Acids, Bases, and Salts

A pH of 9 indicates that the soil is alkaline. Adding slaked lime or sodium hydroxide would increase alkalinity, and soap solution would also be unsuitable. Properly decomposed organic matter can gradually improve soil structure and, through biological decomposition, help moderate excessive alkalinity when used with soil testing and agricultural advice. Therefore option A is the most suitable choice among those given.

The governing idea is dilution. Adding water increases the solution’s volume, so the concentration of hydrogen ions decreases and the pH moves toward 7. However, dilution does not automatically remove every hydrogen ion or create hydroxide ions in excess. Therefore the solution can remain acidic, although weaker. Option A is correct; water does not turn every acid basic, destroy all ions, or leave pH unchanged.

An electric current in a solution requires mobile charged particles. When an acid is diluted with water, it ionises to form hydronium ions and corresponding negative ions; for example, HCl + H₂O → H₃O⁺ + Cl⁻. These ions move toward opposite electrodes and carry charge through the solution. Colour, sour taste, or temperature does not produce electrical conduction. Therefore the presence of mobile ions makes option A correct.

Dilution of a concentrated acid is highly exothermic: heat is released when acid particles become solvated by water. Adding acid slowly to a larger quantity of water allows the released heat to disperse through the water and permits controlled stirring, reducing the chance of sudden boiling and splashing. Adding a small amount of water to concentrated acid can create intense local heating and dangerous splashes. The acid does not freeze or become a base, so option A is correct.

When carbon dioxide is first passed through limewater, insoluble calcium carbonate forms and causes milkiness: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. If excess CO₂ is bubbled through, the calcium carbonate reacts further with carbon dioxide and water to form soluble calcium hydrogen carbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. The precipitate dissolves, so option A is correct.

The pH scale expresses hydrogen-ion concentration on a logarithmic scale. Values below 7 indicate acidity, and a higher pH means a lower hydrogen-ion concentration and therefore weaker acidity. A rise from pH 3 to pH 6 represents a 1000-fold decrease in hydrogen-ion concentration, although the solution remains acidic because 6 is still below 7. Thus option A is correct, while option C is false.

Hydrogen chloride shows acidic behaviour only after dissolving in water and ionising to produce hydronium ions. Dry HCl gas passing over dry blue litmus has no available water, so the ions needed for the colour change are not formed and the paper remains blue. In moist conditions HCl would turn blue litmus red. Therefore option B is the precise conclusion.

The pH scale expresses hydrogen-ion concentration: values below 7 indicate acidic solutions, 7 is neutral, and values above 7 indicate basic solutions. Therefore X, with pH 3, is acidic, while Y, with pH 11, is basic. The numerical separation also shows that they do not have the same acidic character. Hence the correct statement is option B; option A reverses the meanings.

Gypsum is CaSO4·2H2O. Controlled heating removes only part of its water of crystallisation and produces plaster of Paris, CaSO4·1/2H2O. If heating is excessive, the remaining water can also be driven off, producing anhydrous calcium sulphate, CaSO4, commonly called dead-burnt plaster. This material does not rehydrate and set properly, so option A explains the loss of the desired product.

The governing concept is electrolytic conductivity, which depends mainly on the concentration and mobility of ions in solution. Hydrochloric acid is a strong acid and ionises almost completely in water, producing a much larger concentration of mobile H+ and Cl− ions. Ethanoic acid is weak and ionises only partially at the same formal concentration, so fewer charge carriers are available. Therefore hydrochloric acid conducts better; option A is correct.

Initially, carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate, which causes milkiness: Ca(OH)2 + CO2 → CaCO3 + H2O. When excess CO2 is passed, the calcium carbonate reacts further and forms soluble calcium hydrogen carbonate: CaCO3 + CO2 + H2O → Ca(HCO3)2. Thus option A is correct; calcium carbonate is the first, insoluble product.

An acid shows its characteristic behavior in aqueous medium, where it ionizes and produces hydronium ions, H3O+. Dry blue litmus contains insufficient water for the acidic gas to ionize effectively, so no colour change is observed. Moist litmus provides water; the gas dissolves, forms hydronium ions, and changes blue litmus to red. Therefore option A is correct, while the other statements confuse the role of moisture.

Ammonia itself is a covalent gas, but in the presence of water it dissolves and reacts reversibly to form ammonium and hydroxide ions: NH3 + H2O ⇌ NH4+ + OH−. The hydroxide ions turn moist red litmus blue. Dry litmus lacks the water needed for this ion formation, so it shows no change. Thus option A is correct; the observation does not mean ammonia is a strong acid or always neutral.

