound is a well-known indication of the presence of hydrogen gas. To test for hydrogen, a burning or glowing splint is inserted into a test tube containing the gas sample. If the gas in the tube is hydrogen, it will ignite with oxygen, producing a "squeaky pop" sound. This sound is caused by the combustion of hydrogen and oxygen, which is triggered by the heat energy from the flame.
The chemical reaction between hydrogen and oxygen produces two molecules of water and releases a significant amount of energy. This energy is manifested as both light (from the flame) and sound (the "pop"). The reaction is exothermic, meaning it releases a large amount of energy. Hydrogen is also known as "water-former" due to this reaction.
Hydrogen is a highly reactive, diatomic, lightweight, and combustible gas. It is the first element on the periodic table and the most abundant chemical substance in the universe. Hydrogen is a promising source of clean fuel as it contains no carbon molecules. However, the current process of producing hydrogen using natural gas is not environmentally friendly, and researchers are working on finding alternative methods.
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Carbon dioxide turns limewater cloudy
The presence of a gas can be tested in several ways, including the use of a glowing splint, litmus paper, or limewater. One of the most effective ways to test for carbon dioxide (CO2) gas is by using limewater.
Limewater, or calcium hydroxide solution, is a clear solution. When carbon dioxide is bubbled through limewater, it turns cloudy, milky, or white. This occurs due to the reaction between carbon dioxide and calcium hydroxide, which produces calcium carbonate (CaCO3) and water. The chemical equation for this reaction is:
Ca(OH)2 + CO2 → CaCO3 + H2O
In this equation, Ca(OH)2 represents calcium hydroxide, CO2 represents carbon dioxide, CaCO3 represents calcium carbonate, and H2O represents water. Calcium carbonate is a white solid that appears as a precipitate in the solution, giving it a cloudy or milky appearance. This reaction is specific to carbon dioxide, and no other gas will cause this change in limewater. Therefore, the appearance of cloudiness or milkiness in limewater is a definitive indication of the presence of carbon dioxide gas.
To perform this test, one can bubble a gas through a test tube containing clear limewater. If the gas is carbon dioxide, the limewater will turn cloudy. This test is useful because carbon dioxide is a colourless and odourless gas, making it challenging to detect without a specific test like the limewater test. The limewater test provides a clear and unique result for carbon dioxide, allowing for its definite identification.
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Chlorine bleaches damp blue litmus paper white
A positive test for the presence of a gas is one that gives a clear, definite, and unique result for the target gas. For example, the presence of hydrogen gas is confirmed when a lighted splint ignites with a squeaky pop. Similarly, the presence of oxygen is confirmed when a glowing splint held inside a test tube relights.
Chlorine (Cl2) is an acidic gas that acts as a bleach. When damp blue litmus paper is exposed to chlorine gas, it turns white. This colour change occurs due to the following reaction:
Cl2(g) + H2O(l) → HCl(aq) + HClO(aq)
In this reaction, chlorine gas reacts with water molecules present in the damp blue litmus paper to form hydrochloric acid (HCl) and hypochlorous acid (HClO). The hypochlorous acid then further reacts with the litmus dye, causing it to lose its colour and turn white. This process is known as bleaching.
The bleaching effect of chlorine is so strong that it can turn the litmus paper white, regardless of whether the paper was initially blue or red. It is important to note that the colour change in litmus paper is typically used to test for acids (turning blue litmus paper red) or alkalis (turning red litmus paper blue).
Other gases, such as fluorine and bromine, can also bleach damp litmus paper. However, they are not commonly used for this purpose due to their high reactivity and low availability.
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Sulphur dioxide turns orange acidified potassium dichromate green
The presence of certain gases can be determined through specific tests. For instance, hydrogen ignites in air, and when a lighted splint is held near the mouth of a test tube containing hydrogen, it ignites with a "squeaky pop". Similarly, oxygen supports combustion, and a glowing splint held inside a test tube with oxygen relights.
Sulphur dioxide (SO2) turns orange acidified potassium dichromate green. This colour change occurs when sulphur dioxide gas is passed through an acidified potassium dichromate solution. The orange colour of the Cr (+6) ion changes to green, which is the colour of the Cr (+3) ion. This reaction can be represented by the equation:
> ${K_2}C{r_2}{O_7} + 3S{O_2} + {H_2}S{O_4} \to C{r_2}{(S{O_4})_3} + {K_2}S{O_4} + {H_2}O$
Other tests for gases include:
- Carbon dioxide (CO2) turns limewater cloudy or milky/cloudy white.
- Chlorine (Cl2) is a yellow-green gas with a pungent smell that acts as a bleach. It turns damp blue litmus paper white and then red.
- Ammonia (NH3) turns damp red litmus paper blue.
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Frequently asked questions
If hydrogen is present in a test tube, a lighted splint held near its mouth will ignite with a squeaky pop.
If oxygen is present in a test tube, a glowing splint held inside will relight.
If carbon dioxide is present, limewater will turn cloudy or milky.
