Reactivity series

Rusting is a corrosion process that needs both oxygen from air and water or moisture at the iron surface. Oil spreads over the metal and forms a continuous protective coating, so air and moisture cannot easily reach the iron. Thus rusting is prevented or greatly slowed. Oil does not chemically change existing rust into iron, increase reactivity, or supply extra oxygen, so option A is correct.

A displacement reaction occurs when a more reactive metal removes a less reactive metal from its salt solution. If A displaces B from an aqueous solution of a B salt, A must be above B in the reactivity series and therefore is more reactive. Hence option D is correct. If B were more reactive, it would remain in solution while A could not displace it; equal reactivity would not produce this displacement.

A metal can displace another metal from its salt solution only when it is more reactive. Since A displaces B, the reactivity order is A > B. The fact that B cannot displace A gives the same conclusion and rules out equal reactivity. Thus, A lies higher than B in the reactivity series, so option A is correct. This is a displacement-reaction application of the reactivity series.

The governing rule is that a more reactive metal displaces a less reactive metal from its salt solution. Iron is above copper in the reactivity series, so iron atoms enter solution as iron(II) ions while copper ions gain electrons and deposit on the nail: Fe + CuSO₄ → FeSO₄ + Cu. The blue solution may become pale green. Hence option A corrects the statement; this is a displacement reaction, not decomposition.

A displacement reaction follows the reactivity series: a more reactive metal can remove 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. The order is therefore A > B > C, making C the least reactive metal. Hence option C is correct; option D ignores the evidence from both displacement reactions.

Option A is a single-displacement reaction. Zinc lies above copper in the reactivity series, so Zn can replace Cu from copper sulphate solution: Zn + CuSO₄ → ZnSO₄ + Cu. Option B is decomposition because one compound splits into simpler substances. Option C is neutralisation, and option D is a combination reaction. Thus only A shows a more reactive metal displacing a less reactive one.

The reactivity series predicts whether one metal can displace another from its salt solution. A displacement reaction occurs only when the free metal is more reactive than the metal ion in the compound. Since A is less reactive than B, A cannot remove B from its aqueous salt; under the stated conditions, no displacement reaction occurs. Option B would be possible only if A were more reactive than B, so option A is correct.

A metal can displace another metal from its compound only when it is more reactive. Since A displaces B, the order begins A > B. Since B displaces C, the order continues B > C. Combining both statements gives A > B > C in reactivity. Therefore C is the least reactive metal. Option D contradicts the displacement rule, while A and B are more reactive than C.

A more reactive metal displaces a less reactive metal from its salt solution. Iron is more reactive than copper, so iron atoms lose electrons and enter solution as Fe²⁺, while Cu²⁺ ions gain electrons and become copper metal on the nail. The net reaction is Fe + CuSO₄ → FeSO₄ + Cu. Thus A is correct; copper does not displace iron, and water or sulphate gas is not responsible.

The displacement rule states that a metal can displace another metal from its salt only when it is more reactive than that metal. Since A displaces B, A is more reactive than B. Since B displaces C, B is more reactive than C. Combining both relations gives A > B > C, which is the decreasing order. Therefore option A is correct; the other sequences contradict one of the stated displacement observations.

The governing concept is the reactivity series and displacement reaction. A metal can displace another metal from its salt only when it is more reactive than that metal. Since A is less reactive than B, A cannot remove B from its salt solution. Thus no displacement takes place. Option B would be correct only if A were more reactive, while the other options do not describe a valid chemical change.

The governing idea is the reactivity series. A metal placed above hydrogen can displace hydrogen from a dilute acid, producing hydrogen gas, for example Zn + 2HCl → ZnCl₂ + H₂. Copper lies below hydrogen, so it generally cannot displace hydrogen from dilute hydrochloric or dilute sulphuric acid. Thus option A is correct; the other statements are chemically unrelated or false.

The governing concept is the reactivity series of metals. Under comparable conditions, a metal that reacts more rapidly with an acid generally has a greater tendency to lose electrons and displace hydrogen. Magnesium lies above iron in the reactivity series, so it usually produces hydrogen more quickly. The observation does not make iron incapable of reacting, does not classify magnesium as a non-metal, and does not show equal reactivity.

The gas is generally hydrogen, released when a sufficiently reactive metal displaces hydrogen from a dilute acid. Magnesium lies above hydrogen in the reactivity series, so it reacts readily, for example Mg + 2HCl → MgCl₂ + H₂. Silver lies below hydrogen and usually cannot displace it from dilute non-oxidising acids. Magnesium is a metal, so option A gives the correct reason.

A metal liberates hydrogen from a dilute non-oxidising acid only when it is above hydrogen in the reactivity series. Copper lies below hydrogen, so it cannot normally displace H⁺ ions from dilute hydrochloric or dilute sulphuric acid. Consequently, hydrogen gas is not evolved under these conditions. The result is explained by relative reactivity, not by copper being a gas, converting acid to base, or dissolving instantly in water.

In the usual metal–acid reaction, a metal above hydrogen in the reactivity series displaces hydrogen from a dilute acid: metal + acid → salt + H2. Copper lies below hydrogen in the series, so it cannot normally displace hydrogen ions from dilute hydrochloric or dilute sulfuric acid. Therefore option A is correct. Copper is a metal, does not convert acid into a base, and does not instantly dissolve in water. Special oxidising acids can behave differently, but they are outside this general rule.

The ability of a metal to react with an acid and displace hydrogen depends mainly on its position in the reactivity series. A vigorous reaction indicates greater reactivity under the stated conditions, while no gas may indicate that the metal is below hydrogen or is insufficiently reactive. The observation does not measure colour, smell, or shape. Therefore option A, reactivity of metals, is correct.

The governing concept is the reactivity series. A metal liberates 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 series, so the displacement is not favourable under ordinary conditions and H₂ is generally not evolved. Hydrochloric acid does contain hydrogen, and copper is a solid metal, so the other choices are false.

Pancreatic juice enters the small intestine and contains enzymes such as amylase, trypsin, and lipase. These help digest carbohydrates, proteins, and fats into smaller absorbable substances. If the juice is absent, digestion of several nutrient groups is impaired, although other digestive secretions may still act. Hence option A is correct; the other choices describe unrelated kidney, circulatory, or plant functions.

Sodium is a highly reactive metal. It reacts rapidly with oxygen and moisture in air, and the heat produced may even cause it to catch fire. Kerosene isolates sodium from air and moisture. It is not used to increase sodium’s hardness, colour, or electrical conductivity. Exam tip: Highly reactive metals such as sodium and potassium are stored under kerosene oil.