Acids, Bases, and Salts

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.

Equal volume does not mean equal chemical amount. The base may have a higher concentration, may be stronger, or may provide more moles of effective OH⁻ than the acid provides moles of H⁺. After neutralisation, excess OH⁻ remains, making the mixture basic. Neutralisation does occur, but it may be incomplete with respect to the base. Thus A is correct; the other statements are chemically false.

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.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, so option A gives the correct name and formula description. Bleaching powder is not sodium chloride; it is commonly represented as calcium oxychloride. Plaster of Paris is calcium sulphate hemihydrate, not calcium carbonate. Baking soda is sodium hydrogen carbonate, not calcium sulphate. Therefore only 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.

An active metal reacting with a dilute acid generally produces hydrogen gas and a salt: metal + acid → salt + H₂. Hydrogen is combustible and, when a small sample is brought carefully near a burning splint, it burns with a characteristic ‘pop’ sound. Limewater turning milky identifies carbon dioxide, while turmeric and phenolphthalein are indicators rather than confirmation tests for hydrogen. Hence option B is correct.

When hydrogen ions and hydroxide ions are present in nearly equal effective amounts, their acid and base effects balance, giving an approximately neutral solution. Neutral water has pH close to 7. Universal indicator shows green around pH 7. Red and orange indicate acidic conditions, whereas violet indicates a strongly alkaline condition. Therefore the expected colour is green, making option B correct.

Limewater is a dilute aqueous solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is bubbled through it, the reaction is Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble in water and forms a suspended white precipitate, producing the characteristic milkiness. Calcium hydrogen carbonate is soluble, while the sodium salts listed do not result from this test. Thus option B is correct.

The governing concept is laboratory safety: an unknown acid must never be tasted because even a small amount may burn tissues, poison the body, or cause other harm. Taste also cannot reliably identify an acid or measure its pH. Instead, a teacher-approved indicator such as litmus, universal indicator, or pH paper should be used. Therefore option A is correct; B, C, and D make scientifically unsafe or false claims.

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.

The governing concept is the relation between pH and corrosive hazard. A pH of 13 indicates a strongly basic solution, with a very low hydrogen-ion concentration and substantial hydroxide activity. Such a solution may irritate or burn skin and seriously damage eyes. Gloves, goggles, careful labelling, and approved handling procedures are therefore required. Option A is correct; tasting, direct contact, and beverage storage are unsafe.

The governing concept is environmental acidification. Acid rain contains acids formed when atmospheric pollutants react with water and oxygen. When this rain enters lakes or streams, it can lower their pH. Aquatic organisms may then experience disturbed enzyme activity, reproduction, respiration, and mineral balance; sensitive species can decline or die. Hence option A is correct, while B, C, and D do not describe the chemical or biological effect.

The governing concept is acid-base identification followed by neutralisation. A solution that turns red litmus blue is basic, while one that turns blue litmus red is acidic. When suitable amounts of an acid and a base react, hydrogen ions combine with hydroxide ions to form water, and the remaining ions form a salt: H+ + OH− → H2O. Thus option A is correct; the other products are unsupported.