Acids, Bases, and Salts

Acidic behaviour is observed when the gas dissolves in water and produces ions, especially hydrogen ions or hydronium ions, that interact with the indicator. Moist litmus supplies the necessary water, so the colour changes. Dry litmus has no sufficient water medium for ionisation and the characteristic acidic response is not produced. Thus option A is correct. Dry litmus is not inherently basic, and the other statements are unrelated to indicator chemistry.

Lime water is a dilute solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, the initial reaction is Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. Calcium carbonate is insoluble and forms a white precipitate, making the lime water milky; therefore option B is correct. If excess carbon dioxide continues to pass, the precipitate can dissolve by forming soluble calcium hydrogen carbonate, but that is a later stage, not the first product.

The pH scale describes hydrogen-ion concentration, and a neutral solution has pH 7 at ordinary school-level treatment. A rise from pH 2 to pH 3 means the hydrogen-ion concentration has decreased by a factor of ten, so the solution has become less acidic. However, pH 3 is still below 7, meaning it remains acidic. Thus A is correct; B would require pH above 7, C reverses the ion change, and D ignores the measured change.

Acidic soil contains acidic substances and therefore has a relatively low pH. Slaked lime is calcium hydroxide, Ca(OH)2, a basic compound that supplies hydroxide ions. These ions neutralise acidic hydrogen ions, producing water, while the calcium component forms salts. With a suitable amount, the acidity decreases and the soil pH rises toward a more suitable range. A is correct; B states the opposite, C is chemically unrelated, and D makes an unsupported claim about removing all salts.

The nature of a salt solution depends on the strengths and properties of the acid and base that produced the salt. Sodium chloride from a strong acid and strong base gives an approximately neutral solution; sodium carbonate gives a basic solution, while salts such as ammonium chloride can give an acidic solution. Hence C is correct. A and B incorrectly generalise one nature to every salt, and D confuses salt identity with pH.

Acid rain forms when acidic oxides, especially sulfur and nitrogen oxides, react with atmospheric moisture and produce acidic substances. When this precipitation enters a lake, it can lower the water’s pH. A changed pH can disturb enzyme activity, respiration, reproduction, and mineral balance in aquatic organisms, and may also increase the solubility of harmful metals. Thus A is correct; the other options deny or trivialise the chemical effect.

Red litmus turning blue identifies the first solution as basic, while blue litmus turning red identifies the second as acidic. When an acid reacts with a base, hydrogen ions and hydroxide ions combine to form water, and the remaining ions generally form a salt. This is a neutralisation reaction, although the final mixture’s exact pH depends on the amounts used. Therefore A is correct; metal, oxygen, and sugar do not follow from this test.

A reaction that produces salt and water from an acid and another substance is a neutralisation reaction. In this case the metal oxide neutralises the acid, so it behaves as a base; for example, CuO + 2HCl produces CuCl2 + H2O. Therefore the general school-level conclusion is that metal oxides are basic in nature, making B correct. A is the opposite, while C and D do not describe the observed chemical behaviour.

Lime water is an aqueous solution of calcium hydroxide, Ca(OH)2, and is basic because it provides hydroxide ions. An acidic non-metal oxide or its acidic aqueous product can react with this base in a neutralisation reaction. For example, carbon dioxide reacts with lime water to form calcium carbonate and water: CO2 + Ca(OH)2 → CaCO3 + H2O. Thus A is correct; B confuses a solution with a metal, while C and D are chemically false.

Curd contains lactic acid, so it is mildly acidic. If it remains in contact with copper or brass, the acid may slowly react with the metal surface and produce dissolved metal compounds or salts. In excessive or unsuitable exposure, such contamination can make the food unsafe, which is why reactive metal vessels are avoided for acidic foods. A is correct; B and C describe no valid general reaction, and D is false because curd does contain acid.

The governing concept is dilution of an acid and the colour scale of a universal indicator. Red indicates a strongly acidic solution. Adding water increases the volume, so the concentration of hydrogen ions per unit volume decreases and the acidity becomes weaker. However, dilution does not automatically remove all hydrogen ions or create a base. The orange colour therefore indicates that the solution is still acidic, but less acidic than before. Hence option A is correct; B, C, and D contradict the indicator change.

The governing distinction is between a base and an alkali. A base is a substance that reacts with an acid to form salt and water; it may be soluble or insoluble in water. An alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Since the substance does not dissolve in water, it should be called a base, not an alkali. An acid or indicator does not fit the described reaction, so option A is correct.

