1. Laboratory preparation
Reactants: sodium chloride (rock salt) and concentrated sulphuric acid, heated gently below 200 °C.
- Apparatus: round-bottomed flask with a thistle funnel and delivery tube; wash bottle of concentrated sulphuric acid; upright gas jar.
- Drying agent: concentrated sulphuric acid (absorbs water, does not react). Quicklime and solid caustic soda are not used: they are bases and react with the gas.
- Collection: upward displacement of air (heavier than air); not over water (extremely soluble).
- Jar full? A rod dipped in ammonia solution gives dense white fumes; moist blue litmus turns red.
- Why below 200 °C? At a higher temperature sodium sulphate forms as a hard crust that sticks to the flask; fuel is wasted and the glass may crack.
| Feature | Choice | Reason |
|---|---|---|
| Drying agent | Conc. H₂SO₄ | Does not react with HCl |
| Collection | Upward displacement of air | Heavier than air, very soluble |
| Temperature | Below 200 °C | Avoids hard crust of Na₂SO₄ |
Formulas
- Below 200 °C (used)
- Above 200 °C (avoided)
Key terms
- Upward displacement of air
Collection of a gas heavier than air in an upright jar: the gas enters at the bottom and pushes the air out at the top.
- Drying agent
A substance that absorbs water vapour from a gas without reacting with the gas.
2. Density and solubility; the fountain experiment
- Density: vapour density of HCl = 36.5 ÷ 2 = 18.25, greater than that of air (about 14.4), so it is heavier than air. It can be "poured" from one jar into another.
- Solubility: extremely soluble in water; fumes in moist air.
- Fountain experiment: a dry flask of dry HCl, inverted, with a jet tube dipping into water containing blue litmus. A little water dissolves much gas, the pressure inside falls and atmospheric pressure drives water up as a fountain, which turns red. Shows high solubility and acidic nature.
| Observation | Shows |
|---|---|
| Water rises as a fountain | Gas is extremely soluble |
| Blue litmus turns red | Solution is acidic |
| Gas poured into a lower jar | Gas is heavier than air |
Key terms
- Vapour density
Half the relative molecular mass of a gas; compares the mass of a volume of the gas with the same volume of hydrogen.
- Fountain experiment
An experiment in which a very soluble gas dissolves in a little water, lowering the pressure so that water is forced into the flask as a fountain.
3. Hydrochloric acid and back suction
Hydrochloric acid is made by dissolving hydrogen chloride in water. A delivery tube dipped straight into water would allow back suction (water sucked back into the hot flask). An inverted funnel with its rim just dipping in the water is used:
- if water rises into the funnel, the level in the beaker falls below the rim, air enters and the water falls back;
- the funnel also gives a large surface area for absorbing the gas.
| Arrangement | Result |
|---|---|
| Tube dipped into water | Back suction may crack the flask |
| Inverted funnel, rim just dipping | No back suction; large surface for absorption |
Key terms
- Back suction
Water being drawn back into a hot apparatus when a very soluble gas dissolves rapidly and the pressure falls.
4. Reactions of hydrochloric acid
- With ammonia: dense white fumes of ammonium chloride.
- Active metals: chloride + hydrogen (Zn, Mg, Fe → FeCl₂). Copper, silver, gold do not react.
- Oxides and hydroxides: salt + water (CuO gives a blue-green solution).
- Carbonates and hydrogencarbonates: chloride + water + CO₂ (effervescence; lime water milky).
- Sulphides: chloride + H₂S (rotten-egg smell; lead acetate paper black).
- Sulphites: chloride + water + SO₂ (choking smell; acidified potassium dichromate orange → green).
| Substance | Gas given off | Test for the gas |
|---|---|---|
| Zinc | H₂ | Burns with a pop |
| Marble | CO₂ | Lime water milky |
| Iron(II) sulphide | H₂S | Lead acetate paper black |
| Sodium sulphite | SO₂ | Acidified dichromate green |
Formulas
- Dense white fumes
- Active metal
- Metal oxide
- Carbonate
- Sulphide
- Sulphite
Key terms
- Neutralisation
The reaction of an acid with a base to give a salt and water only.
5. Aqua regia
Aqua regia: 3 volumes of concentrated HCl + 1 volume of concentrated HNO₃. Nitric acid oxidises hydrochloric acid to nascent chlorine, which converts gold and platinum into soluble chlorides. Use: dissolving noble metals (e.g. in testing and purifying gold).
| Feature | Aqua regia |
|---|---|
| Composition | 3 vol conc. HCl : 1 vol conc. HNO₃ |
| Active agent | Nascent chlorine |
| Dissolves | Gold, platinum |
Formulas
- Formation of nascent chlorine
Key terms
- Aqua regia
A mixture of three volumes of concentrated hydrochloric acid and one volume of concentrated nitric acid that dissolves gold and platinum.
- Nascent chlorine
Freshly formed, very reactive atomic chlorine.
6. Precipitation reactions and tests
- Silver nitrate: AgNO₃ + HCl → AgCl + HNO₃; curdy white precipitate, insoluble in dilute HNO₃, soluble in excess NH₄OH.
- Lead nitrate: Pb(NO₃)₂ + 2HCl → PbCl₂ + 2HNO₃; white precipitate, soluble in hot water.
Tests: HCl gas — moist blue litmus turns red; dense white fumes with a rod dipped in ammonia solution. Hydrochloric acid — curdy white precipitate with silver nitrate solution (insoluble in dilute nitric acid).
| Reagent | Observation with HCl |
|---|---|
| Moist blue litmus | Turns red |
| Ammonia rod | Dense white fumes |
| Silver nitrate | Curdy white precipitate, insoluble in dil. HNO₃ |
| Lead nitrate | White precipitate, dissolves on heating |
Formulas
- Curdy white precipitate
- White precipitate, soluble in hot water
Key terms
- Precipitate
An insoluble solid formed when two solutions are mixed.
Exam tips
Common mistakes
- Leaving out "below 200 °C" from the preparation.
- Naming quicklime as the drying agent (it reacts with HCl).
- Saying HCl is collected by downward displacement (that is ammonia).
- Writing FeCl₃ for iron with dilute HCl (it is FeCl₂).
- Reversing the 3 : 1 ratio of aqua regia.
How marks are usually awarded
- Equations (1 mark each): correct formulae, balanced, with conditions where asked.
- Observations (1 mark): colour, smell, effervescence or precipitate, stated clearly.
- Fountain experiment: set-up, reason (pressure) and the colour change.