🧪 Hydrogen Chloride (HCl) – Full Breakdown
1) Preparation of Hydrogen Chloride Gas
a) Laboratory Method
Reactants: Sodium chloride (NaCl) + Concentrated sulphuric acid (H₂SO₄)
Equation:
| Lab Preparation of HCl Gas |
|---|
| NaCl + H₂SO₄ → NaHSO₄ + HCl↑ (at room temp / mild heating — 150°C) |
| NaHSO₄ + NaCl → Na₂SO₄ + HCl↑ (at higher temp — 550°C) |
| NOTES: Conc. H₂SO₄ is used because it's non-volatile and doesn't evaporate with HCl. Two-step reaction — first gives NaHSO₄ (acid salt), second gives Na₂SO₄ (normal salt). |
Apparatus setup:
- Round bottom flask with NaCl + conc. H₂SO₄
- Delivery tube → Woulfe's bottle (empty, safety) → drying bottle (conc. H₂SO₄ / anhydrous CaCl₂) → upward delivery collection
- HCl is collected by upward delivery (it's heavier than air — density ~1.28 × air)
- Not collected over water — HCl is highly soluble in water
b) Industrial Method
Hargreaves Process: NaCl (solid) is heated with SO₂, air, and water vapour in presence of a catalyst.
| Hargreaves Process (Industrial) |
|---|
| 4NaCl + 2SO₂ + 2H₂O + O₂ → 2Na₂SO₄ + 4HCl↑ |
| NOTES: By-product Na₂SO₄ is used in glass and paper industries. Catalyst = Cu or Fe at ~400°C. Made in a vertical cast-iron tower. |
2) Physical Properties of HCl Gas
- Colour: Colourless
- Odour: Pungent, choking smell
- Density: Heavier than air (vapour density = 18.25, molecular mass = 36.5)
- Solubility: Highly soluble in water (one volume of water dissolves ~450 volumes of HCl gas at room temp)
- Fumes in air: HCl gas reacts with moisture in air to form HCl acid droplets — looks like white fumes / fog
- Not a supporter of combustion: A burning candle dipped in HCl gas is extinguished
3) The Fountain Experiment — Most Important Experiment
This proves the high solubility of HCl in water and its acidic nature.
Setup: A round bottom flask filled with dry HCl gas is connected to a beaker of water containing litmus. A jet tube goes from the beaker into the flask.
Procedure: A few drops of water are let into the flask → HCl dissolves → pressure drops inside → water from the beaker rushes up through the jet → forms a fountain.
| Fountain Experiment Observations |
|---|
| Water rushes into the flask as a fountain through the jet tube |
| The water in the flask turns red (acidic — blue litmus turns red) |
| If red litmus was used, it stays red (already acidic) |
| NOTES: Proves: (1) HCl is highly soluble in water. (2) HCl solution is acidic. |
4) Chemical Properties of HCl
a) Action on Dry Litmus
Dry HCl gas does NOT change the colour of dry litmus. HCl needs water to produce H⁺ ions, which are responsible for the acidic behaviour. Dry HCl has no H⁺ ions. This is a classic 2-mark question.
b) Reaction with Ammonia
| HCl + NH₃ → NH₄Cl |
|---|
| HCl (g) + NH₃ (g) → NH₄Cl (s) — dense white fumes of ammonium chloride |
| NOTES: This is the test for HCl gas (and also for NH₃ gas). A glass rod dipped in NH₄OH held near HCl gas gives white fumes. The white solid collects on the rod. |
c) Reaction with Metals
| HCl + Metal → Metal Chloride + H₂↑ |
|---|
| 2HCl + Zn → ZnCl₂ + H₂↑ |
| 2HCl + Mg → MgCl₂ + H₂↑ |
| 2HCl + Fe → FeCl₂ + H₂↑ (ferrous chloride, not ferric) |
| NOTES: H₂ gas burns with pop sound. Fe gives FeCl₂ (not FeCl₃) — Fe is oxidised to Fe²⁺ by HCl, not to Fe³⁺. |
d) Reaction with Carbonates and Bicarbonates
| HCl + Carbonate / Bicarbonate → Salt + CO₂↑ + H₂O |
|---|
| 2HCl + Na₂CO₃ → 2NaCl + CO₂↑ + H₂O |
| HCl + NaHCO₃ → NaCl + CO₂↑ + H₂O |
| NOTES: CO₂ turns limewater milky — test for CO₂. Rapid effervescence seen. |
e) Reaction with Bases (Neutralisation)
| HCl + Base → Salt + H₂O |
|---|
| HCl + NaOH → NaCl + H₂O |
| 2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O |
| NOTES: Exothermic. Salt + water formed. Phenolphthalein turns from pink to colourless at endpoint. |
f) Reaction with Oxidising Agents
| HCl acts as a reducing agent with strong oxidisers |
|---|
| MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂↑ (on heating — lab preparation of Cl₂) |
| Pb₃O₄ + 8HCl → 3PbCl₂ + 4H₂O + Cl₂↑ ("red lead" — red to white) |
| K₂Cr₂O₇ + 14HCl → 2KCl + 2CrCl₃ + 7H₂O + 3Cl₂↑ (orange solution turns green) |
| KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 8H₂O + 5Cl₂↑ (purple solution turns colourless) |
| NOTES: In all these reactions, HCl is oxidised to Cl₂. HCl is acting as a reducing agent (it gets oxidised). This is a common board topic. |
5) Hydrochloric Acid as a Reducing Agent
This is an important concept. HCl can donate electrons (get oxidised to Cl₂) in the presence of strong oxidising agents. The Cl⁻ ion is easily oxidised to Cl₂.
