The Four Unlabelled Bottles and the Acid Spill
During a monsoon stock check, a Class 10 laboratory assistant finds four reagent bottles whose labels have washed off. The stock register says the four must be dilute hydrochloric acid, sodium hydroxide solution, sodium hydrogencarbonate solution and sodium chloride solution. The assistant runs a quick test on each and records:
- A: pH paper reads 2; blue litmus turns red; a drop added to a pinch of sodium hydrogencarbonate fizzes steadily.
- B: pH paper reads 13; red litmus turns blue; the liquid feels slippery between the fingers.
- C: pH paper reads 8; litmus papers barely change; it fizzes only after a few drops of A are added.
- D: pH paper reads 7; no litmus change; adding silver nitrate gives a white precipitate that turns grey in sunlight.
The bottles must be relabelled before the board practical. The teacher also wants to know which of the four should be kept ready to treat a small acid spill on the demonstration bench, and which must never be used for that purpose.
1. Identify each bottle and justify every identification with at least two independent observations from the list. Which single test in the list is the least useful for telling the bottles apart?
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- A = dilute hydrochloric acid. Three independent signs point the same way: pH 2 (strongly acidic), blue litmus turning red, and brisk effervescence with sodium hydrogencarbonate, which is the classic carbonate test giving CO2.
- B = sodium hydroxide solution. pH 13, red litmus turning blue, and the slippery soapy feel caused by saponification of skin oils - a strong alkali, not a carbonate, because no fizzing is reported.
- C = sodium hydrogencarbonate solution. It is only faintly alkaline (pH 8) and does almost nothing to litmus, yet it fizzes with A. A fizz on adding acid proves a carbonate or hydrogencarbonate is present, and of the four candidates only NaHCO3 fits.
- D = sodium chloride solution. Neutral pH, no litmus change, and a white precipitate with silver nitrate that darkens in light - AgCl decomposing to grey silver. No other candidate gives this.
Least useful test: the litmus test. It cannot separate C from D, because pH 8 and pH 7 both leave litmus essentially unchanged. It also cannot separate A from any acid, or B from any alkali. Identification here rests on the carbonate and chloride tests, with litmus only confirming the extremes.
2. For a small acid spill on the bench, should the assistant reach for B or for C? Give the chemistry and the safety reasoning, and describe how the treatment would be carried out.
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Use C, sodium hydrogencarbonate. Reasoning:
- Strength of the reagent. C is only weakly basic (pH 8) and is a mild, food-grade solid. B is a strong alkali at pH 13, corrosive to skin and eyes, and it attacks the bench surface and clothing.
- Built-in end point. With C the neutralisation is visible: fizzing continues while acid remains and stops when the acid is used up. With B there is no visible signal, so it is very easy to overshoot and leave a corrosive puddle.
- Heat. A strong acid plus a strong base releases a large amount of heat quickly and can spit droplets; a hydrogencarbonate reacting with dilute acid is far gentler.
Procedure: ventilate and clear the area, dilute the spill with plenty of water first, sprinkle solid sodium hydrogencarbonate until fizzing stops, check with pH paper that the patch is near pH 7, then mop up and wash the area with water. Never pour strong base directly onto a strong acid spill in a confined space, and never use the strong alkali on skin - flush with running water for at least 15 minutes instead.
3. A classmate argues: "C must be neutral, because litmus paper shows no colour change." Refute this using the recorded data and explain why a hydrogencarbonate solution behaves this way.
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The claim confuses a weak response from an indicator with a neutral solution. The pH paper records 8 for C and 7 for D - that difference is the whole point. A solution at pH 8 has more hydroxide ions than hydrogen ions and is mildly basic; a genuinely neutral solution is at pH 7, which is what D shows.
The reason is hydrolysis. Sodium hydrogencarbonate is the salt of a strong base (NaOH) and a weak acid (H2CO3). In water the hydrogencarbonate ion reacts with water, taking a proton to form undissociated H2CO3 and leaving OH⁻ behind, so the solution turns faintly alkaline. The effect is small, so litmus - which changes only over a narrow band well away from neutrality - barely responds.
Good practice is therefore to use universal indicator or a pH meter rather than relying on litmus for anything near neutral. Litmus is a yes/no test for clear acidity or alkalinity, not a measuring instrument. The fizzing of C with A is further independent proof that C is a carbonate-type species, not a neutral salt like D.