Acids, Bases, and Salts

The governing concept is safe laboratory identification of acids and bases. Unknown laboratory chemicals must never be tasted because even a small amount may be corrosive, poisonous, or otherwise harmful. Litmus, universal indicator, or pH paper can provide evidence without direct contact; a pH value also requires suitable measurement conditions. Therefore option A is the correct exam response. The remaining choices promote unsafe practice or make false claims about indicators and taste.

The governing concept is recognising characteristic acid reactions. Zinc, a reactive metal, reacts with a dilute acid to form a salt and hydrogen gas, for example Zn + 2HCl → ZnCl2 + H2. Limestone is mainly calcium carbonate; with acid it produces a salt, water, and carbon dioxide, for example CaCO3 + 2HCl → CaCl2 + H2O + CO2. Thus option A correctly identifies the gases. The reversed pair and chlorine or oxygen alternatives do not fit these reactions.

The governing concept is the logarithmic pH scale, defined as pH = −log10[H+]. A difference of one pH unit represents a tenfold difference in hydrogen-ion concentration. The values 1 and 4 differ by three units, so the ratio is 10^3 = 1000. Therefore the pH 1 solution has one thousand times the hydrogen-ion concentration of the pH 4 solution, making option C correct. It is not merely three times.

The governing concept is salt hydrolysis and the strength of the parent acid and base. A pH of 9 and the change of red litmus to blue show that the salt solution is basic. Salts formed from a weak acid and a strong base commonly give basic solutions because the conjugate base of the weak acid hydrolyses water and produces hydroxide ions. Therefore option B is correct. A strong acid-weak base salt is generally acidic, while a strong acid-strong base salt is approximately neutral.

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.

Litmus changes colour because of acidic or basic species present in an aqueous medium, not simply because a dry gas touches the paper. Turning red litmus blue demonstrates a basic effect, while the absence of a change on dry litmus shows that moisture is required for the gas to dissolve, ionise, or otherwise produce the relevant basic species. The safest conclusion is therefore that the gas forms a basic solution in moisture, making option A correct. The observation does not prove that the gas is a metal or always neutral.

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.

The pH scale is logarithmic, so a three-unit decrease corresponds to a factor of 10³ in hydrogen-ion concentration. For basicity, the relevant hydroxide-ion concentration changes oppositely: pH 14 corresponds to pOH 0, while pH 11 corresponds to pOH 3. Thus [OH⁻] falls from about 1 M to 10⁻³ M, a thousandfold decrease. The solution remains basic, but much less strongly basic; A is correct.

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.