Study of Compounds — Hydrogen Chloride
Introduction
Hydrogen chloride (HCl) is an important acidic compound. In ICSE Class 10 Chemistry, you study its laboratory preparation, unique properties demonstrated by the fountain experiment, and its extensive industrial uses.
Laboratory Preparation of HCl Gas
Principle: Heating sodium chloride with concentrated sulphuric acid.
Equation:
NaCl + H₂SO₄ (conc.) → NaHSO₄ + HCl (g) (at ~150°C)
If the reaction is continued at a higher temperature:
NaHSO₄ + NaCl → Na₂SO₄ + HCl (g) (at ~550°C)
Overall: 2NaCl + H₂SO₄ (conc.) → Na₂SO₄ + 2HCl (g)
Apparatus Setup
- Reactants: Common salt (NaCl) and concentrated H₂SO₄.
- Heating: A round-bottom flask fitted with a delivery tube.
- Drying agent: Concentrated H₂SO₄ (not CaO or P₂O₅ — they react with HCl).
- Collection: Upward delivery of gas (HCl is heavier than air).
- Precautions:
- HCl should not be collected over water (very soluble).
- Use an anti-suckback device.
Physical Properties
| Property | Observation |
|---|---|
| Colour | Colourless gas |
| Odour | Pungent, choking smell |
| Density | Heavier than air (vapour density = 18.25) |
| Solubility | Extremely soluble in water (~500 volumes of HCl in 1 volume of water at STP) |
| Effect on dry litmus | No effect (only acidic in solution) |
Fountain Experiment
The fountain experiment demonstrates the extremely high solubility of HCl gas in water.
Procedure:
- A round-bottom flask is filled with dry HCl gas.
- A dropper containing water is fitted at the mouth.
- When water is squeezed into the flask, HCl dissolves rapidly, creating a partial vacuum.
- Water from the trough below rushes up through the jet, forming a fountain.
With blue litmus solution: The fountain turns red (showing acidic nature).
Chemical Properties
Reaction with Water
HCl(g) + H₂O → H₃O⁺ + Cl⁻ (fuming hydrochloric acid)
Reaction with Ammonia
NH₃(g) + HCl(g) → NH₄Cl(s)
Observation: Dense white fumes of ammonium chloride.
Reaction with Metals
Zn + 2HCl → ZnCl₂ + H₂↑ (with reactive metals)
Reaction with Silver Nitrate
AgNO₃ + HCl → AgCl↓ + HNO₃
Observation: White precipitate of AgCl (soluble in NH₄OH).
Uses of HCl
- In the manufacture of chlorine, ammonium chloride, and glucose.
- As a laboratory reagent.
- For cleaning metal surfaces (pickling).
- In the production of PVC (vinyl chloride monomer).
- In the pharmaceutical industry.
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Using an inverted trough for collection | HCl is collected by UPWARD DELIVERY (downward displacement of air) |
| Drying HCl with CaO | CaO is basic — reacts with HCl. Use conc. H₂SO₄ as drying agent |
| Thinking dry HCl turns blue litmus red | Dry HCl does NOT change litmus — H⁺ ions are needed |
| Confusing HCl gas with hydrochloric acid | HCl(g) is molecular; HCl(aq) is ionic (H⁺ + Cl⁻) |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: laboratory preparation, fountain experiment, chemical properties (especially NH₃ + HCl reaction), uses.
Marks Blueprint: Preparation/Setup — 2 marks, Fountain experiment — 2 marks, Properties and reactions — 2 marks.
Self-Test Questions
-
Describe the laboratory preparation of HCl gas. Write the necessary equations.
-
Why is HCl gas dried using concentrated H₂SO₄ and not CaO?
-
Explain the fountain experiment. What does it demonstrate?
-
What is observed when HCl gas reacts with NH₃? Write the equation.
-
Why does dry HCl gas not affect dry litmus paper?
-
State three important uses of hydrogen chloride.
