Acids, Bases, and Salts

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 first observation is decisive: blue litmus turns red in an acidic medium. The second observation is consistent with it because phenolphthalein is colourless in acidic solutions, whereas it becomes pink in a basic range. Together, the tests support the conclusion that the unknown solution is acidic, so option B is correct. Colourlessness with phenolphthalein alone could also occur in a neutral solution, but the litmus result rules neutrality out in this case.

Red litmus turning blue is evidence of a basic medium. Phenolphthalein provides a second confirming observation because it becomes pink in a basic solution and remains colourless in acidic or neutral conditions. Since both tests point to the same chemical nature, the solution is basic and option C is correct. A neutral solution would not normally change red litmus or produce the pink phenolphthalein colour, while the last option is contradictory and unsupported.

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.

According to the Arrhenius concept, a base increases the concentration of hydroxide ions in aqueous solution. Sodium hydroxide dissociates almost completely in water: NaOH(aq) → Na⁺(aq) + OH⁻(aq), so it is a strong base and clearly gives OH⁻ ions. Hydrochloric acid, lemon juice, and vinegar are acidic substances that increase hydrogen-ion concentration instead. Therefore, option C is correct.

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.

The name describes two important products of brine electrolysis. Chlorine gas is released at the anode, while sodium hydroxide, which is an alkali, is obtained in the solution; hydrogen is also produced at the cathode. Therefore, option A is correct because “chlor” refers to chlorine and “alkali” refers to sodium hydroxide. The other options incorrectly say that only salt, only water, or no chemical product is formed.

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.