Acids, Bases, and Salts

Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonia, a weak base. In water, the ammonium ion reacts partially with water: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. The hydronium ions make the solution acidic, allowing it to turn blue indicator paper red. It is not sodium hydroxide, is not generally a basic salt, and it dissolves readily enough in water to produce an aqueous solution.

A pH greater than 7 indicates a basic solution containing a greater effective concentration of hydroxide ions than hydrogen ions. Sodium carbonate is a salt of a strong base and a weak acid; carbonate ions hydrolyse in water and generate some OH⁻ ions, so its solution is basic. Sodium chloride is approximately neutral, ammonium chloride is acidic because of ammonium-ion hydrolysis, and pure water is approximately neutral at ordinary conditions. Thus sodium carbonate is the only suitable choice.

A solution with pH less than 7 is acidic. Ammonium chloride is produced from a strong acid, hydrochloric acid, and a weak base, ammonia. In water, NH₄⁺ undergoes partial hydrolysis and forms H₃O⁺ ions, giving the solution an acidic character. Sodium chloride is approximately neutral, while sodium carbonate and washing soda, sodium carbonate decahydrate, give basic solutions because carbonate ions produce OH⁻ through hydrolysis. Therefore ammonium chloride is the correct answer.

The governing concept is water of crystallisation in hydrated salts. Blue copper sulphate is CuSO₄·5H₂O. On heating, it loses its fixed water molecules and becomes white anhydrous CuSO₄. Adding water restores the hydrated form and its blue colour. Therefore option A is correct; the other options do not explain the reversible colour change or involve conversion of copper, sulphate, or acidity.

The governing concept is the test for carbon dioxide. On heating, sodium hydrogen carbonate decomposes: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The released CO₂ reacts with calcium hydroxide in lime water to form insoluble calcium carbonate, CaCO₃, which makes the solution milky. Thus option A is correct; blue colour and a hydrogen pop test are unrelated.

The governing concept is the acid–carbonate reaction used in baking. Baking powder contains baking soda and a mild edible acid. When moistened and heated, the acid reacts with sodium hydrogen carbonate and releases CO₂, whose bubbles expand and make the batter rise. Without enough acid, less CO₂ is produced, so option A is correct; chlorine formation, salt conversion, and metallic water are irrelevant.

The governing concept is the fire triangle: combustion needs fuel, heat, and oxygen. Carbon dioxide is denser than air and does not support combustion, so it spreads over the burning material and displaces or limits the oxygen reaching the flame. Removing oxygen interrupts combustion, making option A correct. It does not strengthen fire, convert fuel into acid, or burn water.

The governing concept is the stability and storage of bleaching powder. Moisture and carbon dioxide from air can gradually decompose or deactivate its available chlorine, reducing its bleaching and disinfecting action. Consequently, bleaching powder should be kept in a dry, tightly closed container. Option A correctly states the practical effect; the other options describe impossible transformations and do not explain why dry storage is required.

The governing concept is the setting reaction of plaster of Paris. Calcium sulphate hemihydrate, CaSO₄·½H₂O, combines with water and changes into hard gypsum, CaSO₄·2H₂O. Because this hydration and setting begin soon after mixing, the paste must be shaped and applied promptly. Option A is correct; it does not escape as a gas, acidify water, or become a metal.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and produces plaster of Paris, CaSO₄·½H₂O, which sets after mixing with water. Excessive heating can remove nearly all the water and form over-burnt anhydrous calcium sulphate, which hydrates very slowly and therefore loses useful setting ability. Thus option A is correct; the other options describe impossible products.

The pH scale shows acidity inversely: a lower pH means a greater concentration of hydrogen ions and a more acidic environment. Acids produced by bacteria can react with and dissolve minerals in tooth enamel, especially when the mouth remains acidic for a long time. This softens the enamel and promotes cavities. Hence A is correct; low pH is not basic, does not make teeth metallic, and is not harmless.

A soil pH of 5 indicates that the soil is acidic. Slaked lime, Ca(OH)₂, is a basic substance; when applied in a suitable quantity, it neutralises some excess acidity and moves the soil pH toward a range more favourable for many crops. Adding acid or vinegar would increase acidity. Large additions of ammonium chloride are not a suitable correction because such fertiliser can contribute to soil acidification. Thus A is correct.

