Lab Prep: Ammonia Gas
React ammonium chloride with slaked lime to make gas
What this lesson covers
Why it matters
Heat a mixture of ammonium chloride and slaked lime, and the air fills with a sharp, pungent sting.
The idea in plain words
Analyze the double decomposition: ammonium ions swap partners with calcium ions, releasing ammonia gas and water.
Predict first
Why do we heat solid ammonium salts with a strong base to make ammonia?
Heat provides the energy to break bonds and release ammonia gas, which escapes because it is volatile.
- To melt the salts into a liquid mixture
- To drive off the volatile ammonia gas from the reaction — correct
- To dissolve the ammonium chloride in water
- To decompose calcium hydroxide into calcium oxide
What you do
Heat the solid mixture. Collect the gas and test it with moist red litmus paper.
Check yourself
What is the correct chemical equation for this preparation?
The reaction requires two moles of ammonium chloride for every mole of calcium hydroxide to balance the atoms, producing two moles of ammonia and two moles of water.
How do you confirm the gas produced is ammonia?
Ammonia is alkaline; it turns moist red litmus blue. It also has a characteristic pungent smell.
Why is calcium hydroxide used instead of sodium hydroxide in this lab preparation?
NaOH is deliquescent and attacks glass on heating; slaked lime, Ca(OH)₂, is cheap, easy to handle and does not attack glass.
- NH₄Cl + Ca(OH)₂ → CaCl₂ + NH₃ + H₂O
- 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + 2H₂O — correct
- 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₄OH
- NH₄Cl + CaO → CaCl₂ + NH₃ + H₂O
- It burns with a blue flame
- It turns moist red litmus paper blue — correct
- It turns moist blue litmus paper red
- It relights a glowing splint
- Sodium hydroxide is too expensive
- Sodium hydroxide is deliquescent and attacks/corrodes glass apparatus — correct
- Calcium hydroxide reacts faster
- Sodium hydroxide does not react with ammonium salts