Three Plots, Three Bags, Two Water Sources
Students of a school eco-club test the soil of three plots and the water from two sources. Plot P, under trees where leaf litter has collected for years, shows pH 5.1; plot Q, watered for years from the borewell, shows pH 7.0; plot R, beside a heap of cement bags and rubble, shows pH 8.4. The club wants to grow tomatoes, which do best between pH 6.0 and 6.8. The store has only three bags - calcium oxide, calcium carbonate and gypsum (calcium sulphate) - and nothing else. The club also notices that the borewell water needs a lot of soap before any lather forms, while rainwater collected in a drum lathers with very little soap. The teacher asks the club to plan the treatment of the plots using only what is in the store, to justify every choice with a test or a reason, and to put a short note on the two water sources on the school notice board.
1. Which plot or plots need treatment, in which direction, and which bag or bags should be used? Give the chemistry behind each choice.
Read the worked answer
Plot P at pH 5.1 is too acidic and must be raised towards 6.0–6.8; plot R at pH 8.4 is too alkaline and must be lowered; plot Q at 7.0 is already in the acceptable band and needs no chemical treatment.
For plot P, add a base. Calcium oxide is the strongest option: CaO + H2O → Ca(OH)2, and the hydroxide neutralises soil acids. It is caustic, so it must be weighed out in small amounts, spread evenly and mixed in at least a week before planting, otherwise seedlings are burnt. Calcium carbonate is milder and slower (CaCO3 + 2H+ → Ca2+ + H2O + CO2) and is the safer first choice for a school plot, with a second pH reading after two weeks. Gypsum is a neutral salt; it supplies calcium and improves the structure of sodic clay soils but does not raise pH, so it is the wrong bag for plot P.
For plot R, do not add lime. Add compost or leaf litter, which releases acids as it decays, and leach with good water; a little gypsum helps where sodium has built up. Always average at least three samples per plot before deciding.
2. A teacher suspects the "calcium oxide" bag has been standing open for months. Describe a simple laboratory test to check this, and say whether the spoiled material is still useful.
Read the worked answer
Open calcium oxide slowly takes up moisture and carbon dioxide from the air: CaO + CO2 → CaCO3, and CaO + H2O → Ca(OH)2. The bag is therefore partly converted to carbonate and hydroxide.
Test: put a spatula of the powder in a dry test tube and add dilute hydrochloric acid. Fresh CaO dissolves with hissing and much heat but gives no gas (CaO + 2HCl → CaCl2 + H2O). A spoiled sample gives brisk effervescence of CO2 (CaCO3 + 2HCl → CaCl2 + H2O + CO2), and the gas turns lime water milky. A second, independent check: add a few drops of water - fresh CaO crackles, swells and the tube becomes distinctly hot, and the paste turns red litmus blue strongly; a heavily spoiled sample shows little heat.
Judgement: the spoiled material is not useless. It is largely calcium carbonate, which is exactly the mild liming material recommended for a school plot; but it acts slowly and is needed in larger quantity, so it cannot be substituted gram for gram for CaO. It should be relabelled and dosed by pH testing, not by the original instruction on the bag.
3. Explain the difference in soap behaviour of the borewell water and the rainwater, describe a quick test to confirm it, and state one practical consequence for the school.
Read the worked answer
Borewell water has percolated through rock and carries dissolved calcium and magnesium salts, so it is hard. Soap is the sodium salt of a long-chain fatty acid; its anions meet Ca2+ and Mg2+ and form an insoluble scum before any lather can appear, so soap is wasted. Rainwater has evaporated from the sea, condensed and fallen almost free of dissolved salts, so it is soft and a little soap lathers at once.
Quick test: take equal volumes of the two waters in stoppered bottles, add soap solution drop by drop, shake ten times after each drop, and count the drops needed for a lather that lasts half a minute. The water needing more drops is the harder one.
Practical consequence: hard water is safe to drink and to irrigate most crops, but on boiling it deposits scale of CaCO3 and Mg(OH)2 inside kettles, geysers and boiler tubes. Scale insulates the heating element, so fuel or electricity is wasted and tubes can choke; the school geyser will need periodic descaling or a water softener, while the garden can use the borewell water as it is.