A pH of 9 indicates that the soil is alkaline. During decomposition, organic manure can produce humus and various weakly acidic substances. These may react with or buffer some alkaline components, helping to reduce excessive basicity and improve soil conditions. The change is gradual and does not make the pH zero. Organic manure does not turn soil into metal and does not only increase alkalinity. Therefore A is correct.

Marble is composed mainly of calcium carbonate, CaCO₃. Acid rain contains dissolved acidic substances, commonly including sulfuric or nitric acid formed from atmospheric pollutants. An acid reacts with calcium carbonate, producing a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Repeated reaction dissolves and roughens the marble surface, so A is correct.

Phenolphthalein is colourless in an acidic or neutral medium and pink in a sufficiently basic medium. The initial pink colour therefore shows that the original solution was basic. Adding acid consumes hydroxide ions through neutralisation, lowering the pH; when the solution leaves the indicator’s pink transition range, the colour disappears. Thus A is correct. The observation does not show that the original solution was strongly acidic.

A salt solution need not always be neutral because the ions may react with water, a process called hydrolysis. Ammonium chloride is formed from a strong acid and a weak base; its ammonium ion reacts with water and produces hydronium ions, so its solution is acidic. Sodium chloride, from a strong acid and strong base, is approximately neutral. Pure water and dry sand are not counterexamples to the statement about salts.

The reaction between an acid and a base is called neutralisation. In a typical strong-acid and strong-base reaction, hydrogen ions combine with hydroxide ions to form water, and energy is released to the surroundings: H⁺ + OH⁻ → H₂O. This released energy raises the solution temperature, so the process is exothermic in this case. The words “always absorbs” and “never react” are incorrect.

Ions are electrically charged particles. When an ionic substance dissolves in water, its ions may become mobile; their movement between electrodes allows the solution to conduct electric current. Therefore electrical conductivity is strong experimental evidence for mobile ions, although the degree of conductivity depends on ion concentration and mobility. Colour, taste, or apparent temperature does not demonstrate the presence of ions and may be unrelated to composition.

Methyl orange is red in an acidic medium and yellow in a neutral or alkaline medium. Adding enough base consumes hydrogen ions through neutralisation, so the pH rises and the solution moves from the acidic indicator range toward the alkaline range. Consequently, the observed colour changes from red toward yellow. The other choices describe the reverse change or colours that do not match methyl orange in this situation.

Acids ionise or dissociate when dissolved in water, producing hydronium ions and corresponding negative ions. These charged particles are mobile in the aqueous solution and carry charge when an electric potential is applied, allowing current to flow. The strength of conduction depends on the number and mobility of ions. Colour, taste, or floating solid particles is not the reason for electrical conduction. Thus option A is correct.

The pH scale is defined by hydrogen-ion concentration: pH = −log[H⁺]. Thus a decrease of one pH unit represents a tenfold increase in hydrogen-ion concentration. Among pH 2, 7, 10, and 12, pH 2 is the lowest and therefore has the greatest [H⁺]. The pH 7 solution is neutral, while pH 10 and pH 12 are alkaline and have lower hydrogen-ion concentrations. Hence option A is correct.

The governing concept is that acids show their characteristic behaviour through hydrogen ions, more precisely hydronium ions in aqueous solution. Water allows the acid to ionise and permits the indicator molecules to respond with a colour change. A dry acid may therefore show little or no indicator effect. Indicators do not work only in metals, acids do not become bases without water, and dry acid still contains atoms.

Zinc is above hydrogen in the reactivity series, so with a suitable dilute acid it displaces hydrogen: Zn + 2HCl → ZnCl₂ + H₂. Carbon dioxide would turn lime water milky by forming calcium carbonate, but hydrogen does not. Thus the absence of milkiness supports hydrogen rather than CO₂; the other statements contradict the reaction or the limewater test. A burning-splint pop test can provide confirmation.

A base is a substance that can neutralise an acid, but it need not dissolve in water. An alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Therefore, alkalis form a subset of bases: every alkali is a base, but every base is not necessarily an alkali. The other choices describe incorrect universal claims or reverse the behaviour of bases.

Hydrogen chloride gas itself does not release hydrogen ions effectively without water. When it dissolves, HCl + H₂O → H₃O⁺ + Cl⁻, and the hydronium ions give the solution acidic properties. Moist blue litmus contains the needed water and therefore turns red. Dry blue litmus does not show this reliable change, red litmus turning blue indicates a base, and limewater milkiness is a carbon-dioxide test.