Acids, Bases, and Salts

During electrolysis of brine, chloride ions move towards the positively charged anode. At the anode they lose electrons according to the oxidation process 2Cl⁻ → Cl₂ + 2e⁻. The chlorine atoms combine to form chlorine gas. Hence option A, the anode, is correct. Hydrogen is produced at the cathode, so option B confuses the two electrode reactions, while C and D are inconsistent with electrolysis.

In the chlor-alkali process, water is reduced at the negatively charged cathode. The cathodic reaction can be represented as 2H₂O + 2e⁻ → H₂ + 2OH⁻, so hydrogen gas is released at the cathode. Therefore option B is correct. Chlorine is formed at the anode by oxidation of chloride ions; the remaining choices do not describe an electrode reaction and cannot produce hydrogen.

Very acidic soil can harm roots and reduce the availability of some nutrients. Slaked lime, Ca(OH)₂, is a basic substance that reacts with excess acidic components and raises the soil pH toward a more suitable range. This is an agricultural application of neutralisation, so option A is correct. Adding more acid would worsen the condition, while salt and sugar do not provide the required neutralising action.

Excessively basic soil can be improved by adding organic manure. As organic matter decomposes, it produces humus and mild acidic substances that can gradually reduce the soil's alkalinity. Thus option A is the most appropriate choice in the school-level context. Lime, sodium hydroxide, and washing soda are basic or alkaline materials, so adding them would generally increase rather than correct the basic nature of the soil.

Acidic strength in water is related to the concentration of hydronium ions produced by ionisation. Hydrochloric acid is a strong acid and ionises almost completely, whereas acetic acid is a weak acid and ionises only partially at the same concentration. Consequently, HCl produces more H₃O⁺ ions and shows a stronger acidic effect. Therefore option A is correct; colour, carbon content, and physical state are irrelevant distractors.

A solution conducts electricity when it contains mobile charged particles. Hydrogen chloride produces hydronium and chloride ions when dissolved in water, and its aqueous solution has a high concentration of ions because HCl is a strong acid. Therefore option C is the best answer. Dry HCl gas and dry sugar lack freely moving ions, while pure water has only a very small concentration of ions and conducts much less effectively.

Turning blue litmus red is a characteristic test for an acidic solution. In aqueous school-level chemistry, acids produce hydrogen ions, more precisely hydronium ions formed when H⁺ associates with water. These ions account for acidic properties, and their movement also helps the solution conduct electricity. Therefore option A is correct. Hydroxide ions indicate basicity, while sodium and carbonate ions do not produce the stated acidic behaviour.

A soda-acid fire extinguisher uses sodium hydrogen carbonate, NaHCO₃, together with an acid. Their reaction produces carbon dioxide, for example NaHCO₃ + HCl → NaCl + H₂O + CO₂. The released CO₂ does not support combustion and forms a layer that helps cut off oxygen from the fire. Therefore option A is correct. Sodium hydroxide and calcium oxide are bases, while sodium chloride does not release CO₂ with acid.

Hard water contains dissolved calcium and magnesium ions, which react with soap to form scum and prevent good lather. Washing soda, sodium carbonate (Na₂CO₃), supplies carbonate ions. These combine with calcium and magnesium ions to form insoluble carbonates that can precipitate and be removed. Thus A is correct; washing soda does not acidify water, convert it into gas, or generate chlorine.

Marble is composed mainly of calcium carbonate, CaCO₃. Acids present in polluted rainwater react with this carbonate, producing a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Repeated reaction gradually dissolves or erodes the surface, so A is correct. Water alone is not the key cause, and acid rain neither makes metal nor is completely blocked.

Both reactions release carbon dioxide because the carbonate or hydrogen carbonate ion reacts with hydrogen ions from the acid. The general equations are metal carbonate + acid → salt + water + CO₂ and metal hydrogen carbonate + acid → salt + water + CO₂. Therefore A is the common product. Hydrogen is associated with some acid-metal reactions, while oxygen and ammonia are not products of these carbonate reactions.

An acid reacting with a metal carbonate produces all three named products: salt, water, and carbon dioxide. For example, 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. An acid and a metal generally produce salt and hydrogen; an acid-base reaction produces salt and water; and an acid-metal oxide also generally produces salt and water. Hence A is the only complete match.

For an aqueous solution, pH = −log[H⁺]. Adding water dilutes the solution, lowering the hydrogen-ion concentration and increasing its pH from 2 to 4. Because pH 4 is still below 7, the solution remains acidic, but it is less acidic than before. It has not necessarily become basic, and dilution does not remove all ions. Thus A is the most accurate statement.

