Acids, Bases, and Salts

Neutrality and electrical conductivity describe different properties. A neutral sodium chloride solution has nearly equal concentrations of hydrogen and hydroxide ions, so its pH is close to 7. However, dissolved sodium chloride dissociates into mobile Na+ and Cl− ions. These charged particles move towards opposite electrodes and carry electric current. Therefore option A is correct; being neutral does not mean that a solution lacks ions. The other options are chemically incorrect.

Marble consists mainly of calcium carbonate, CaCO3. Acid rain contains acids such as sulfuric acid and nitric acid, which react with carbonate minerals. The reaction consumes calcium carbonate and produces soluble salts, water, and carbon dioxide; repeated reaction gradually erodes the polished surface. Thus option A is correct. Sodium chloride and copper sulphate are salts, while sodium hydroxide is a base and is not the principal constituent responsible for marble damage.

Baking soda is sodium hydrogen carbonate, NaHCO3. On heating, it undergoes thermal decomposition: 2NaHCO3 → Na2CO3 + CO2 + H2O. The solid residue is therefore sodium carbonate, Na2CO3, and its aqueous solution is alkaline because carbonate ions undergo hydrolysis and produce hydroxide ions. Option A is correct. Sodium chloride is neutral, calcium sulphate is a different salt, and ammonium chloride gives an acidic solution.

pH is related to hydrogen-ion concentration: pH = −log[H+]. An acidic solution contains excess H+ ions. When OH− ions are added, they neutralise these ions according to H+ + OH− → H2O. The hydrogen-ion concentration consequently decreases, so the solution becomes less acidic and its pH increases. Option A correctly states this neutralisation. The other options do not describe the relevant ionic reaction or pH change.

The pH scale is logarithmic, not linear. A change from pH 13 to pH 10 is a decrease of three pH units. For hydrogen-ion concentration, this represents a 10^3-fold increase in H+. Relative to the alkaline character, the solution is therefore approximately one thousand times less basic in the usual school-level interpretation. It remains basic because its pH is still above 7. Hence option A, not the linear distractor B, is correct.

A basic oxide reacts with an acid in a neutralisation reaction to form salt and water. For example, CuO + 2HCl → CuCl2 + H2O. Since copper oxide consumes the acid and produces these two products, it shows basic character and is classified as a basic oxide. Option A is correct. An acidic oxide generally reacts with a base, while copper oxide is neither a gas nor an ordinary salt in this reaction.

Indicators change colour in characteristic media. Turmeric turns reddish-brown in the presence of a base, and phenolphthalein becomes pink in a basic solution, generally when the pH is above its transition range. Since both independent observations indicate alkalinity, the solution is basic. Option B is correct. An acidic solution would not produce this combination of colours, and neutral or metallic are not suitable descriptions of the solution’s acid–base nature.

Neutralisation depends on the amounts of reacting H+ and OH−, not merely on the volumes mixed. Equal volumes can contain different numbers of reactive particles if the concentrations differ; strong and weak substances can also contribute ions differently. If the acid supplies more effective H+ than the base can neutralise with OH−, an excess remains and the final solution is acidic. Thus option A is correct. Equal volume does not guarantee neutral pH.

An aqueous solution of sodium chloride is called brine. Electrolysis of brine is the chlor-alkali process. At the cathode, water produces hydrogen gas and hydroxide ions; sodium ions remain in solution and combine with hydroxide ions to form sodium hydroxide. Chlorine gas is released at the anode. Therefore option A is correct. Calcium carbonate, ammonium chloride, and copper oxide are not products formed as the important base in this process.

The pH scale is logarithmic, and pH = −log[H⁺]. Therefore, an increase of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 4 to pH 7 is three units, so the concentration changes by 10³ = 1000 times in the decreasing direction. Hence option C is correct; the other numerical options do not apply the logarithmic scale correctly.

Bleaching powder works mainly because it contains available chlorine, which gives it bleaching and disinfecting properties. Moisture and exposure to air can cause gradual decomposition and reduce the amount of effective chlorine. A dry, tightly closed container limits this deterioration and preserves its activity. Thus option A is correct; the other choices describe unrelated or chemically incorrect changes.

Many acids react with metals above hydrogen in the reactivity series and release hydrogen gas: metal + acid → salt + H₂. Magnesium is sufficiently reactive, so brisk bubbles appear, whereas silver is below hydrogen and normally does not displace it from a dilute non-oxidising acid. The observation therefore demonstrates different metal reactivities, making option A correct.

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.

Acidic behaviour is produced by ions in an aqueous medium, especially hydrogen or hydronium ions. A dry acidic gas cannot ionise sufficiently on dry litmus, so no colour change is observed. Moisture dissolves the gas and permits ion formation; these ions cause blue litmus to turn red. Hence option A is correct, while the other explanations are scientifically irrelevant or false.

Plaster of Paris is calcium sulphate hemihydrate, written as CaSO₄·½H₂O, so option B is correctly matched. Bleaching powder is not sodium chloride; baking soda is sodium hydrogen carbonate, not calcium sulphate; and washing soda is sodium carbonate decahydrate, not calcium carbonate. These comparisons establish that only option B is correct.

On the pH scale, a value of 12 is far above 7 and therefore indicates a strongly basic solution. A slippery or soapy feel is also commonly associated with bases, although touching an unknown chemical is unsafe. Such a solution may be caustic and can damage skin, so it should be handled with protective equipment. Thus option A is the safest conclusion.

An active metal reacts with an acid to form a salt and hydrogen gas: metal + acid → salt + H₂. When a burning splint is brought near hydrogen, the gas burns rapidly and produces the characteristic ‘pop’ sound. Limewater turning milky identifies carbon dioxide, while the other tests do not identify hydrogen. Therefore option A is correct.

When hydrogen-ion and hydroxide-ion effects are approximately balanced, the solution is nearly neutral. Its pH is close to 7 because neither acidic nor basic character predominates. Universal indicator displays green around neutral pH. Red and orange indicate acidic conditions, whereas violet indicates a strongly basic condition. Therefore option A, green, is the correct answer.

Limewater is an aqueous solution of calcium hydroxide. When carbon dioxide is passed through it, the reaction is Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble in water and appears as a white suspension, producing the milky appearance. If excess carbon dioxide is passed, soluble calcium hydrogen carbonate may form and the milkiness can disappear. Thus option A is correct.

The governing concept is laboratory safety and the use of chemical indicators. Many acids are corrosive or toxic, so tasting even a small amount may burn tissues or cause poisoning. Litmus paper, universal indicator, or a pH strip can test acidic character without direct contact. Therefore option A is correct; B is false because acids may have tastes, while C and D incorrectly treat taste or indicators as unsuitable tests.