Acids, Bases, and Salts

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 reaction pattern is acid plus carbonate, which produces a salt, water, and carbon dioxide: carbonate + acid → salt + water + CO₂. Hydrogen is commonly released when a suitable active metal reacts with a dilute acid, not when a carbonate reacts. Carbon dioxide can be confirmed because it turns limewater milky. Thus option A correctly replaces the student’s statement; chlorine and oxygen are not products of this reaction, and option D wrongly denies gas evolution.

Both observations identify the test solution as acidic: blue litmus turns red in acid, and methyl orange is red in a sufficiently acidic medium. Neutralisation requires an acid to react with a base, forming salt and water. Sodium hydroxide solution is a base and supplies hydroxide ions that combine with hydrogen ions. Lemon juice, vinegar, and hydrochloric acid are acidic, so they would not appropriately neutralise this solution. Therefore option A is correct.

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.

The governing idea is that the colour of a substance alone is not a reliable chemical test. Many acids and bases are colourless, and the colours of solutions can vary for reasons unrelated to acidity or basicity. Litmus, universal indicator, or pH paper gives an observable response linked to hydrogen-ion concentration and pH. Therefore option A is the scientifically safe and exam-correct answer. Options B and C make unjustified generalisations, while D rejects a useful measurement.

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 relationship is pH = −log[H⁺]. A tenfold decrease in hydrogen-ion concentration raises pH by one unit. Ten thousand equals 10⁴, so making the solution 10,000 times less acidic means [H⁺] decreases by four powers of ten and pH rises by four units. Starting from pH 2: 2 + 4 = 6. Thus option B is correct. This assumes the stated change refers to hydrogen-ion concentration and ignores minor water-ion effects.

A pH of 6 is below 7, and the slight red colour of blue litmus confirms that the salt solution is mildly acidic. In school-level salt hydrolysis, a salt formed from a strong acid and a weak base generally gives an acidic solution because the conjugate acid of the weak base hydrolyses in water. A strong base with a weak acid usually gives a basic solution, while strong acid plus strong base is approximately neutral. Hence option B is correct.

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.

In the chlor-alkali process, electrolysis of concentrated brine produces chlorine at the anode, hydrogen at the cathode and sodium hydroxide in solution. These products must be separated because chlorine can react with sodium hydroxide, and chlorine and hydrogen can form a hazardous mixture under suitable conditions. A membrane or another separation arrangement limits such contact. Thus unwanted reactions and safety risks, not insolubility or freezing, explain option A.

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.