Acids, Bases, and Salts

Bleaching powder, commonly represented as calcium oxychloride, releases chlorine or chlorine-containing disinfecting species in water. These substances oxidise and destroy many harmful microorganisms, making the water safer. Baking soda and washing soda are carbonates used for other purposes, while blue vitriol is copper sulfate and is not the standard school-level disinfectant for drinking-water treatment. Thus option A is correct.

Bleaching powder is used as a bleaching agent because it releases chlorine or active chlorine in suitable conditions. This active chlorine oxidises many coloured substances, reducing or removing their colour from cloth and wood pulp. Sodium chloride is common salt, ammonium chloride is used in several laboratory and industrial applications, and gypsum is a calcium sulfate mineral. Therefore only option A matches this use.

Sodium hydrogen carbonate reacts with an acid in an acid–carbonate reaction. For example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. The carbon dioxide is released rapidly as gas bubbles, and these bubbles produce visible effervescence. No oxygen is absorbed, water does not freeze as the cause, and sodium metal is not formed because the sodium remains in the salt solution as ions.

The governing concept is hydration of salts. Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The dot followed by 10H₂O shows that each formula unit of sodium carbonate is associated with ten water molecules of crystallisation. Therefore, option A is correct. Two, five, and half do not match the established formula of washing soda and refer to no correct hydration state here.

The governing concept is the industrial use of products obtained from brine electrolysis. The chlor-alkali process produces sodium hydroxide, chlorine, and hydrogen. Sodium hydroxide is an important alkali used in soap manufacture and in the paper industry, including processing plant materials. Therefore, option A is correct. The other choices are not recognised uses of sodium hydroxide and do not describe chemical industries based on this product.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, whose chemical formula is Ca(OH)₂. The reaction produces bleaching powder, commonly represented as CaOCl₂, along with water. Therefore, chlorine is the required gas. Hydrogen, nitrogen, and carbon monoxide do not participate in this standard preparation and are not suitable distractors for the reaction.

Bleaching powder releases active chlorine in moist conditions or in water. This chlorine oxidises and destroys many disease-causing microorganisms, so the substance is used as a disinfectant for water in controlled quantities. Its action is not related to sweetness, conversion of water into oil, or changing minerals into gold. Hence option A correctly identifies the relevant property.

On the pH scale, a value of 7 represents a neutral solution at ordinary school-level conditions. Values greater than 7 indicate a basic or alkaline solution, and pH 11 is therefore basic. The change from 7 to 11 shows that the solution has changed from neutral to basic. It does not become acidic, because acidic solutions have pH values below 7.

Neutralisation is the reaction between an acid and a base in which salt and water are formed. Hydrochloric acid reacts with sodium hydroxide according to HCl + NaOH → NaCl + H₂O. The other pairs represent dissolution or no relevant acid-base reaction under ordinary conditions: sodium chloride and sugar dissolve in water, while copper does not neutralise water. Hence A is correct.

The governing concept is acid strength, which depends on the extent of ionisation in water. A weak acid, such as ethanoic acid, ionises only partially and establishes an equilibrium between molecules and ions. Therefore, option A is correct. Weak does not mean insoluble, neutral, or basic; a weak acid can dissolve and still produce hydrogen or hydronium ions and show acidic properties.

The governing concept is the colour range of an acid–base indicator. Phenolphthalein remains colourless in acidic and approximately neutral conditions, but its structure changes in a sufficiently basic aqueous medium, producing a pink colour. Thus option A is correct. Acidic solutions do not turn it pink, and a neutral salt solution normally gives no pink colour; the dry-acid choice is not a valid indicator test.

The governing concept is the use of natural acid–base indicators. Turmeric contains curcumin, which normally appears yellow but turns reddish-brown in a basic medium. Soap solution is generally basic because of its alkaline components. Consequently, option A correctly explains the stain. Soap is not an acid, turmeric does not become a metal, and the colour is not produced by carbon dioxide.

The governing concept is indicator behaviour: an indicator changes colour according to whether the medium is acidic or basic. Red cabbage juice contains anthocyanin pigments whose molecular form changes with pH, so different colours appear in acidic, neutral, and basic solutions. Option A is therefore correct. The juice does not transform substances into gold or gas and is not restricted to testing metals.

