Acids, Bases, and Salts

Baking powder contains sodium hydrogen carbonate and a suitable edible acid. In the dry mixture, ions have limited mobility, so the reaction is slow. Water dissolves the components and allows ionic contact; on heating, carbon dioxide is released more rapidly: acid + hydrogen carbonate → salt + water + CO₂. The CO₂ bubbles expand and raise the dough. Therefore option A is correct.

Copper oxide is a metal oxide and shows basic character. The governing reaction is a neutralisation: basic metal oxide + acid → salt + water. For example, CuO + H₂SO₄ → CuSO₄ + H₂O; the dissolved copper salt gives the solution its characteristic colour. Formation of salt with an acid demonstrates basicity, so option B is correct. It is not an acidic oxide, neutral gas, or salt-free material.

Phenolphthalein is pink in a basic medium and colourless in an acidic or neutral medium. Methyl orange is yellow in a neutral or basic medium, so the two observations together indicate that the solution is basic. In a basic aqueous solution, hydroxide ions are present in excess relative to hydrogen ions. Chloride and sulphate ions do not determine this indicator combination. Therefore option B is correct.

Neutralisation depends on the stoichiometric amounts of hydrogen and hydroxide ions, not merely on the volumes mixed. Equal volumes can have different concentrations or different strengths, so one reactant may remain in excess. For example, equal volumes of 0.1 M acid and 0.01 M base cannot neutralise completely. The final pH therefore need not be 7. Hence option A is the scientific reason.

Aqueous common salt, sodium chloride, undergoes electrolysis in the chlor-alkali process: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. The sodium hydroxide formed is a strong base used in making soap, paper, and several chemicals. Calcium carbonate, ammonium chloride, and copper sulphate are different compounds and are not the important base obtained in this process. Therefore option A is correct.

The governing relation is pH = −log[H⁺]. Therefore, an increase of one pH unit means a tenfold decrease in hydrogen-ion concentration. Here pH rises from 2 to 4, an increase of two units, so [H⁺] decreases by 10² = 100 times. Thus option A is correct. Option B reverses the logarithmic relationship, while C treats pH as a linear scale and D ignores the change in pH.

Bleaching powder, commonly represented as CaOCl₂, owes its bleaching and disinfecting action to available chlorine. On prolonged exposure to atmospheric moisture and carbon dioxide, it gradually decomposes and loses available chlorine, reducing its effectiveness. A closed container limits contact with air and moisture, so option A is correct. The other options describe unrelated products or an impossible purpose; storage does not convert bleaching powder into salt, gypsum, or food.

Blue litmus turning red is a standard indication that the solution is acidic. However, hydrogen evolution depends on the reactivity of the metal as well as the acid. Metals above hydrogen in the reactivity series can usually displace hydrogen from dilute acids, whereas copper is below hydrogen and generally does not do so. Therefore option A correctly combines both observations; B and D contradict the litmus test, and C is chemically false.

A pH of 9 indicates that the soil is distinctly basic or alkaline. To move its pH toward a range more suitable for many crops, the general principle is to add a suitable mildly acidic amendment, often organic matter such as compost or well-managed manure, according to soil testing. Therefore option A is the best general answer. Slaked lime and sodium hydroxide would increase alkalinity, while soap is not an appropriate soil amendment.

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.

Washing soda is sodium carbonate decahydrate, with the formula Na₂CO₃·10H₂O; hence option A gives a correct common-name and chemical-name pair. Baking soda is sodium hydrogen carbonate, bleaching powder is not sodium chloride, and Plaster of Paris is calcium sulphate hemihydrate rather than calcium carbonate. These comparisons eliminate B, C, and D and confirm A unambiguously.

A slippery or soapy feel is a common physical indication of a basic solution, although it should not be tested by touching unknown chemicals in practice. Turmeric is a natural indicator: it remains yellow in acidic or neutral conditions but turns reddish-brown in the presence of a base. Both observations point to alkalinity, so option B is correct. Acidic and neutral solutions do not produce this combined response, and being salt-free is not a measure of acid-base nature.

An active metal reacting with a dilute acid commonly produces a salt and hydrogen gas, for example Zn + 2HCl → ZnCl₂ + H₂. Hydrogen is combustible and, when brought near a burning splint, burns rapidly with the characteristic ‘pop’ sound. Therefore option A is the correct conclusion. Carbon dioxide extinguishes a flame, while chlorine and nitrogen do not give this standard pop test for hydrogen.

The pH scale describes the acidic or basic character of an aqueous solution. At approximately 25°C, pH 7 represents neutrality, where hydrogen-ion and hydroxide-ion concentrations are approximately equal. Universal indicator normally appears green near neutral pH, so both pieces of evidence support option B. Highly acidic solutions have low pH and red or orange colours, whereas highly basic solutions have high pH and blue or violet colours; metallic is not an acid-base classification.

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 governing principle is laboratory safety: no chemical should be tasted because even a small quantity of an acid may burn tissues or be poisonous. Acids also differ in concentration, so taste cannot reliably identify their strength or exact pH. Instead, litmus, universal indicator, or pH paper can test acidity without direct contact. Therefore option A is correct; B is false because acids may have tastes, C confuses taste with measurement, and D wrongly limits indicators to metals.

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.

On the usual aqueous pH scale, values near 14 indicate a very high concentration of hydroxide ions and strong basic character. Such a solution may be caustic, meaning it can damage skin, eyes, or other tissues. The pH value alone does not make a substance pure, edible, or acidic, so careful handling with goggles, gloves, and proper laboratory practice is required. Therefore A is correct; B, C, and D contradict the meaning of high pH.