Chemistry (9202) | Identification of common gases

The Chemistry Detective: Identifying Common Gases

Hello future chemist! Welcome to one of the most practical and exciting topics in chemical analysis: learning how to identify invisible gases. Think of yourself as a detective! You can't see the gas, but by observing how it reacts with specific chemicals or flames, you can figure out exactly what it is.

This chapter is crucial for your exams because these tests are definitive proof. Once you master these five simple tests, you'll be able to identify the most common gases produced in the lab!

Safety First! The Golden Rules of Gas Testing

Because some gases are toxic or highly flammable, safety is key. Always use small amounts of gas and perform the tests in a well-ventilated area or fume cupboard, especially when dealing with gases like Chlorine.

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Section 1: The Flammable Gases (Hydrogen and Oxygen)

These two gases are often tested using a simple piece of wood: the splint.

1. Hydrogen Gas (\(\text{H}_2\))

Hydrogen is the lightest gas and is highly flammable. We test for it using a burning flame.

The Hydrogen Test (The 'Pop' Test)

• Equipment Needed: A lit splint (a thin piece of wood, burning).
• Step 1: Collect the gas in a test tube.
• Step 2: Bring the lit splint quickly to the mouth of the test tube.
• Observation: If Hydrogen is present, you will hear a sharp, squeaky 'pop' sound, and the small amount of gas will burn up rapidly.

Why does it 'pop'? The reaction is rapid combustion, where Hydrogen reacts explosively with Oxygen in the air, creating water vapor and releasing a lot of heat and sound energy.

$$2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l) \text{ (+ energy)}$$

Memory Aid: H is for Hydrogen, H is for High-pitched POP!

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2. Oxygen Gas (\(\text{O}_2\))

Oxygen is essential for combustion (burning), but it does not burn itself. It acts as an oxidiser.

The Oxygen Test (The 'Relight' Test)

• Equipment Needed: A glowing splint (a splint that has been lit and then blown out so only the tip is still glowing red).
• Step 1: Collect the gas in a test tube.
• Step 2: Insert the glowing splint into the test tube.
• Observation: If Oxygen is present, the splint will relight (burst back into flame).

Why does it relight? The concentration of Oxygen gas in the test tube is much higher than in the air. This abundance of Oxygen supports the combustion reaction much more vigorously, providing enough energy for the glowing ember to reignite.

Memory Aid: O is for Oxygen, O is for Oh, it’s back! (The flame comes back to life).

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Section 2: The Acidic and Neutral Gases

These gases are tested using liquids or special indicator paper, as they don't produce characteristic fire results.

3. Carbon Dioxide Gas (\(\text{CO}_2\))

Carbon Dioxide is a common product of combustion, respiration, and the reaction between acids and carbonates.

The Carbon Dioxide Test (The Limewater Test)

• Equipment Needed: Limewater (A solution of Calcium Hydroxide, \(\text{Ca}(\text{OH})_2\)).
• Step 1: Bubble the unknown gas through the limewater solution, or shake the gas with the limewater.
• Observation: If Carbon Dioxide is present, the clear limewater solution will turn milky or cloudy (a white precipitate forms).

The Chemistry: Carbon Dioxide reacts with the dissolved Calcium Hydroxide to form insoluble Calcium Carbonate, which is the white solid that makes the solution cloudy.

$$\text{Ca}(\text{OH})_2(aq) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$$

Common Mistake to Avoid! If you bubble $\text{CO}_2$ through the limewater for too long, the $\text{CaCO}_3$ reacts further to form soluble Calcium Hydrogen Carbonate. The milky solution will become clear again! Always stop the test as soon as cloudiness appears.

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4. Chlorine Gas (\(\text{Cl}_2\))

Chlorine is a greenish-yellow gas that is toxic and corrosive. Always handle this test with extreme care, ideally in a fume cupboard.

The Chlorine Test (The Bleaching Test)

• Equipment Needed: Damp litmus paper (usually blue, but red or universal paper works too).
• Step 1: Hold the damp litmus paper at the mouth of the container holding the gas.
• Observation: Chlorine gas will first turn the paper slightly red (because chlorine reacts with water to form acidic compounds), and then it will quickly bleach the paper white (turn it colourless).

Why does it bleach? Chlorine acts as a strong oxidising agent. It reacts with the water on the damp paper to form Hypochlorous acid (\(\text{HOCl}\)), which is the active bleaching agent that destroys the dye in the litmus paper.

Did you know? This same chemical principle is why chlorine is added to swimming pools and used in household cleaning products!

Memory Aid: $\text{Cl}_2$ for Chlorine, $\text{C}$ for Colourless (the bleached paper).

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5. Ammonia Gas (\(\text{NH}_3\))

Ammonia is a pungent gas (it smells sharp, like urine or certain cleaning products). It is an alkaline gas.

The Ammonia Test (The Basic Test)

• Equipment Needed: Damp red litmus paper.
• Step 1: Hold the damp red litmus paper at the mouth of the container holding the gas.
• Observation: Because Ammonia is an alkaline gas, it will react with the water on the paper to form an alkaline solution, causing the red litmus paper to turn blue.

Alternative Test (High Ability Bonus): Ammonia gas can also be tested using a rod dipped in concentrated Hydrochloric Acid ($\text{HCl}$). When the rod is brought near the ammonia gas, dense white fumes of solid Ammonium Chloride ($\text{NH}_4\text{Cl}$) are formed.

$$\text{NH}_3(g) + \text{HCl}(g) \rightarrow \text{NH}_4\text{Cl}(s)$$

Memory Aid: $\text{A}$ for Ammonia, $\text{A}$ for Alkaline (turns red blue).

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Section 3: Summary Table and Key Takeaways

Use this table as your ultimate quick reference guide for exam revision. Memorising the chemical name, test reagent, and observation is essential!

Key Takeaway

Understanding gas tests is the foundation of analytical chemistry. These simple observations—a sound, a relight, a change in colour, or cloudiness—give you 100% certainty about which gas you are working with. Keep practicing the link between the gas and its unique result!