Acids, Bases, and Salts

Turmeric contains a natural indicator substance called curcumin. In an alkaline or basic medium, curcumin changes from its usual yellow colour to reddish-brown. Thus, when turmeric paper or turmeric paste contacts a base, the reddish-brown colour appears, so option B is correct. Acidic, neutral, and ordinary sugar solutions do not normally produce this characteristic turmeric change.

Phenolphthalein is an acid–base indicator whose colour depends on the medium. It remains colourless in acidic or neutral conditions but changes to pink, often bright pink in a sufficiently basic solution. Therefore option B is correct. Colourless is associated with its acidic form, whereas red and green are not the standard phenolphthalein colour changes used in this test.

Methyl orange is an acid–base indicator with two familiar colour forms. It appears red in an acidic solution and yellow in a basic or nearly neutral solution. Since the question specifies an acidic medium, the correct observation is red, so option A is correct. Yellow is the likely basic-medium distractor, while blue and green are not the standard colours of methyl orange.

The governing concept is indicator colour change with pH. Methyl orange is red in an acidic medium, but in a basic medium its colour is yellow. Therefore option B is correct. Red is the relevant distractor because it is the colour seen in acid, while black and blue are not the characteristic colours of methyl orange in this test. The observation helps distinguish acidic and basic solutions.

Baking soda is the common name of the salt sodium hydrogen carbonate, whose formula is NaHCO₃. It reacts with acids and releases carbon dioxide, which explains its use in baking and some antacid preparations. Therefore option A is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium carbonate is a different salt found in materials such as limestone.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Sodium carbonate is the solid residue, while carbon dioxide and water are gaseous or vapour products under the usual conditions. Hence option A is correct; the other choices are unrelated salts and are not formed in this decomposition.

Washing soda is the hydrated salt sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The ten water molecules are called water of crystallisation and are part of the crystal structure. Therefore option A is correct. Sodium hydrogen carbonate is baking soda, sodium chloride is common salt, and calcium sulphate hemihydrate is plaster of Paris, so these alternatives describe different substances.

Hard water contains dissolved calcium and magnesium ions, which react with soap to form scum and reduce lather. Washing soda supplies carbonate ions that precipitate these ions as insoluble carbonates, such as calcium carbonate. Removing the ions reduces hardness, so option A is correct. It does not soften water by forming hydrogen or chlorine gas, and making water acidic would not be the governing process.

The governing concept is the removal of water of crystallisation by controlled heating. Gypsum, CaSO4·2H2O, loses part of its combined water when heated moderately and forms calcium sulphate hemihydrate, CaSO4·½H2O, which is plaster of Paris. Common salt, baking soda and washing soda do not give plaster of Paris in this preparation.

Plaster of Paris is the partially dehydrated form of gypsum. Gypsum has the formula CaSO4·2H2O, whereas controlled heating produces CaSO4·½H2O. This compound is called calcium sulphate hemihydrate, so option A is correct. Calcium carbonate is limestone or chalk, sodium carbonate decahydrate is washing soda, and sodium chloride is common salt.

The governing concept is rehydration followed by setting. Plaster of Paris is calcium sulphate hemihydrate, CaSO4·½H2O. When water is added, it combines with the compound and reforms gypsum, CaSO4·2H2O. This hydration reaction produces a hard solid mass. It does not form sodium chloride, gas or sugar, so A is the only suitable answer.

The governing concept is water of crystallisation in hydrated salts. Blue copper sulphate crystals are CuSO4·5H2O, so each formula unit contains a fixed number of water molecules within the crystal structure. This hydrated form has the characteristic blue appearance. Sodium ions, chlorine gas and carbon dioxide are not responsible for the colour of copper sulphate crystals.

Blue copper sulphate crystals are hydrated CuSO4·5H2O. On heating, they lose their water of crystallisation and become anhydrous copper sulphate, which is white. Thus the colour disappears because the hydrated crystal structure changes, not because sodium or chlorine is added or carbon dioxide is formed. Adding water can restore the blue hydrated form.

The governing preparation is the reaction of chlorine with dry slaked lime. Dry slaked lime is calcium hydroxide, Ca(OH)2, and passing chlorine over it produces bleaching powder, commonly represented as CaOCl2, along with water under the usual school-level description. Moisture is avoided because the lime must remain dry for proper preparation. Therefore A is correct.

The governing concept is chemical disinfection by available chlorine. When bleaching powder comes into contact with water, it releases chlorine or chlorine-containing species that form hypochlorous acid. These reactive species damage and kill many disease-causing microorganisms. The effect is not related to sweetness, conversion into a metal, or freezing, so option A correctly explains its use.

Common salt is the familiar crystalline ionic compound represented by the formula NaCl. It contains sodium ions, Na+, and chloride ions, Cl−, in a regular lattice, so its chemical name is sodium chloride. Sodium carbonate is a different salt, calcium sulphate occurs in gypsum and plaster-related compounds, and sodium hydroxide is a strong alkali. Hence A is correct.

Brine is a concentrated aqueous solution of sodium chloride. During electrolysis, chloride ions produce chlorine at the anode, water produces hydrogen at the cathode, and sodium hydroxide remains in the solution. Because the process yields chlorine and an alkali, sodium hydroxide, it is called the chlor-alkali process. Neutralisation, evaporation and filtration are different processes.

The governing reaction is electrolysis of aqueous sodium chloride, or brine. Chloride ions are oxidised at the anode to form chlorine gas, while water is reduced at the cathode to form hydrogen gas and hydroxide ions. Sodium ions combine with hydroxide ions in solution to give sodium hydroxide. Therefore A lists all three main products; the other sets belong to unrelated processes.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions lose electrons: 2Cl⁻ → Cl₂ + 2e⁻, so chlorine gas is released there. At the cathode, water produces hydrogen gas, while sodium hydroxide remains in solution. Therefore option B is correct; hydrogen belongs at the cathode, not the anode.

In the chlor-alkali process, brine is electrolysed. At the cathode, water molecules gain electrons and form hydrogen gas: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Chloride ions form chlorine at the anode, not at the cathode. The hydrogen can be tested with a lighted splint, producing a characteristic pop. Therefore option A is correct.