SwaPrep presents · A chapter in Science
Every sour taste and every soapy touch is the same old rivalry — H⁺ against OH⁻ — and the salts are its peace treaties.
6 acts · 13 scenes · scroll to play↓
I
Every acid tells the same story in water: it releases hydrogen ions — H⁺ — and that single act explains everything else about it. It is why acids taste sour, and why they turn blue litmus paper red. Strength is simply a measure of how completely the story is told: strong acids like hydrochloric, sulphuric and nitric acid ionise completely in solution, while weak acids — the acetic acid in vinegar, carbonic acid, citric acid — only partially let go of their hydrogen.
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Chemists split the acid family in two. The mineral acids — hydrochloric, sulphuric and nitric — are the inorganic heavyweights, and sulphuric acid is old enough to carry an alias: 'oil of vitriol'. The organic acids live in your kitchen and garden: acetic acid in vinegar, citric acid in lemon, lactic acid in curd, oxalic acid in tomato, tartaric acid in tamarind — and formic acid, delivered by the sting of an ant.
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Acids announce themselves in their reactions, too. Drop a metal into an acid and you get a salt plus hydrogen gas; pour acid over a carbonate and carbon dioxide bubbles out, turning lime water milky — the classic test. And when concentrated hydrochloric and nitric acids are mixed in a 3:1 ratio, they become aqua regia — a liquid powerful enough to dissolve gold and platinum.
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Bases are the acids' mirror image: in water they release hydroxide ions — OH⁻ — taste bitter, feel soapy, and turn red litmus blue. The strong ones carry industrial names: caustic soda, caustic potash and slaked lime. The weak ones are gentler — ammonium hydroxide, and the magnesium hydroxide sold as milk of magnesia. One definition wins marks by itself: an alkali is a base that dissolves in water, so every alkali is a base, but not every base is an alkali.
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II
Mix an acid with a base and the rivalry ends at once: the two neutralize each other, producing nothing more dangerous than a salt and water. The reaction gives off heat — it is exothermic. The textbook example is the simplest one of all: hydrochloric acid plus sodium hydroxide gives common salt and water.
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III
In any liquid, the balance between acid and base can be read off a single number. The pH scale — 'potential of Hydrogen' — measures the concentration of H⁺ ions and runs from 0 to 14. Below 7 a solution is acidic; at exactly 7 it is neutral; above 7 it is basic, or alkaline.
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The body runs on tightly managed pH. Gastric juice in the stomach sits near a ferocious 1.2, lemon juice near 2.2, and milk just below neutral at about 6.5. Pure water marks the exact centre at 7.0 — while human blood is held inside the razor-thin band of 7.35 to 7.45. Farmers and dentists live by the same ruler: most crops grow best between pH 6 and 7, and tooth enamel begins to corrode once the mouth drops below pH 5.5.
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IV
Acids and bases hide in clear liquids, so chemists use indicators — dyes whose colour change confesses what the liquid is. Litmus, extracted from lichen, goes blue-to-red in acid and red-to-blue in base. Phenolphthalein stays colourless in acid but flushes pink in base, while methyl orange shows red-orange in acid and yellow in base. Even kitchen turmeric is an indicator — yellow in acid, red-brown in base — which is why a turmeric stain turns red the moment soap touches it.
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V
Two of the most-tested salts in the syllabus come from the same industrial source — the Solvay process. Baking soda, sodium bicarbonate (NaHCO₃), leavens food in the kitchen, settles stomachs as an antacid, and fills fire extinguishers. Its tenfold-hydrated cousin washing soda, Na₂CO₃·10H₂O, makes glass, softens hard water, and cleans.
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Three more salts run whole industries. Bleaching powder, CaOCl₂, is made by passing chlorine over slaked lime; it bleaches textiles and paper and purifies water. Plaster of Paris, CaSO₄·½H₂O, is born by heating gypsum to exactly 373 K, and then goes on to set fractured bones, shape sculptures, and finish buildings. Caustic soda, NaOH, comes from the chlor-alkali electrolysis of brine and ends up making soap and degreasing metal.
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Some salts lock water molecules inside their own crystals — the water of crystallisation — and that hidden water gives them their colour. Copper sulphate, CuSO₄·5H₂O, is the famous blue vitriol: brilliantly blue with its water, white without it. Iron sulphate, FeSO₄·7H₂O, is green vitriol, and calcium sulphate with two waters, CaSO₄·2H₂O, is gypsum — the very stone that Plaster of Paris is roasted from.
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Water that refuses to lather is hard, and the exam wants you to know its two varieties. Temporary hardness comes from the bicarbonates of calcium and magnesium, and simple boiling removes it. Permanent hardness comes from their chlorides and sulphates — boiling cannot touch it, and only washing soda or an ion-exchange treatment will soften the water.
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VI
This chapter reliably yields one to two APPSC questions, and its favourite answers are already on record. Blood pH is 7.35 to 7.45; aqua regia mixes HCl and HNO₃ in 3:1; Plaster of Paris comes from gypsum heated to 373 K. Phenolphthalein turns pink in base, the acid in curd is lactic, sulphuric acid is 'oil of vitriol' — and a turmeric stain that turns red has simply met soap.
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End of chapter
The salts table — formula, preparation, uses — and the pH values are the most tested lines in this chapter. Open the full notes and drill them.
Practice MCQs on this chapter →