Two Unlabelled Bottles and a Sample of Hard Water
Two colourless liquids, A and B, have lost their labels in the school chemistry store. The teacher allows a student only these tests: smell, blue and red litmus paper, sodium hydrogencarbonate solution, a few drops of concentrated sulphuric acid under supervision, distilled water, and a sample of hard water with a bottle of soap solution C.
The student's record reads:
- A has a pleasant smell, is neutral to both litmus papers, and burns with a clean blue flame.
- B has a sharp, sour smell and turns blue litmus red; a pinch of sodium hydrogencarbonate added to B gives brisk effervescence, and the gas turns limewater milky.
- A gives no effervescence with sodium hydrogencarbonate.
- Warming A and B together with a few drops of concentrated sulphuric acid produces a sweet, fruity smell.
- Soap solution C lathers freely in distilled water but forms a white scum in the hard water sample.
The student concludes that B is a 'strong acid' because it bubbles with sodium hydrogencarbonate while A does not.
1. Identify A and B, giving two observations for each. Write the equation for the reaction of B with sodium hydrogencarbonate and name the compound responsible for the fruity smell.
Read the worked answer
A is ethanol, C2H5OH. It is neutral to litmus, burns with a clean blue flame, gives no CO2 with sodium hydrogencarbonate, and forms an ester with B and concentrated sulphuric acid. Its pleasant smell is also characteristic.
B is ethanoic acid, CH3COOH. It is sour, turns blue litmus red, and releases CO2 from sodium hydrogencarbonate - the standard test for a carboxylic acid.
Reaction with sodium hydrogencarbonate: CH3COOH + NaHCO3 → CH3COONa + H2O + CO2↑. The gas turns limewater milky, confirming CO2.
Esterification: CH3COOH + C2H5OH → CH3COOC2H5 + H2O (warm, concentrated H2SO4 as catalyst/dehydrating agent). The product is ethyl ethanoate, the sweet, fruity-smelling compound.
2. Is the student right to call B a 'strong acid' on the basis of the bubbling? Explain what the observations actually show, and describe a fair test to compare acid strength.
Read the worked answer
The observation is correct but the conclusion is wrong. Sodium hydrogencarbonate reacts only with acids, so the test shows that B is acidic and A is not. It does not compare the strength of two acids, because A is an alcohol, not an acid.
Ethanol has an -OH group whose hydrogen is not released to any significant extent in water. Ethanoic acid has a -COOH group, which loses H+ and sets up an equilibrium: CH3COOH ⇌ CH3COO- + H+. Even so, ethanoic acid is a weak acid - only partly ionised - not a strong acid like HCl.
A fair comparison needs solutions of the same concentration and a measurement that reflects ionisation: pH (lower pH means more acidic), electrical conductivity, or the volume of CO2 collected in a fixed time under identical conditions. Watching bubbles only shows 'acid present', not 'how strong'.
3. Explain, with an equation, why soap C forms scum in hard water. Would a detergent be a better choice for this village? Give one advantage and one disadvantage.
Read the worked answer
Soap is the sodium salt of a long-chain fatty acid, for example sodium stearate, C17H35COONa. Hard water contains Ca2+ and Mg2+ ions. These displace sodium and form an insoluble calcium or magnesium stearate:
2C17H35COONa + Ca2+ → (C17H35COO)2Ca↓ + 2Na+
That white, sticky precipitate is the scum. Part of the soap is used up before any lather appears, so washing wastes soap and the scum deposits on cloth and skin.
A detergent is usually a sodium salt of a long-chain sulphonic acid or sulphate. Its calcium and magnesium salts are soluble, so it lathers even in hard water.
Advantage: it works in hard water, does not form scum and therefore saves soap. Disadvantage: many detergents, especially those with branched chains, are poorly biodegradable; they cause persistent foam and pollution in ponds and rivers and can be harsh on skin. A biodegradable detergent is the better compromise for this village.