Four unlabelled bottles and one set of litmus papers
Four colourless liquids in unlabelled bottles are left in a school laboratory after an exhibition. The teacher tells the science club that they are dilute hydrochloric acid, sodium hydroxide solution, sodium hydrogen carbonate solution and sodium chloride solution, but does not say which is which. The only things available are blue litmus paper, red litmus paper, test tubes, a dropper, a spirit lamp and a burning splint. The club secretary writes in the plan: 'Litmus will identify all four liquids; the gas test is unnecessary.' The club also has a dry cell, a bulb and two carbon rods, and finds that the bulb glows when the rods dip into each of the four solutions, but not when they are pressed into dry sodium chloride crystals taken from a reagent bottle. One member concludes that the dry crystals must be a covalent compound, because covalent compounds do not conduct electricity.
1. Design a sequence of tests, using only the materials listed, that identifies all four liquids. State the observation that settles each identification.
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Step 1 – Blue litmus. Only one liquid turns blue litmus red: the dilute hydrochloric acid. The other three leave it blue.
Step 2 – Red litmus. Sodium hydroxide and sodium hydrogen carbonate turn red litmus blue (both are basic); sodium chloride leaves red litmus red. So sodium chloride is the liquid that changes neither paper.
Step 3 – Separate the two bases with the acid already identified. Add a few drops of the hydrochloric acid to a fresh sample of each basic liquid:
- Sodium hydrogen carbonate gives brisk effervescence; the gas turns a burning splint out with a pop-like extinguishing, showing carbon dioxide: NaHCO₃ + HCl → NaCl + H₂O + CO₂.
- Sodium hydroxide gives no gas; the tube only becomes warm as it is neutralised: NaOH + HCl → NaCl + H₂O.
A good answer confirms the end point by showing the fizzing sample is neutral to litmus once the acid is in slight excess. Litmus alone stops at step 2; the gas test is what finishes the job.
2. Judge the secretary's claim that litmus alone identifies all four liquids.
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The claim is false.
- Litmus can give only three outcomes: blue litmus turns red, red litmus turns blue, or neither changes. Four liquids cannot be separated by three outcomes.
- Sodium hydroxide and sodium hydrogen carbonate share the same litmus result (red → blue), because both produce hydroxide or carbonate ions that make the solution basic. Litmus reports only whether a solution is acidic or basic; it cannot distinguish one base from another.
- A further limitation: litmus is a single indicator with no graduation, so it also cannot rank two acids of different strength.
Therefore at least one extra, chemically different observation is needed. The cheapest is a gas-evolution test with the acid, because carbonate and hydroxide behave differently towards an acid even though both are basic. This is a good example of a test being necessary but not sufficient.
3. Explain the conduction results and evaluate the member's conclusion about the dry sodium chloride crystals.
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Why the solutions conduct: conduction in these liquids is ionic, not electronic. The bulb glows because mobile ions move to the carbon electrodes: H₃O⁺ and Cl⁻ in the acid, Na⁺ and OH⁻ in the alkali, Na⁺ and HCO₃⁻ in the hydrogen carbonate solution, Na⁺ and Cl⁻ in the salt solution.
Why dry sodium chloride does not conduct: sodium chloride is an ionic solid. Na⁺ and Cl⁻ are held in a rigid three-dimensional lattice by strong electrostatic forces, so the ions are not free to move. On melting or dissolving, the lattice breaks down and the ions become mobile, and conduction begins.
Judging the member's conclusion: it is wrong on two counts.
- Sodium chloride is ionic, not covalent; the missing conductivity comes from the fixed ions, not from bond type.
- The rule 'covalent compounds do not conduct' is itself unreliable. Hydrogen chloride is covalent, yet its aqueous solution conducts well because it ionises in water to give H₃O⁺ and Cl⁻. The reliable rule is: a substance conducts when it contains free charge carriers, whatever kind of bond it has.