Testing Liquids with a Bulb: Ions, Weak Acids and Electrolysis
A Class 10 science club builds a conductivity tester: a 6 V battery, a 6 V/0.3 A bulb, two carbon rods recovered from old dry cells, and 100 mL beakers. Each liquid is tested with the rods 2 cm apart, rinsed and dried between tests.
Observations:
- Distilled water: no glow.
- Ethanol: no glow.
- Glucose solution (10 g in 100 mL water): no glow.
- Dilute hydrochloric acid: bright glow.
- Dilute acetic acid: faint orange glow.
The club then adds sodium hydroxide solution drop by drop to the acetic acid beaker, stirring, until the glow becomes as bright as the hydrochloric acid beaker. A label on the acetic acid bottle reads "CH₃COOH, 0.1 M"; the hydrochloric acid bottle reads "HCl, 0.1 M".
In a second experiment the club electrolyses concentrated sodium chloride solution with the same carbon rods, adding a few drops of phenolphthalein. After five minutes, a colourless gas collects at one rod, a pale yellow-green gas at the other, and the liquid around one rod turns pink.
1. Explain the glow pattern across the five liquids in terms of particles. Why do ethanol and glucose fail to conduct even though both contain hydrogen atoms?
Read the worked answer
Rule being applied: the bulb glows only if charged particles - free ions - can move through the liquid. A covalent molecular substance, however many hydrogen atoms it contains, gives no ions and so no current.
- Distilled water: ionises only to about 10⁻⁷ mol/L, so the ion count is negligible - no glow.
- Ethanol (C₂H₅OH): a covalent molecule; the O–H hydrogen is not released as H⁺ in water, so there are no ions - no glow. "Contains hydrogen" is irrelevant; what matters is whether that hydrogen is released as an ion.
- Glucose: also covalent; it dissolves as intact molecules, so no ions - no glow.
- Hydrochloric acid: a strong acid, essentially 100% ionised, so 0.1 M gives about 0.1 mol/L H⁺ plus 0.1 mol/L Cl⁻ - a high ion count, bright glow.
- Acetic acid: a weak acid, only about 1% ionised at this concentration, so the ion count is perhaps a hundred times lower - faint glow only.
Both acids are 0.1 M, so the difference is degree of ionisation, not concentration. The faint glow proves ions exist; the brightness measures how many.
2. Explain, with an equation, why adding sodium hydroxide solution makes the bulb as bright as in the hydrochloric acid beaker, even though the acid is being used up.
Read the worked answer
Equation: CH₃COOH + NaOH → CH₃COONa + H₂O
What changes: before the addition, the beaker contains mostly un-ionised CH₃COOH molecules with only a small number of CH₃COO⁻ and H⁺ ions. Adding NaOH converts the weak acid into sodium acetate, an ionic salt that dissociates completely into Na⁺ and CH₃COO⁻. The OH⁻ from the base removes H⁺ as water, pulling the weak-acid equilibrium over to the ionised side and, at the same time, replacing a barely-ionised acid with a fully dissociated salt.
Result: the total number of free ions in the beaker rises sharply - roughly two ions per formula unit of acetate formed - so the current and the brightness rise with it. This is why the glow strengthens even though the amount of "acid" is falling.
Judgement to note: brightness here is a measure of ion concentration, not of acidity. A solution can be far less acidic yet a much better conductor, and a solution can be strongly acidic yet a poor conductor if the acid is weak and undissociated.
3. In the electrolysis of concentrated sodium chloride solution, identify the gas at each rod and explain why the pink colour appears around one rod only.
Read the worked answer
At the cathode (negative rod): H⁺ ions from water are discharged in preference to Na⁺, giving hydrogen, a colourless gas: 2H₂O + 2e⁻ → H₂ + 2OH⁻.
At the anode (positive rod): Cl⁻ ions are discharged in preference to OH⁻ because the solution is concentrated, giving chlorine, a pale yellow-green gas: 2Cl⁻ → Cl₂ + 2e⁻.
Why pink appears at the cathode only: each hydrogen ion reduced at the cathode leaves behind one OH⁻ ion, so the solution around the cathode becomes progressively alkaline. Phenolphthalein is colourless in acid or neutral solution and pink above about pH 8.2, so the pink zone grows around the negative rod. Around the anode no OH⁻ accumulates - chloride is oxidised, and any chlorine that dissolves forms hypochlorous and hydrochloric acid, making that region acidic, which keeps phenolphthalein colourless.
The pink is therefore a location marker for the cathode: it shows where reduction is happening, and it is a useful, cheap alternative to collecting gas when the current is small.