The governing concept is the metal reactivity series. A metal releases hydrogen from a dilute non-oxidising acid only when it is above hydrogen in the series and can displace H⁺ ions. Copper lies below hydrogen, so it does not normally react with dilute hydrochloric acid to produce H₂. It does not dissolve simply in water, hydrochloric acid is an acid, and the statement that copper always forms gas is false. Thus option A is correct.

The governing concept is the two-stage reaction of carbon dioxide with limewater. Initially, CO₂ reacts with calcium hydroxide to form insoluble calcium carbonate, Ca(OH)₂ + CO₂ → CaCO₃ + H₂O, causing milkiness. When excess CO₂ is bubbled through, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium hydrogen carbonate is soluble, so the precipitate dissolves and the solution becomes clear. Option A is correct.

The governing concept is the acidic nature of many non-metal oxides. Such oxides can react with bases in a neutralisation-type reaction to form a salt and water. For example, CO₂ + 2NaOH → Na₂CO₃ + H₂O, while CO₂ with limewater can form calcium carbonate. Hydrogen gas and a metal are not compulsory products, and it is incorrect to say that no reaction ever occurs. Therefore, option A is correct.

The governing concept is amphoteric behaviour, meaning that a substance can react with both acids and bases. Aluminium oxide reacts with an acid such as HCl to form aluminium chloride and water: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. It also reacts with a strong base such as NaOH to form sodium aluminate in aqueous conditions. Thus it is neither exclusively acidic nor exclusively basic, so option A is correct.

The governing concept is acid strength, which depends on the extent of ionisation in water, not merely on the solution label or concentration. Hydrochloric acid is a strong acid and ionises almost completely, producing a higher concentration of H₃O⁺ ions at the same molar concentration. Acetic acid is weak and ionises only partially, so it produces fewer hydronium ions and a weaker acidic effect. Therefore, option A is correct.

The governing concept is pH and the action of acids on tooth enamel. Bacteria use sugars and food particles to produce acids, lowering the mouth’s pH. When the pH becomes sufficiently low, the mineral-rich enamel begins to lose minerals and becomes weaker, increasing decay. Therefore, option A is correct. Bases do not immediately dissolve teeth, salt does not make teeth metallic, and ordinary water is not the main cause.

The governing concept is acid–base neutralisation in soil treatment. Slaked lime, Ca(OH)₂, is a basic substance. When it is mixed with highly acidic soil, it reacts with excess acidic components and reduces their effect, bringing the soil closer to a suitable pH for plant growth. Therefore option A is correct. The treatment does not increase acidity, manufacture chlorine, or convert soil into salt.

The governing concept is correction of soil pH through natural chemical changes. Organic manure decomposes through microbial activity and may produce organic acids and other mildly acidic substances. These can react with or lessen excess basicity, improving conditions for plant growth. Hence option A is correct. Manure does not produce sodium metal, remove every drop of water, or transform soil into glass; those claims are unrelated to decomposition.

The governing concept is the reaction of a strong base with an acidic oxide. Sodium hydroxide absorbs carbon dioxide from air and can form sodium carbonate, for example: 2NaOH + CO₂ → Na₂CO₃ + H₂O. This consumes some NaOH, so less active base remains and its measured strength may decrease. Option A is correct; the other choices describe impossible or irrelevant changes.

The governing concept is the colour change of acid–base indicators. Red litmus or a comparable red indicator turns blue in a basic medium. Sodium carbonate is a salt of a strong base and a weak acid; its carbonate ions hydrolyse in water and generate hydroxide ions, so the solution is basic. Therefore option A is correct. It is not strongly acidic, ion-free, or pure water.

The governing concept is the nature of salts formed by neutralisation. Sodium chloride can be obtained from hydrochloric acid and sodium hydroxide: HCl + NaOH → NaCl + H₂O. Both parent substances are strong, and Na⁺ and Cl⁻ do not significantly hydrolyse water, so the aqueous solution is generally near neutral. Thus option A is correct; the other descriptions do not explain its pH.

The governing concept is salt hydrolysis. Ammonium chloride is formed from strong hydrochloric acid and the weak base ammonium hydroxide. In water, NH₄⁺ reacts partially with water: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. The hydronium ions make the solution acidic, so option A is correct. It is not a pure base, and the salt does ionise in water.