Test for oxidising agents using HCl: If HCl turns into Cl₂ gas (greenish-yellow, pungent) when heated with a substance, that substance is an oxidising agent.
6) Uses of HCl
- Making chlorides (ZnCl₂, NH₄Cl, CaCl₂ etc.)
- Making chlorine gas (from MnO₂ + HCl)
- Cleaning metal surfaces before electroplating (pickling — removes rust/oxide layer)
- As a laboratory reagent (acid-base titrations, etc.)
- Making aqua regia (HCl + HNO₃ in 3:1 ratio — dissolves gold and platinum)
- Digesting food in the stomach (as gastric juice)
7) Distinction Between HCl Gas and HCl Acid
| Property | HCl Gas | HCl Acid (aq) |
|---|---|---|
| Nature | Covalent molecule | Ionic (H⁺ + Cl⁻ in water) |
| Litmus | No effect on dry litmus | Turns blue litmus red |
| Conductivity | Does not conduct | Conducts electricity (has H⁺ and Cl⁻) |
| Reaction with metals | No reaction (dry) | React to give H₂ gas |
8) Top ICSE Reasoning Questions
Q1: Why is HCl gas collected by upward delivery and not over water?
Answer: HCl gas is highly soluble in water (~450 volumes in 1 volume water). It would dissolve if collected over water. It is collected by upward delivery because it is heavier than air (vapour density = 18.25, more than air's 14.4).
Q2: Why does dry HCl gas not change the colour of dry blue litmus?
Answer: HCl is a covalent molecule in the gaseous state. It does not produce H⁺ ions unless dissolved in water. The acidic behaviour (litmus colour change) is due to H⁺ ions. Dry HCl has no H⁺ ions — so no colour change.
Q3: In the reaction between NaCl and H₂SO₄, why is conc. H₂SO₄ used?
Answer: Conc. H₂SO₄ is a non-volatile acid. It stays in the flask while HCl gas is driven off. If a volatile acid like HNO₃ were used, it would also vaporize and contaminate the HCl gas. Also, conc. H₂SO₄ has a high H⁺ concentration which drives the reaction forward.
Q4: What happens when a glass rod dipped in NH₄OH is brought near HCl gas?
Answer: Dense white fumes of NH₄Cl are formed. NH₃ (from NH₄OH) reacts with HCl to form solid NH₄Cl particles that appear as white fumes. This is a test for HCl gas.
Q5: In the fountain experiment, why does water rush into the flask?
Answer: HCl gas is extremely soluble in water. When a few drops of water enter the flask, a large volume of HCl dissolves instantly. This creates a vacuum (very low pressure) inside the flask. The atmospheric pressure outside pushes water from the beaker up through the jet tube into the flask, forming a fountain.
Q6: How would you prove that HCl acts as a reducing agent?
Answer: Heat HCl with MnO₂. Greenish-yellow Cl₂ gas is produced. MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂↑. HCl is oxidised to Cl₂ (loses H or gains Cl) — this shows HCl acts as a reducing agent. The Cl₂ gas can be identified by its pungent smell and its ability to bleach litmus (turns blue litmus white, not red).
Q7: Why does Fe react with HCl to give FeCl₂ and not FeCl₃?
Answer: HCl is a mild oxidising agent. It can only oxidise Fe to Fe²⁺ (ferrous), not to Fe³⁺ (ferric). To get FeCl₃, you need a stronger oxidising agent like chlorine gas: 2Fe + 3Cl₂ → 2FeCl₃.
1. The fountain experiment is the #1 most asked question from this chapter — learn the setup, observation, and explanation.
2. Dry HCl vs aqueous HCl distinction is a free 2 marks — remember: dry = covalent, no H⁺, no litmus change.
3. HCl as a reducing agent with MnO₂ is another favourite — know the equation and the colour changes.
4. White fumes with NH₃ = test for both HCl and NH₃. Super common.
5. Conc. H₂SO₄ is used because it's non-volatile — this is the key reason for every preparation question.