Sodium hydroxide is deliquescent, so it absorbs moisture from air. It also reacts with atmospheric carbon dioxide to form sodium carbonate: 2NaOH + CO2 → Na2CO3 + H2O. Absorbed water changes the sample’s concentration, while carbonate formation changes its composition; both effects reduce purity. Therefore option A is correct. The other options describe chemically impossible transformations under ordinary storage conditions.

Bleaching powder, commonly represented as CaOCl2, works mainly because it releases available chlorine, which provides oxidising and disinfecting action. In a moist, open environment it may undergo decomposition and react with moisture and carbon dioxide, so the amount of available chlorine decreases over time. Consequently its bleaching and disinfecting power falls. Therefore option A is correct, and it should be stored in a dry, tightly closed container.

The pH scale is logarithmic: pH = −log[H⁺]. A rise of one pH unit means the hydrogen-ion concentration, and therefore acidity, decreases by a factor of 10. The change from pH 3 to pH 6 is three units, so the decrease is 10 × 10 × 10 = 1000 times. Thus option A is correct; three, one hundred, and ten times do not match the three-unit logarithmic change.

The relevant principle is the metal reactivity series. A metal above hydrogen can displace hydrogen from a dilute acid, producing hydrogen gas; for example, metal + acid can form a salt and H₂. Copper is below hydrogen in the usual school-level reactivity series and generally does not release hydrogen from dilute non-oxidising acids. Thus A follows. The gas is not always carbon dioxide, and the observation does not make copper highly reactive.

A blue indicator paper turning red is evidence that the solution is acidic. Many acids react with carbonates to release carbon dioxide, so bubbling would normally be expected when the reagent and conditions are suitable. However, no bubbles can result from an unsuitable or insufficient carbonate, low acid concentration, poor contact, or an experimental fault. Thus the cautious conclusion in A is best; the observation does not prove that the solution is basic, neutral, or only sugar.

The experiment demonstrates the role of water in producing acidic ions. Dry hydrogen chloride gas does not provide mobile H⁺ or hydronium ions to the dry indicator, so no red colour appears. On moist paper, HCl dissolves in water and ionises, forming hydronium ions; these give the acidic indicator colour. Therefore A is correct. The result does not show that HCl is basic, that dry paper is defective, or that water alone is an acid.

The governing concept is the pH scale: a lower pH means a greater hydrogen-ion concentration. At the same formal concentration, the solution with pH 2 has more H⁺ ions than the solution with pH 5, so it shows the stronger acidic effect and is more extensively ionised under the stated comparison. In fact, the difference of three pH units corresponds to about a thousandfold difference in hydrogen-ion concentration. Therefore A is correct.

Amphoteric substances react with both acids and bases. Zinc reacts with a dilute acid to release hydrogen, for example, zinc plus hydrochloric acid forms zinc chloride and hydrogen. It also reacts with concentrated sodium hydroxide solution to form a soluble zincate compound and hydrogen. Thus the observation demonstrates amphoteric behaviour, not merely acidic, neutral, or oxidising behaviour. Option A is correct.

Carbon dioxide is an acidic oxide, so it reacts with the hydroxide ions supplied by sodium hydroxide. With limited carbon dioxide, sodium carbonate may form: 2NaOH + CO₂ → Na₂CO₃ + H₂O; with excess carbon dioxide, sodium hydrogen carbonate can form. In either case, hydroxide ions are consumed, so the solution becomes less basic. Therefore, option A is correct.

The gas is identified as carbon dioxide because it turns limewater milky by forming insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. When excess CO₂ is passed, the precipitate reacts further to form soluble calcium hydrogencarbonate, so the milkiness disappears. Acids release CO₂ from carbonates and hydrogencarbonates. Thus option A is correct.

The governing concept is electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions are oxidised to chlorine; at the cathode, water is reduced to hydrogen and hydroxide ions. The remaining sodium ions combine with hydroxide ions to form sodium hydroxide. Therefore option A lists all three useful products, while the other choices omit or misidentify them.

The governing electrochemical rule is oxidation at the anode, meaning loss of electrons. In brine, chloride ions migrate toward the anode and are oxidised according to 2Cl⁻ → Cl₂ + 2e⁻, releasing chlorine gas. Hence option A is correct. Hydrogen is produced at the cathode, sodium metal is not deposited from aqueous brine, and sodium hydroxide remains in solution rather than precipitating as stated.