Metals and Non-metals

A metal higher in the reactivity series can displace a less reactive metal from its salt solution. Since A displaces B, A is more reactive than B; since B displaces C, B is more reactive than C. Therefore, the order is A > B > C, making C the least reactive. Option B is intermediate, while option C is the most reactive.

The reaction is Fe + CuSO₄ → FeSO₄ + Cu. Iron lies above copper in the reactivity series, meaning iron loses electrons more readily and can displace copper ions from solution. Copper is then reduced to copper metal and deposits on the nail. Therefore, the governing principle is that a more reactive metal displaces a less reactive metal. The other statements are not general chemical principles.

Galvanisation is the coating of iron or steel with zinc to prevent corrosion. Rusting requires iron to come into contact with oxygen in air and water or moisture. The zinc coating acts as a barrier, so air and moisture cannot readily reach the iron surface; zinc also offers sacrificial protection if scratched. Therefore option A is correct. Sound, light, salt, sugar, hydrogen and nitrogen are not the required pair for ordinary rusting.

Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from dilute sulphuric acid: Zn + H₂SO₄ → ZnSO₄ + H₂. Hence option A is correct. An acid reacting with sodium carbonate releases carbon dioxide, not hydrogen. Carbon dioxide passed into limewater forms calcium carbonate, while heating copper sulphate does not produce hydrogen gas. The type of acid and the metal’s reactivity are decisive.

The governing concept is the acid-base nature of oxides. Many metal oxides react with acids to form salt and water, so they are generally basic; for example, CuO reacts with hydrochloric acid. Many non-metal oxides react with water to form acids or with bases to form salts, so they are generally acidic; carbon dioxide is a common example. The word generally matters because exceptions such as amphoteric oxides exist. Thus option A is correct.

The governing concept is the acidic nature of many non-metal oxides. When a suitable non-metal oxide reacts with water, it can form an oxyacid; for example, CO2 + H2O ⇌ H2CO3. The resulting solution contains acidic species and may turn blue litmus red. Therefore option A is correct. Options B and C reverse the usual behaviour, and D is too absolute because some non-metal oxides dissolve or react with water.

The governing concept is the reaction of acids with metals. Curd contains lactic acid and is therefore acidic. During prolonged storage, the acid may react with copper, producing soluble copper compounds that can contaminate the food and may be harmful in excess. The vessel is consequently unsuitable for curd. Option A is correct; B and C describe impossible changes, while D incorrectly denies the presence of acid in curd.

Most metal oxides are basic because they react with acids to form salt and water. For example, copper(II) oxide reacts with hydrochloric acid: CuO + 2HCl → CuCl2 + H2O. Some metal oxides, such as aluminium oxide and zinc oxide, are amphoteric and react with both acids and bases, so “generally” is important. Nevertheless, the broad school-level classification is basic, making option B correct.

During chlor-alkali electrolysis of concentrated brine, chloride ions move to the anode and lose electrons by oxidation: 2Cl− → Cl2 + 2e−. Consequently, chlorine gas is released at the anode. Hydrogen is formed at the cathode, while oxygen and nitrogen are not the expected products in this school-level process. Hence option A is correct. The chlorine obtained can also be used in making bleaching powder and other compounds.

A burning splint producing a characteristic pop confirms that the gas is hydrogen. In the usual reaction of a metal with a dilute acid, the metal displaces hydrogen: metal + acid → salt + H₂. Such displacement is possible when the metal is above hydrogen in the reactivity series. Therefore option A is correct; carbonate is a compound, not a metal, and the other statements conflict with the test.

A metal releases hydrogen from a dilute non-oxidising acid only when it is more reactive than hydrogen and can displace H⁺ ions. Copper lies below hydrogen in the usual reactivity series, so it cannot normally displace hydrogen from dilute hydrochloric or dilute sulphuric acid. Consequently, hydrogen gas is not evolved under ordinary conditions, making option A correct. The other options do not describe the displacement rule.

The governing concept is selective reabsorption during urine formation. Filtration first places water and small solutes in the nephron tubule, after which useful water and substances are normally returned to the blood. If reabsorption becomes very low, less water is recovered, so more filtrate remains and urine volume may increase. Therefore option A is correct; the other choices describe unrelated or biologically impossible effects.