Acids, Bases, and Salts

Correct answer: A. Hydrogen

An active metal displaces hydrogen from a dilute acid, producing a salt and hydrogen gas. For example, zinc reacts with hydrochloric acid as Zn + 2HCl → ZnCl₂ + H₂↑. Hence option A is correct. Carbon dioxide is commonly released when an acid reacts with a carbonate, whereas oxygen and nitrogen are not the usual products of an acid–active-metal reaction.

Correct answer: B. By a pop sound near a burning splint

Hydrogen is a combustible gas, so its confirmatory school-level test is the pop test. When a burning splint is brought near the mouth of a vessel containing hydrogen, the gas burns rapidly with a characteristic ‘pop’ sound; therefore option B is correct. Limewater turning milky indicates carbon dioxide, while turmeric and phenolphthalein colour changes indicate alkaline conditions, not hydrogen.

Correct answer: B. Carbon dioxide

Acids react with carbonates to produce a salt, water, and carbon dioxide gas. The general reaction is acid + carbonate → salt + H₂O + CO₂; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Thus option B is correct. Hydrogen is released in many acid–metal reactions, whereas chlorine and ammonia require different reactants and are not general products here.

Correct answer: A. Milky

Carbon dioxide is identified by passing it through lime water, which is an aqueous solution of calcium hydroxide. The reaction is CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. The calcium carbonate produced is insoluble and forms a fine suspension, making the solution appear milky. Therefore, option A is correct; the other colours are not the characteristic result of this test.

Correct answer: A. Calcium carbonate

Lime water is a clear solution of calcium hydroxide. When carbon dioxide is passed through it, calcium carbonate is formed: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble in water, so its suspended particles scatter light and produce the milky appearance. Thus option A is correct. Sodium chloride and sodium hydroxide do not cause this observation, and hydrogen is not the precipitate formed.

Correct answer: A. Acidic

Non-metal oxides are generally acidic in nature. Many of them react with water to form acids; for instance, CO₂ + H₂O → H₂CO₃, and SO₂ + H₂O → H₂SO₃. They may also react with bases to produce salt and water. Therefore option A is correct. Basic, metallic and salty are not the general classifications for non-metal oxides, although a few oxides can show special or amphoteric behaviour.

Correct answer: B. Basic oxide

Copper oxide, CuO, is a metal oxide. Its reaction with hydrochloric acid is CuO + 2HCl → CuCl₂ + H₂O. A substance that reacts with an acid to form salt and water is showing basic oxide behaviour through neutralisation. Hence option B, basic oxide, is correct. It is not an acidic oxide, a gas or a common salt; those options do not describe the stated reaction or the substance’s composition.

Correct answer: B. Acidic oxide

Carbon dioxide, CO₂, is an oxide of carbon, and carbon is a non-metal. It is classified as an acidic oxide because it reacts with water to form carbonic acid: CO₂ + H₂O ⇌ H₂CO₃. It also reacts with bases such as lime water. Therefore option B is correct. It is not a basic oxide, a metal oxide or a salt; those choices conflict with its composition and reactions.

Correct answer: A. Sodium hydrogen carbonate

Baking soda is the common name for sodium hydrogen carbonate, with the formula NaHCO₃. It is a mild basic salt and reacts with acids to release carbon dioxide, for example NaHCO₃ + HCl → NaCl + H₂O + CO₂. Thus option A is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium carbonate is found in substances such as limestone and chalk.

Correct answer: C. Sodium carbonate decahydrate

Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The ten water molecules are called water of crystallisation, so option C is correct. Sodium hydrogen carbonate, NaHCO₃, is baking soda; sodium chloride is common salt; and calcium sulphate hemihydrate is plaster of Paris. These distinctions are based on the chemical composition and names of the salts.

Correct answer: A. Sodium chloride

Common salt is sodium chloride, whose chemical formula is NaCl. It is an ionic compound formed from sodium ions and chloride ions and is widely used in food. Therefore option A is correct. Sodium carbonate is a different sodium salt, sodium hydroxide is a strong base, and calcium carbonate is a calcium salt found in materials such as limestone and chalk. Their ions and properties differ from NaCl.

Correct answer: A. Calcium sulphate hemihydrate

Plaster of Paris is calcium sulphate hemihydrate, represented by CaSO₄·½H₂O, so option A is correct. It is prepared by carefully heating gypsum, CaSO₄·2H₂O, to remove part of its water of crystallisation. When water is added, it changes back to gypsum and sets into a hard mass. Sodium chloride, washing soda and calcium carbonate are different salts with different formulas and uses.