The governing pattern is: active metal + dilute acid → salt + hydrogen gas. For example, magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen: Mg + 2HCl → MgCl₂ + H₂. Therefore, option A is correct. Carbon dioxide is normally produced when an acid reacts with a carbonate or hydrogen carbonate, not with an ordinary active metal; water and oxygen or a base and chlorine are not the general products here.

Magnesium and zinc are both metals above hydrogen in the usual reactivity series, so each can displace hydrogen from a dilute non-oxidising acid. The general reaction is metal + acid → salt + H₂; for example, Zn + 2HCl → ZnCl₂ + H₂. Thus hydrogen is common to both reactions, making option A correct. Carbon dioxide would suggest a carbonate, chlorine requires a different chemical process, and ammonia is not produced in this reaction.

Indicators provide the governing evidence. An acidic solution changes blue litmus to red, while phenolphthalein remains colourless in an acidic or sufficiently neutral medium. The two observations together therefore identify the solution as acidic, so option A is correct. A basic solution would turn red litmus blue and generally make phenolphthalein pink. The test does not justify claiming that the solution must specifically be sodium chloride.

The governing concept is the indicator test for a base. A basic solution changes red litmus to blue and turns colourless phenolphthalein pink. Sodium hydroxide dissolves in water to give an alkaline solution, so option A satisfies both observations. Lemon juice and vinegar contain acids and would show the opposite litmus behaviour, while soda water is mildly acidic because dissolved carbon dioxide forms carbonic acid; it would not normally turn phenolphthalein pink.

According to the Arrhenius concept, a base is a substance that produces hydroxide ions, OH⁻, in aqueous solution. Many soluble bases also feel slippery or soapy because they react with oils on the skin; however, unknown substances should never be tested by touch. Acids produce H⁺ ions, a neutral salt is not necessarily hydroxide-producing, and a metal is a broader element class. Therefore option A is correct.

Neutralisation is the reaction of an acid with a base, usually producing salt and water. Hydrochloric acid supplies H⁺ ions and sodium hydroxide supplies OH⁻ ions; they combine as H⁺ + OH⁻ → H₂O, while Na⁺ and Cl⁻ form sodium chloride: HCl + NaOH → NaCl + H₂O. Mixing salt, sand, or sugar with water is dissolution or suspension, not an acid-base reaction. Thus A is certain.

Diluting a concentrated acid with water is strongly exothermic. If water is poured rapidly into acid, the small amount of water can heat suddenly, boil, and eject corrosive acid droplets. The safe laboratory rule is to add acid slowly to a larger volume of water while stirring continuously and using suitable protection. This spreads the heat through the water. Therefore option A is correct; heating the acid or avoiding stirring increases danger.

Plaster of Paris is calcium sulfate hemihydrate, CaSO4·1/2H2O. It reacts with water or atmospheric moisture and changes into gypsum, CaSO4·2H2O. This hydration causes the material to set and become hard, so it may lose its usefulness during storage. The other choices describe no relevant property: it neither forms a metal from oxygen, turns black in ordinary light, nor explodes with nitrogen.

Sodium hydrogen carbonate, or baking soda, decomposes on heating: 2NaHCO3 → Na2CO3 + H2O + CO2. The released carbon dioxide forms bubbles in dough or batter, and these bubbles expand during cooking, making the food rise and become porous. Oxygen is not the gas responsible, and water removal or hardening does not explain the leavening effect.

Chlorine is a strong disinfectant because it reacts with water to form substances such as hypochlorous acid, which can destroy many microorganisms. Controlled chlorination is therefore used to make drinking water safer. Antacids reduce stomach acidity, carbon dioxide makes cakes rise, and chlorine is not normally selected simply to acidify soil. Hence option A correctly matches the property with its use.

Hydrochloric acid reacts with sodium hydroxide in a neutralisation reaction: HCl + NaOH → NaCl + H2O. A measurable temperature rise means that the reaction releases heat to the surroundings, so it is exothermic. The formation of water and salt confirms a chemical change. Therefore option A is correct; an endothermic process would absorb heat and usually lower the temperature without external heating.

Washing soda is the common name for hydrated sodium carbonate. The sodium carbonate part is Na₂CO₃, and the crystal contains ten molecules of water of crystallisation, so its complete formula is Na₂CO₃·10H₂O. Sodium bicarbonate, NaHCO₃, is baking soda. CaSO₄·½H₂O is plaster of Paris, whereas CaSO₄·2H₂O is gypsum. Hence option A is unambiguously correct.