Acids, Bases, and Salts

Baking soda is sodium hydrogen carbonate, NaHCO₃. When it reacts with an acid, the hydrogen carbonate ion forms carbonic acid, which quickly decomposes into carbon dioxide and water. For example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. Effervescence is caused by the released CO₂, so option B is correct.

Baking powder contains sodium hydrogen carbonate along with a mild edible acid. When the mixture becomes moist and is heated, the acid reacts with the bicarbonate and releases carbon dioxide gas. The gas gets trapped as bubbles in the dough or batter; on heating, these bubbles expand and make the food rise. Hydrogen, chlorine, and nitrogen are not the gases responsible for this leavening action.

On heating, baking soda, or sodium hydrogen carbonate (NaHCO₃), undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Sodium carbonate is the main solid residue, while carbon dioxide escapes as a gas and water is produced as vapour. Sodium chloride, calcium carbonate, and ammonium chloride are not products of this decomposition reaction, so option A is correct.

The chlor-alkali process is the electrolysis of brine, which is a concentrated aqueous solution of sodium chloride. During electrolysis, chloride ions produce chlorine at the anode, water produces hydrogen at the cathode, and sodium hydroxide remains in solution. Sugar water is not an electrolyte that gives these products, while lime water and vinegar are not the industrial feedstock for this named process.

In the chlor-alkali process, aqueous sodium chloride (brine) is electrolysed. Chloride ions are discharged at the anode to form chlorine gas, water is reduced at the cathode to form hydrogen gas, and sodium hydroxide remains in the solution. Thus the three main products are chlorine, hydrogen, and sodium hydroxide. The other combinations belong to unrelated reactions.

The chlor-alkali process electrolyses brine. At the anode, oxidation occurs: chloride ions lose electrons and form chlorine gas, represented by 2Cl⁻ → Cl₂ + 2e⁻. Hydrogen is formed at the cathode by reduction of water. Oxygen and carbon dioxide are not the characteristic anode products in this process, so chlorine, option B, is correct.

In chlor-alkali electrolysis, the cathode is the site of reduction. Water molecules gain electrons there and produce hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Chlorine is released at the anode from chloride ions. Nitrogen and sulphur dioxide do not arise from brine electrolysis, so hydrogen is the correct answer.

Tasting a laboratory chemical is unsafe because acids and bases may be corrosive, poisonous, or irritating even in small quantities. Their taste is therefore not an acceptable identification method. A suitable indicator, pH paper, or another supervised laboratory test should be used instead. Option B is false because acids and bases are not always sweet; options C and D are also scientifically incorrect.

Hydrogen chloride shows acidic behaviour only after dissolving in water, where it ionises and produces hydronium ions, H₃O⁺. Dry HCl gas cannot ionise on completely dry blue indicator paper, so the acid-sensitive colour change does not occur. It does not become a base, blue paper is not limited to bases, and gases still contain particles. Therefore option A is correct.

Moisture supplies the water needed for hydrogen chloride to dissolve and ionise: HCl + H₂O → H₃O⁺ + Cl⁻. The hydronium ions give the solution acidic character, so the blue indicator changes to red. Moisture does not turn a gas into a metal, wet paper is not naturally always red, and water does not make every gas basic. Thus option A explains the observation.

Turning blue indicator paper red is a direct indication that the solution is acidic. The second observation supports this conclusion because magnesium reacts with an acid to form a salt and release hydrogen gas, for example Mg + 2HCl → MgCl₂ + H₂. A basic or neutral solution would not produce this combined pattern, and being salty alone does not determine acidity.

The governing concept is the reaction of an active metal with a dilute acid. Zinc displaces hydrogen from the acid, for example: Zn + 2HCl → ZnCl₂ + H₂. Hydrogen is combustible and, when tested with a burning splint, burns rapidly with a characteristic pop sound. Carbon dioxide extinguishes a flame, oxygen relights it, and chlorine has different bleaching and toxic properties. Therefore, option A is correct.

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 acid–carbonate reaction and the limewater test. An acid reacts with a carbonate to form a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. When CO₂ is passed through limewater, it forms insoluble calcium carbonate, CaCO₃, which makes the solution milky. Hydrogen, nitrogen, and ammonia do not give this characteristic test. Therefore, 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 neutralisation. Most metal oxides are basic oxides, so they react with acids to produce a salt and water. For example, CuO + 2HCl → CuCl₂ + H₂O. This is not a metal-displacement reaction, so metal and hydrogen are not the general products. Carbon dioxide is usually associated with an acid reacting with a carbonate, not an ordinary metal oxide. Hence 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 the definition of an amphoteric oxide. If an oxide reacts with an acid as well as a base, it displays dual acid–base behaviour and is called amphoteric. Zinc oxide reacts with acids to form zinc salts and water, and with strong bases to form zincate species. Therefore, it cannot be classified as only acidic, only basic, or neutral on the evidence given. Option A correctly names the property proved by the observation.

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.