Acids, Bases, and Salts

Phenolphthalein is an acid–base indicator that becomes pink in a sufficiently basic solution, while it remains colourless in acidic and neutral solutions. Therefore, colourlessness alone cannot prove that the solution is strongly acidic or neutral; it only rules out the usual pink basic range. Hence option B is the most careful conclusion. The other choices either reverse the indicator result or make unrelated claims.

The governing concept is the ionisation of acids in water. An acid produces hydrogen ions in aqueous solution; more precisely, these hydrogen ions associate with water to form hydronium ions, H₃O⁺. Therefore the concentration of the acidic hydrogen-ion species increases, making option B correct. Hydroxide ions are associated with basicity, carbonate ions are not produced by every acid, and chloride ions occur only with particular acids such as hydrochloric acid.

The pH scale describes hydrogen-ion activity: neutral water is approximately pH 7, acidic solutions are below 7, and basic solutions are above 7. Dissolving a base in water increases hydroxide-ion concentration and makes the solution alkaline. Thus its pH moves to a value above 7, so option C is correct. It does not move below 7 or remain exactly neutral; the actual value depends on the amount and strength of the base.

A pH of 4 identifies the solution as acidic because it is below the neutral value of 7. Adding water dilutes the acid, so the hydrogen-ion concentration decreases and the acidity becomes weaker. Consequently, the pH rises toward 7, although it would not normally jump above 7 or instantly reach another extreme. Therefore option A is correct; options B and C point in the wrong direction, while option D ignores dilution.

A pH of 12 indicates a basic solution because it is well above neutral pH 7. Adding water dilutes the base, lowering the concentration of hydroxide ions and reducing the solution’s basicity. As a result, the pH decreases toward 7, although sufficient dilution would be needed to approach neutrality and it would not instantly become acidic. Therefore option B is correct; the other choices reverse or exaggerate the effect of dilution.

On the pH scale, only pH 5 is acidic; pH 7 is neutral, while pH 9 and 11 are basic. Therefore, if the question asks specifically for the acidic solution, option A is the only valid choice. Tooth enamel begins to face increased risk in sufficiently acidic conditions, commonly discussed near pH 5.5, so a lower pH is generally more harmful. The wording should not be read as saying pH 5 is harmless; it is simply the only acidic option listed.

The governing test is that carbon dioxide turns limewater milky because it reacts with calcium hydroxide to form insoluble calcium carbonate. An acid reacting with a carbonate releases carbon dioxide: sodium carbonate + dilute acid → salt + water + CO₂. Zinc with dilute acid mainly releases hydrogen, while sodium hydroxide with water and common salt with water do not produce carbon dioxide. Therefore, option B is correct.

The relevant gas-identification principle is the pop test. Metals that are above hydrogen in the reactivity series generally react with dilute acids to form a salt and hydrogen gas, for example, Zn + 2HCl → ZnCl₂ + H₂. Hydrogen burns rapidly and produces a characteristic pop near a burning splint. Carbon dioxide extinguishes a flame, oxygen relights it, and chlorine has different tests. Thus option C is correct.

The governing concept is the reactivity series. A metal can displace hydrogen from a dilute non-oxidising acid only when it is more reactive than hydrogen. Copper lies below hydrogen in the series, so it cannot normally reduce hydrogen ions from dilute hydrochloric acid to H₂. Option B is false because copper does not simply dissolve in water; option C misidentifies HCl, and option D is an unjustified absolute statement. Hence A is correct.

Neutralisation is the reaction of an acid with a base to produce salt and water. Sulphuric acid supplies the sulphate ion, SO₄²⁻, while sodium hydroxide supplies Na⁺ and OH⁻. Complete neutralisation is represented by H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. Thus the salt is sodium sulphate. Sodium chloride requires hydrochloric acid, sodium carbonate requires carbonate ions, and calcium sulphate requires calcium ions. Option B is correct.

This is an acid-base neutralisation reaction. Hydrochloric acid provides chloride ions, and calcium hydroxide provides calcium ions and hydroxide ions. The balanced equation is Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O, so calcium chloride is the salt formed. Calcium carbonate would require carbonate ions, whereas sodium and magnesium are not present among the reactants. Therefore, option A is the only chemically consistent answer.

The governing idea is concentration, which compares the amount of solute with a given amount of solution. A dilute acid contains a smaller proportion of acid and a larger proportion of solvent, so option A is correct. “Dilute” does not mean basic, and a concentrated acid can still contain some water. Concentration is also different from strength: a concentrated acid may be strong or weak depending on its ionisation in water.