Correct answer: B. Chlorine

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, which is calcium hydroxide. The reaction produces bleaching powder containing available chlorine, so chlorine is the required reactant. Hydrogen and nitrogen do not form bleaching powder, while carbon dioxide is not the bleaching or disinfecting gas involved in this preparation. Therefore, option B is correct.

Correct answer: A. Dry slaked lime

The standard preparation of bleaching powder involves passing chlorine over dry slaked lime, whose chemical formula is Ca(OH)₂. Dry conditions are specified because moisture can interfere with the desired preparation. Common salt is a raw material for producing chlorine industrially, but it is not the substance over which chlorine is passed here. Thus option A is correct.

Correct answer: A. It can provide available chlorine

Bleaching powder is useful for water disinfection because it releases available chlorine when it reacts with water. This chlorine forms oxidising substances, including hypochlorous acid, which damage and kill many disease-causing microorganisms. It does not sweeten water, convert water into a metal, or freeze it. Therefore option A correctly states the reason.

Correct answer: A. Plaster of Paris

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses part of its water of crystallisation and forms calcium sulphate hemihydrate, CaSO₄·½H₂O, called plaster of Paris. Washing soda, baking soda, and common salt are different salts and are not produced by heating gypsum. Hence option A is correct.

Correct answer: A. It changes into gypsum

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. When water is added, it combines with the powder and re-forms gypsum, CaSO₄·2H₂O. The new gypsum crystals interlock and produce a hard solid, which is why plaster is used for casts and moulds. No sodium chloride, gas, or sugar is formed. Option A is correct.

Correct answer: A. Water of crystallisation

Hydrated copper sulphate crystals are represented as CuSO₄·5H₂O. The fixed water molecules in this crystalline structure are called water of crystallisation and are associated with the characteristic blue appearance of the hydrated salt. Sodium ions are absent, and chlorine or hydrogen gas is not responsible for the colour. Therefore option A is the intended correct answer.

Correct answer: A. Water of crystallisation is removed

Blue copper sulphate crystals contain water of crystallisation and can be written as CuSO₄·5H₂O. On heating, this water is driven out, producing anhydrous copper sulphate, which is white. Adding water restores the blue hydrated form. Sodium, chlorine, and sugar are not formed in this change, so option A gives the correct explanation.

Correct answer: A. Fixed amount of water in salt crystals

Water of crystallisation is the definite number of water molecules chemically associated with one formula unit of certain crystalline salts. It is part of the crystal structure, not merely water absorbed on the surface. For example, CuSO₄·5H₂O contains five such water molecules per formula unit. Hence option A gives the correct definition.

Correct answer: A. Calcium and magnesium ions

Hardness of water is mainly caused by dissolved calcium and magnesium ions. Washing soda, sodium carbonate decahydrate, supplies carbonate ions that react with them to form insoluble calcium carbonate and magnesium carbonate. These precipitates can be removed, reducing hardness. Sodium and chloride ions do not cause ordinary hardness, so option A is correct.

Correct answer: B. Carbon dioxide

Baking soda is sodium hydrogen carbonate, NaHCO₃. When it reacts with an acid, the hydrogen carbonate ion forms carbonic acid, which quickly decomposes into carbon dioxide and water. For example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. Effervescence is caused by the released CO₂, so option B is correct.

Correct answer: A. By producing carbon dioxide

Baking powder contains sodium hydrogen carbonate along with a mild edible acid. When the mixture becomes moist and is heated, the acid reacts with the bicarbonate and releases carbon dioxide gas. The gas gets trapped as bubbles in the dough or batter; on heating, these bubbles expand and make the food rise. Hydrogen, chlorine, and nitrogen are not the gases responsible for this leavening action.

Correct answer: A. Sodium carbonate

On heating, baking soda, or sodium hydrogen carbonate (NaHCO₃), undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Sodium carbonate is the main solid residue, while carbon dioxide escapes as a gas and water is produced as vapour. Sodium chloride, calcium carbonate, and ammonium chloride are not products of this decomposition reaction, so option A is correct.

Correct answer: A. Brine

The chlor-alkali process is the electrolysis of brine, which 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 solution. Sugar water is not an electrolyte that gives these products, while lime water and vinegar are not the industrial feedstock for this named process.