Acids, Bases, and Salts

The governing concept is the reaction of carbon dioxide with lime water, which is aqueous calcium hydroxide. At first, CO₂ reacts as Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Insoluble calcium carbonate particles scatter light and make the solution milky. When excess CO₂ is passed, CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium hydrogen carbonate is soluble, so the suspended solid disappears and the milkiness is removed. Therefore option A is correct; the other options describe unrelated products.

The relevant concept is the degree of ionisation of acids in water. For equal formal concentrations, a strong acid ionises almost completely and produces a larger concentration of H⁺ ions than a weak acid. The higher H⁺ concentration gives a lower pH, while the greater number of mobile ions generally produces higher electrical conductivity. Thus the described sample is most likely a strong acid, so option B is correct. A weak acid ionises only partially, and the salt or base choices do not fit both observations.

The governing safety principle is that dilution of a concentrated acid is exothermic: hydration of the ions releases considerable heat. If a small amount of water is poured into concentrated acid, that water can heat rapidly, boil locally, and eject corrosive acid droplets. Adding acid slowly to a larger volume of water allows the released heat to spread through the water and permits controlled stirring and cooling. Therefore option A is correct. The other choices give chemically false reasons.

In the chlor-alkali process, concentrated brine is electrolysed. At the cathode, water is reduced and hydrogen gas is released: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Hydrogen is combustible, so bringing a burning splint or flame close to the collected gas produces the characteristic ‘pop’ sound. Hence option B is correct. Lime water is a test for carbon dioxide, turmeric indicates a base under suitable conditions, and blue litmus turning red indicates acidity; none identifies hydrogen.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses only one and a half molecules of water and forms plaster of Paris, CaSO₄·½H₂O: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. This hemihydrate mixes with water and sets into a hard solid. Excessive heating can remove all the water of crystallisation and produce anhydrous calcium sulphate, which does not set properly. Therefore option A is correct.

Indicator behaviour shows that the solution is acidic: methyl orange is red in a strongly acidic medium, and phenolphthalein remains colourless in acidic and neutral media. To neutralise an acid, a suitable base must be added so that H⁺ and OH⁻ react to form water: H⁺ + OH⁻ → H₂O. Sodium hydroxide solution supplies OH⁻ ions and is therefore the correct choice, option B. Adding more acid would increase acidity, while sodium chloride or water alone does not provide the required neutralising base.

Sodium hydroxide is deliquescent, meaning that solid NaOH absorbs water vapour from humid air and may dissolve in the absorbed moisture; this explains why pellets become wet. It also reacts with acidic carbon dioxide in air: 2NaOH + CO₂ → Na₂CO₃ + H₂O. Formation of sodium carbonate changes the composition and lowers the purity of the stored hydroxide. Thus option A correctly explains both observations. The other options describe impossible transformations under ordinary open-air conditions.

The decisive test is the reaction with an acid. Carbonates react with acids to release carbon dioxide; for example, Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Sodium carbonate is also a basic salt because the carbonate ion hydrolyses in water and produces OH⁻ ions, making its aqueous solution alkaline. Therefore option C satisfies both observations. Sodium chloride is essentially neutral, ammonium chloride gives an acidic solution, and copper sulphate does not give carbon dioxide in this test.

The governing concept is acid attack on tooth enamel. Enamel is mainly a calcium-containing mineral, and acids can dissolve it when the mouth becomes sufficiently acidic. In school-level chemistry, the commonly cited critical pH is about 5.5; below this value, demineralisation of enamel becomes more likely and repeated exposure can contribute to tooth decay. Therefore the mouth should preferably remain above about 5.5, making option B correct. The other values are far more acidic and would not provide protection.

Neutrality and electrical conductivity describe different properties. Neutrality refers mainly to the balance of acidic and basic effects, whereas conductivity requires charged particles that can move. When sodium chloride dissolves, it separates into mobile Na⁺ and Cl⁻ ions. These ions carry charge through the solution, so it conducts electricity. Options B, C, and D incorrectly confuse neutrality with the absence of ions or with metallic behaviour.

Marble consists mainly of calcium carbonate, CaCO₃. Acids present in acid rain react with this carbonate, producing a salt, water, and carbon dioxide; for example, an acid reacting with CaCO₃ gradually dissolves or roughens the surface. Sodium chloride and ammonium chloride do not represent the main reactive component of marble, while sodium hydroxide is a base and is not the substance responsible for marble’s vulnerability. Therefore A is correct.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it decomposes according to 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The solid residue is therefore sodium carbonate, Na₂CO₃. Its aqueous solution is basic because carbonate ions hydrolyse water to produce hydroxide ions, so it turns red litmus blue. Sodium chloride is neutral, while the other choices do not match this decomposition.