The standard preparation of bleaching powder involves passing chlorine gas over dry slaked lime, calcium hydroxide, Ca(OH)₂. The product is commonly represented as calcium oxychloride, CaOCl₂, though commercial bleaching powder can have a more complex composition. Drying is important because the process is carried out with dry slaked lime. Sodium chloride, gypsum, and washing soda are not the reactants used in this preparation. Therefore option A is correct.

The governing concept is dilution. Adding a large amount of water lowers the concentration of hydrogen ions in the acidic solution, so its pH increases and its acidity decreases. It does not normally remove every hydrogen ion or add enough hydroxide ions to make the solution basic immediately. The pH therefore moves toward 7 rather than suddenly crossing to the basic side. Option A gives this correct reason; B, C, and D describe false effects of dilution.

Universal-indicator colours represent approximate pH ranges. Red shows strong acidity, while yellow represents a less strongly acidic range. Adding water dilutes the solution and lowers the hydrogen-ion concentration per unit volume, so the colour shifts toward green, which represents neutrality. Because the result is yellow rather than green, neutralisation has not been reached; the solution remains acidic. Thus option A is correct, while B, C, and D are inconsistent with the observation.

The governing concept is that electrical conductivity of an aqueous acid depends mainly on the concentration of mobile ions. At equal analytical concentration, an acid that ionises to a greater extent produces more H⁺ and corresponding negative ions, so it carries current better. Therefore, the first acid is most likely a more highly ionised strong acid. A weak acid gives fewer ions, while a neutral salt or insoluble solid does not fit the stated acid behaviour.

The governing reaction is the test for carbon dioxide. Carbon dioxide first reacts with lime water, Ca(OH)₂, to form insoluble calcium carbonate, CaCO₃, which makes the solution milky. When excess CO₂ is passed, the precipitate reacts further with water and carbon dioxide to form soluble calcium hydrogen carbonate, Ca(HCO₃)₂, so the milkiness disappears. Hence the correct sequence is carbonate followed by hydrogen carbonate.

Turning red litmus blue establishes that the solution is basic. The additional observation excludes carbonate and hydrogencarbonate salts, because both release carbon dioxide when treated with an acid: carbonate gives CO₂ through acid decomposition, and hydrogencarbonate does so as well. Sodium hydroxide is a strong base and neutralises an acid without producing CO₂. Therefore option C is the best answer; the carbonate choices fail the gas test.

The governing concept is water of crystallisation in hydrated salts. Blue copper(II) sulphate crystals are commonly represented as CuSO₄·5H₂O. On heating, some or all of the bound water is removed and anhydrous copper(II) sulphate becomes white. Adding water reforms the hydrated salt and restores the blue colour. This reversible colour change supports option A; it does not indicate free sodium, chlorine gas or common salt.

Baking powder contains sodium hydrogencarbonate together with a suitable edible acid. In the dry mixture, the particles have limited contact and ions cannot move freely, so the acid–hydrogencarbonate reaction is slow. Moisture dissolves the ingredients and permits ion movement and effective contact; CO₂ is then released, causing dough to rise. Therefore option A gives the best explanation, while moisture does not prevent CO₂.

Neutralisation is governed by the amounts and effective concentrations of H⁺ and OH⁻, not simply by the volumes poured. Equal volumes can contain unequal mole amounts if the solutions have different concentrations, and a weak base may also provide fewer effective OH⁻ ions than a strong acid provides H⁺ ions. If H⁺ remains in excess after reaction, the final solution is acidic. Thus option A is correct.

Sodium hydroxide is a strong base, while carbon dioxide is an acidic oxide present in air. When solid sodium hydroxide is exposed to air, it absorbs carbon dioxide and reacts with it: 2NaOH + CO₂ → Na₂CO₃ + H₂O. Thus sodium carbonate is formed. Nitrogen and oxygen do not produce this salt, and water does not become an acid in this process, so option A is correct.

The pH scale identifies solutions with pH above 7 as basic. Both 13 and 11 are above 7, so the solution remains basic after dilution. Adding water lowers the concentration of hydroxide ions and reduces alkalinity, causing the pH to move toward 7. It does not make the solution acidic or exactly neutral, and hydroxide concentration does not increase. Therefore option A is correct.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On strong heating it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The evolved carbon dioxide turns limewater milky because calcium carbonate forms, while the solid residue is sodium carbonate. Sodium chloride, calcium sulphate, and ammonium chloride are not products of this decomposition, so option A is correct.

Acid rain introduces acidic substances into the lake and lowers its pH. Lime is a basic material, commonly represented by calcium oxide or calcium hydroxide in this context. It reacts with acidic substances and reduces their acidity, thereby raising the pH toward a less acidic value. This acid–base reaction is neutralisation, not evaporation, condensation, or decomposition. Hence option A is correct.