Describe tests to identify the gases: (a) ammonia, $mathrm{NH}_3$, using damp red litmus paper (b) carbon dioxide, $mathrm{CO}_2$, using limewater (c) chlorine, $mathrm{Cl}_2$, using damp litmus paper (d) hydrogen, $mathrm{H}_2$, using a lighted spli

Experimental Techniques and Chemical Analysis – Identification of Gases

(a) Ammonia – \$NH_3\$

Ammonia is a colourless gas with a sharp, pungent smell, like that of cleaning products. When it comes into contact with damp red litmus paper, it turns blue, showing its basic nature.

  1. Moisten a strip of red litmus paper with water.

  2. Hold the paper near the source of the gas.

  3. Observe the colour change from red to blue.

🔬 Analogy: Think of red litmus paper as a mood ring that changes colour when it feels the “basic” vibes of ammonia.

Exam tip: Remember that only damp red litmus reacts with \$NH_3\$ – dry paper will not change colour.

(b) Carbon Dioxide – \$CO_2\$

Carbon dioxide is a colourless, slightly acidic gas. It reacts with limewater (a dilute solution of calcium hydroxide) to produce a milky cloud, indicating the formation of calcium carbonate.

  1. Pour a small amount of limewater into a test tube.

  2. Introduce the gas into the solution.

  3. Watch for the solution to turn cloudy.

🌫️ Analogy: The cloud is like a foggy day – it’s a visual cue that \$CO_2\$ is present.

Exam tip: The milky cloud appears instantly; if it takes time, the gas may not be pure \$CO_2\$.

(c) Chlorine – \$Cl_2\$

Chlorine is a greenish-yellow gas with a strong, irritating odor. It turns damp litmus paper red, showing its acidic character.

  1. Moisten a strip of litmus paper (red or blue).

  2. Expose the paper to the chlorine gas.

  3. Observe the colour change to red.

🟢 Analogy: Think of chlorine as a “colour‑changing” gas that tells you it’s acidic by turning the paper red.

Exam tip: Use damp paper – dry paper will not react. Red litmus is the safest choice for detecting \$Cl_2\$.

(d) Hydrogen – \$H_2\$

Hydrogen is a colourless, flammable gas. When a lighted splint is introduced into a hydrogen atmosphere, it produces a distinct “pop” sound, indicating combustion.

  1. Light a splint and hold it in a small glass tube.

  2. Introduce the hydrogen gas into the tube.

  3. Observe the “pop” sound as the splint burns.

💥 Analogy: The pop is like a tiny fireworks display – a clear sign that hydrogen is present.

Exam tip: Ensure the splint is fully lit before introducing the gas; a weak splint may not show the pop.

(e) Oxygen – \$O_2\$

Oxygen is a colourless gas that supports combustion. A glowing splint (a splint that is still glowing after being held in a flame) will reignite when placed in an oxygen-rich environment.

  1. Light a splint and let it burn for a few seconds.

  2. Remove the splint and hold it near the gas source.

  3. Observe the splint reigniting.

🔥 Analogy: Oxygen is like a “fuel enhancer” – it makes the splint’s flame come back to life.

Exam tip: The splint must be glowing, not just warm, to confirm the presence of \$O_2\$.

(f) Sulfur Dioxide – \$SO_2\$

Sulfur dioxide is a colourless gas with a sharp, irritating smell. When it is bubbled through an acidified aqueous potassium manganate(VII) solution, the green solution turns yellow, indicating the reduction of \$\ce{MnO4^{2-}}\$ to \$\ce{MnO4^-}\$.

  1. Prepare a dilute \$\ce{K2MnO4}\$ solution and acidify it with a few drops of \$\ce{HCl}\$.

  2. Introduce the gas into the solution.

  3. Observe the colour change from green to yellow.

🟢➡️🟡 Analogy: The green turning yellow is like a mood shift – the gas is changing the mood of the solution.

Exam tip: The acidification is essential; without it, the colour change may not occur.

GasTestObservation
\$NH_3\$Damp red litmus paperTurns blue (basic)
\$CO_2\$LimewaterMilky cloud (CaCO₃)
\$Cl_2\$Damp litmus paperTurns red (acidic)
\$H_2\$Lighted splint“Pop” sound (combustion)
\$O_2\$Glowing splintReignites (supports combustion)
\$SO_2\$Acidified \$\ce{K2MnO4}\$ solutionGreen → Yellow (reduction)