An acidic solution contains a relatively high concentration of hydrogen or hydronium ions. Added hydroxide ions react with them in the neutralisation process: H⁺ + OH⁻ → H₂O. As H⁺ concentration falls, the negative logarithm used to define pH increases, so the solution becomes less acidic and its pH rises. The other options describe no valid consequence of adding hydroxide ions.

Copper(II) oxide reacts with an acid such as hydrochloric acid: CuO + 2HCl → CuCl₂ + H₂O. Formation of salt and water from an acid and a metal oxide is a neutralisation reaction, showing that the oxide has basic character. Most metal oxides are basic, although a few may be amphoteric. Copper oxide is not an acidic oxide, a gas, or a salt, so option B is correct.

Turmeric is a natural indicator that changes from yellow to reddish-brown in a basic medium. Methyl orange is yellow in a neutral or basic medium and becomes red in a sufficiently acidic medium. Therefore the two observed colours are consistent with an alkaline solution. The evidence does not support acidity, definite neutrality, or a gaseous state. Hence option B is the only appropriate conclusion.

Aqueous sodium chloride is called brine. In the chlor-alkali process, electrolysis of brine produces chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide in the solution: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Because the dissolved sodium ions combine with hydroxide ions, the base obtained is sodium hydroxide. None of the calcium, ammonium, or magnesium hydroxides is produced in this process.

The pH relationship is pH = −log[H⁺], so a rise of one pH unit means a tenfold decrease in hydrogen-ion concentration. The pH rises from 2 to 5, a change of three units. Therefore, [H⁺] decreases by 10 × 10 × 10 = 1000 times. It does not decrease merely three times, and it certainly does not increase. Hence option A is correct.

Bleaching powder, commonly represented as calcium oxychloride, is effective because it supplies available chlorine. Moisture, carbon dioxide, and prolonged exposure to air can cause decomposition and reduce this available chlorine, lowering its bleaching and disinfecting power. A dry, tightly closed container limits these effects and preserves the substance. The other options describe no valid storage purpose, so option A is correct.

Blue litmus turning red establishes that the solution is acidic. An acid can liberate hydrogen only from a metal placed above hydrogen in the reactivity series. Magnesium is above hydrogen and reacts with dilute acids, producing hydrogen gas; copper is below hydrogen and normally cannot displace it from a non-oxidising dilute acid. Thus option A correctly explains both observations.

A pH of 8.8 indicates that the soil is distinctly basic. To move its pH toward a range more suitable for many crops, a suitable mildly acidic amendment or organic matter may be applied carefully, preferably after soil testing. Slaked lime and sodium hydroxide are alkaline, so they would increase basicity rather than correct it. Soap solution is also unsuitable and may harm soil structure. Therefore option A is the best choice.

Acidic behaviour is produced by ions in aqueous conditions, especially hydrogen or hydronium ions. The gas cannot ionise the dry litmus paper sufficiently, so no colour change is observed. When the paper is moist, the gas dissolves in water and forms ions, which interact with the indicator and turn it red. Thus the observation demonstrates the role of water in ion formation; option A is correct.

A pH of 12 is far above neutral pH 7 and indicates a strongly basic solution. The slippery or soapy feel is also consistent with an alkali, but it must not be used as a safety test. Concentrated or sufficiently strong bases can break down skin oils and damage tissue, causing irritation or chemical burns. Therefore direct contact is unsafe and protective equipment is needed. Option A is correct.

The governing concept is the logarithmic pH scale, defined as pH = −log[H+]. A rise of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 3 to pH 6 is three units, so acidity decreases by 10 × 10 × 10 = 1000 times. Thus option A is correct. Option B treats pH as an ordinary linear scale, while C and D reverse or ignore the change.

The governing concept is neutralisation. Slaked lime is calcium hydroxide, Ca(OH)2, which is a basic substance. In acidic soil, its hydroxide ions react with acidic hydrogen ions and reduce the effective acidity; in simplified form, H+ + OH− → H2O. As hydrogen-ion concentration falls, pH rises. Therefore option A is correct, whereas B, C, and D contradict the chemical behaviour of lime.

The governing concept is salt hydrolysis and the nature of the parent acid and base. A salt from a strong acid and strong base, such as sodium chloride, gives an approximately neutral solution. A salt from a strong acid and weak base may be acidic, while one from a weak acid and strong base may be basic. Therefore option C is correct; options A and B overgeneralise, and D is false.