Isabella Carter
The identification of gases is a crucial process in various fields, including environmental science, industrial processes, and chemical analysis. Different tests are employed to confirm the presence of specific gases, with distinct outcomes indicating a positive test result. For example, the test for oxygen involves introducing a glowing splint to a gas sample, and if the splint reignites, it confirms the presence of oxygen. On the other hand, hydrogen is identified by igniting the gas, resulting in a distinctive 'pop' sound due to its rapid combustion. This article will delve into the tests and positive indicators for various gases, including hydrogen, oxygen, carbon dioxide, chlorine, ammonia, and sulfur dioxide, providing insight into the methods and observations that constitute a positive test for each gas.
| Characteristics | Values |
|---|---|
| Oxygen | A glowing splint relights when placed in a sample of oxygen gas |
| Hydrogen | Ignites in air with a distinctive 'pop' sound |
| Carbon dioxide | Turns limewater cloudy or milky |
| Chlorine | Bleaches damp blue litmus paper white, turning it red first |
| Ammonia | Turns damp red litmus paper blue |
| Sulfur dioxide | Turns orange acidified potassium dichromate green |
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What You'll Learn
Hydrogen ignites in air with a 'squeaky pop'
To test for the presence of hydrogen gas, a sample of the gas in question is collected and placed in a test tube. The bung is then removed from the test tube, and a burning splint is held at the opening of the test tube. If the gas in the tube is hydrogen, the hydrogen will ignite in the air with a distinctive "squeaky pop" sound. This sound is produced by the combustion of hydrogen with oxygen, which is ignited by the heat energy from the flame. This test is often known as the "'squeaky pop' test".
The "squeaky pop" test is based on the flammability of hydrogen gas, which was dramatically demonstrated in the 1937 Hindenburg disaster. When hydrogen burns, it releases a tremendous amount of energy, even more than petrol or coal. This release of energy during combustion is what causes the distinctive "pop" sound when testing for hydrogen.
The "squeaky pop" test is a simple and widely accepted method for identifying hydrogen gas. It is important to note that hydrogen is a highly flammable gas, so caution must be exercised when performing this test. Proper safety measures, such as handling glassware and lit splints with care and avoiding spillages, are crucial to ensure a safe testing environment.
In addition to the "squeaky pop" test, there are other methods to identify hydrogen gas. One method is to bubble hydrogen into water, which will increase the hydrogen ion concentration and decrease the pH, causing acid/base indicators to change colour. This colour change can be used as an indication of the presence of hydrogen gas.
Furthermore, students with chemistry experience may be familiar with the aqueous hydrogen ion and the associated indicator colour changes as the hydrogen ion concentration varies. Understanding the periodic table can also aid in recognising the highly reactive nature of hydrogen, a diatomic, lightweight gas.
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Oxygen relights a glowing splint
The presence of oxygen gas can be detected using the glowing splint test. This test is widely accepted and easy to perform.
To conduct the test, light a small splint, such as a wooden coffee stirrer, and allow it to burn for a few seconds. Then, blow out the flame or shake it out, leaving only hot embers. Place the still-glowing splint into the unknown gas. If the splint reignites or relights, it indicates the presence of oxygen. This is because oxygen supports combustion, or burning, and will cause the splint to ignite again.
It is important to note that sometimes the splint may not relight, but it will glow very brightly. This is also a positive result for the presence of oxygen. This test is not limited to oxygen and can also be used to detect other oxidizing gases, such as nitric oxide.
Additionally, another test for oxygen involves bringing a burning matchstick near the evolving gas. If the flames glow more brightly, it indicates the presence of oxygen. This is because oxygen enhances combustion, resulting in a brighter flame.
Oxygen is a non-toxic, colourless, and odourless gas that is highly reactive and enters into chemical reactions with combustible agents. It is produced through various reactions, including decomposition and photosynthesis.
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Carbon dioxide turns limewater cloudy
Carbon dioxide (CO2) is a colourless and odourless gas, so it cannot be detected through direct observation. Therefore, a sample of the gas must be collected and tested. One way to collect a sample of carbon dioxide is to react calcium carbonate (or limestone chips) with hydrochloric acid. The reaction produces carbon dioxide, which can be collected in a test tube.
Once a sample of carbon dioxide has been collected, one way to test for its presence is to bubble the gas through limewater. Limewater is a diluted solution of calcium hydroxide (also known as slaked lime or milk of lime). When carbon dioxide is bubbled through limewater, it reacts with the calcium hydroxide to form a solid precipitate of calcium carbonate – chalk or limestone. Since calcium carbonate is insoluble in water, the limewater turns cloudy or milky white. Therefore, if the limewater turns cloudy, it indicates the presence of carbon dioxide.
Another way to test for carbon dioxide is the glowing splint test. A small splint, such as a wooden coffee stirrer, is lit, and the flame is blown out, leaving only glowing embers. The splint is then placed into the unknown gas sample. If the gas is carbon dioxide, the splint will go out completely.
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Chlorine bleaches damp blue litmus paper
Chlorine is a gas with distinct properties that can be used to identify its presence. One of the tests to identify chlorine gas involves using damp blue litmus paper. Here's how it works:
Preparation
Firstly, obtain a test tube containing the sample gas. Remove the bung from the test tube. It is important to note that chlorine gas is poisonous, so the experiment should be conducted in a fume cupboard to ensure safety.
Testing
Insert a piece of damp blue litmus paper into the opening of the test tube. Blue litmus paper is typically used to test for acids, and water on the damp paper allows chlorine to react with the indicator. Observe the litmus paper carefully.
Results
The presence of chlorine gas will cause the damp blue litmus paper to change colour. Initially, the paper may turn red due to the formation of hydrochloric acid from the reaction between chlorine and water. Subsequently, the paper will turn white as the chlorine bleaches it. This bleaching effect is unique to chlorine gas.
Other Identification Methods
In addition to the litmus paper test, chlorine gas can be identified through its distinct smell, described as strong and suffocating, similar to the odour associated with swimming pools. It also has a greenish-yellow colour and exhibits acidic properties, turning moist universal indicator paper red.
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Ammonia turns damp red litmus paper blue
Ammonia is a gas with no colour but a pungent smell. It is alkaline in water, with a pH of 11.5. One of the tests for ammonia involves turning moist red litmus paper blue. This is because ammonia gas is alkaline, and red litmus paper turns blue in response to alkaline substances. Ammonia will also turn moist universal indicator paper blue.
Another test for ammonia gas involves reacting it with hydrogen chloride gas from concentrated hydrochloric acid. If ammonia is present, a white smoke of ammonium chloride is formed. Ammonia gas will also put out a lit splint.
It is important to note that ammonia, NH3, is different from ammonium, NH4+, which refers to the aqueous cation.
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Frequently asked questions
A positive test for carbon monoxide is indicated by a carbon monoxide detector beeping, signalling that it has picked up on the presence of carbon monoxide. Other signs of a gas leak include a hissing sound, white dust clouds, and sulphuric odours.
A positive test for methane is indicated by a concentration of 10% methane or more, which will set off the alarm on a handheld natural gas detector.
A positive test for natural gas is indicated by a natural gas alarm reporting "GAS" or beeping continuously. Natural gas from appliances has an added chemical called mercaptan, which gives it a strong unpleasant sulphuric odour.
A positive hydrogen breath test indicates poor digestion of test sugars. A rise of more than 20 ppm over the baseline is considered a positive test.
A positive radon test is indicated by a reading of 4 pCi/L (picocuries per litre) or higher. This requires a radon detection test to be set up in your home for 48-72 hours.
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