Acid strength is governed mainly by the extent of ionisation in water. A strong acid, such as hydrochloric acid in school-level treatment, ionises nearly completely and produces a high proportion of H₃O⁺ ions. A weak acid, such as acetic acid, ionises only partially and establishes an equilibrium. Colour, container size, and smell do not define strength. Therefore, the scientifically correct choice is option B.

At equal formal concentration, the amount of ionisation determines how many ions are produced in solution. Hydrochloric acid is treated as a strong acid and ionises almost completely, giving H₃O⁺ and Cl⁻ ions. Acetic acid in vinegar is weak and ionises only partially, so fewer particles form ions. Both do produce ions, but not equally under these conditions. Hence hydrochloric acid, option B, can produce more ions.

Sulphur dioxide and nitrogen oxides are acidic pollutants released mainly by combustion and industrial processes. In the atmosphere, they undergo oxidation and react with water to form acids such as sulphuric acid, sulphurous acid, nitric acid, or nitrous acid. These reactions increase hydrogen-ion concentration and lower the pH of rainwater. They do not make rain basic or merely change its colour. Therefore, option A correctly explains acid rain.

The governing concept is the effect of pH on aquatic ecosystems. Acid rain introduces acidic substances into a water body and can lower its pH. If the pH becomes very low, many fish, amphibians, invertebrates, eggs, and aquatic plants may be unable to survive or reproduce normally. Therefore, option A is correct. A pH near 7 is comparatively suitable, while clear appearance does not prove chemical safety and the absence of gas is not the deciding factor.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions lose electrons and form chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻. At the cathode, water gains electrons and produces hydrogen gas. Sodium hydroxide remains in solution. Thus option B gives the correct anode-to-cathode order; the other options reverse or name products that are not formed in this process.

Bleaching powder is effective mainly because it contains available chlorine, which is released during suitable reactions and provides bleaching and disinfecting action. Prolonged exposure to air, moisture, and carbon dioxide can reduce this available chlorine through gradual chemical decomposition. Consequently, its bleaching strength falls, so option A is correct. The other choices describe no relevant chemical change and do not explain loss of effectiveness.

Baking powder contains baking soda along with an edible acid such as tartaric acid. On heating, baking soda can form sodium carbonate, which has a bitter taste. The edible acid neutralises this sodium carbonate and prevents bitterness in the food. Therefore, option A is correct. Option C is incorrect because the carbon dioxide produced helps cakes and bread rise. Exam tip: Link the edible acid in baking powder with the neutralisation of bitter sodium carbonate.

Sodium hydrogen carbonate, NaHCO₃, is a mild base. In the stomach it reacts with excess hydrochloric acid in a neutralisation reaction: NaHCO₃ + HCl → NaCl + H₂O + CO₂. This reduces excessive acidity, so option A is correct. It does not increase stomach acid, make water acidic, or act by dissolving tooth enamel. Its use should still be controlled because the reaction releases carbon dioxide and adds sodium.

Hard water contains dissolved calcium and magnesium ions, commonly supplied by their salts. Soap contains fatty-acid ions that react with Ca²⁺ and Mg²⁺ to form insoluble scum. These precipitates consume soap before it can form lather, so option A identifies the cause. Hard water does contain ions, is not an acidic gas, and soap is not completely unable to dissolve. Washing soda can soften water by removing these ions.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses part of its water of crystallisation and forms plaster of Paris, calcium sulphate hemihydrate, CaSO₄·½H₂O. Thus their principal difference is the amount of chemically combined water, so option A is correct. Metallic shine, chlorine smell, and litmus colour are not the defining chemical differences between these two calcium sulphate forms.

Ammonium chloride is formed from a strong acid, HCl, and a weak base, NH₄OH. In water, NH₄⁺ undergoes hydrolysis and produces H₃O⁺ ions, making the solution acidic; consequently, blue litmus turns red. Therefore, option A is correct. Sodium chloride from a strong acid and strong base is nearly neutral, while sodium carbonate and sodium acetate generally produce alkaline solutions and turn red litmus blue.

The governing concept is the hydrolysis of salts in water. Sodium carbonate is formed from a weak acid, carbonic acid, and a strong base, sodium hydroxide. Its carbonate ions react with water and generate hydroxide ions, so the solution becomes basic. Sodium chloride and potassium nitrate come from strong acid–strong base combinations and are nearly neutral, whereas ammonium chloride, from a weak base and strong acid, is acidic. Therefore, option A is correct.

Salt behaviour depends on the strengths of the acid and base from which the salt is formed. Ammonium chloride comes from hydrochloric acid, a strong acid, and ammonia or ammonium hydroxide, a weak base. The ammonium ion reacts with water and produces hydronium ions, making the solution acidic. Sodium carbonate is basic, while sodium chloride and potassium chloride are nearly neutral. Hence option B is the unambiguous answer.