Which one of the following is the highest reactive metal?
- (a)Sodium
- (b)Potassium
- (c)Calcium
- (d)Iron
Correct — B, Potassium.
Reactivity in a metal means how readily it hands over its outer electron.
Potassium carries its single valence electron farther from the nucleus, behind more inner shells, than sodium does, so less energy is needed to strip it away — a first ionisation enthalpy of roughly 419 kJ/mol against sodium's 496.
The other two are further off. Calcium must surrender two electrons and its first ionisation enthalpy, near 590 kJ/mol, is already higher than either alkali metal's. Iron is a transition metal that is unreactive towards cold water and needs steam before it is oxidised.
The idea to carry away: in group 1, reactivity rises as you move down the group, because the valence electron sits farther out and is better shielded.
That runs against the instinct that a smaller, tighter atom must be the more energetic one.
- (a)Sodium — Sodium is genuinely reactive — its reaction with cold water is violent enough that the hydrogen given off catches fire, and it is kept under kerosene —
but it sits one step below potassium in the activity series, because its valence electron lies closer to the nucleus and costs more energy to remove.
It is the right answer to which of these metals colours a flame golden yellow. Potassium gives lilac and calcium brick red.
- (c)Calcium — Calcium gives up two electrons rather than one, which invites the reading that it must therefore be the more reactive metal.
What decides reactivity is the energy cost of removing those electrons, and calcium's first ionisation enthalpy is above that of both sodium and potassium.
It is the right answer to which metal floats on hydrogen bubbles without the gas igniting — the bubbles cling to its surface, and the heat evolved is not enough to set that hydrogen alight.
- (d)Iron — Iron is the least reactive of the four printed here. It stands below the other three in the activity series, is stable in dry air, and is oxidised by steam rather than by cold or hot water.
It is the right answer to which of these is smelted in a blast furnace by reduction with carbon; the other three sit too high in the series for carbon to reduce their compounds.
A metal's reactivity is its tendency to lose valence electrons and become a positive ion.
The cheaper that loss is in energy, the more readily the metal attacks water, acids, and the salts of metals placed below it.
Chemists arrange metals on exactly this basis in an activity series — the school version is a ranked list headed by potassium and ending at gold.
The series is the spine of school inorganic chemistry. It predicts which metal displaces which from solution, which metals release hydrogen from water or from dilute acid, and which ores can be reduced by carbon rather than by electricity.
When an item names four metals and asks you to rank them, the fastest route is recalling the order itself rather than deriving it.
- The school activity series runs potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, hydrogen, copper, mercury, silver, gold.
- First ionisation enthalpies rise across these four: potassium about 419 kJ/mol, sodium about 496, calcium about 590, iron about 762.
- Potassium and sodium are stored under kerosene because they react with the moisture and oxygen of air.
- Calcium reacts with cold water more gently, giving calcium hydroxide and hydrogen; the metal floats as bubbles stick to it.
- Iron is oxidised by steam: 3Fe + 4H2O gives Fe3O4 + 4H2.
- Flame colours separate the three that are not iron: potassium lilac, sodium golden yellow, calcium brick red.
The four metals in the order the school activity series places them; the keyed answer is the top row.
- Treating a divalent metal as the more reactive one: calcium loses two electrons, but the energy cost of removing them, not their number, sets its place in the series.
- Ranking group 1 by the wrong direction of the periodic trend — reactivity rises down the group, while electronegativity and ionisation enthalpy fall.
- Ordering by standard electrode potential instead of the activity series: calcium's value is more negative than sodium's, though the school series puts sodium higher. Potassium still leads among these four either way.
- Assuming the stem is asking for the most reactive metal in existence; caesium and francium are more reactive than potassium, but neither is on offer here.
- Picking sodium because it is the familiar demonstration metal, when potassium is the one placed above it.
The same ordering can be tested as a displacement item — will zinc displace iron from iron sulphate solution — or as a grading of reactions with water: cold water, hot water, steam, or no reaction at all.
It also underlies extraction, which separates the metals carbon can reduce from those that need electrolysis.
The order can be probed in reverse too: a reaction is described, and you name the metal that behaves that way.
No eligible match among this question's citable set.
- practice — not a real PYQ
Which one of the following metals is kept immersed in kerosene oil to keep it away from the moisture and oxygen of the air?
- (a)Sodium
- (b)Iron
- (c)Copper
- (d)Gold
Answera — Sodium reacts with damp air readily enough to catch fire, so it is stored under kerosene, which seals air and moisture off the surface.Iron corrodes slowly to rust and is stored in the open; copper tarnishes slowly to a green basic carbonate; gold is noble and keeps its lustre in air, so none of the three needs kerosene.
- practice — not a real PYQ
Which one of the following metals does not react with cold or hot water, but reacts with steam to form its oxide and hydrogen?
- (a)Sodium
- (b)Calcium
- (c)Iron
- (d)Potassium
Answerc — Iron is oxidised when steam is passed over it, 3Fe + 4H2O giving Fe3O4 + 4H2, while cold and hot water leave it unchanged.Potassium and sodium react with cold water itself, the reaction with potassium being the more violent. Calcium also reacts with cold water, more gently, forming calcium hydroxide and hydrogen while the metal floats on the bubbles.
- practice — not a real PYQ
The reactivity of the alkali metals increases from lithium to caesium chiefly because, on moving down the group,
- (a)the atomic radius decreases
- (b)the ionisation enthalpy decreases
- (c)the electronegativity increases
- (d)the nuclear charge decreases
Answerb — The valence electron lies in a higher shell and is screened by more inner electrons, so the energy needed to remove it falls, and the metal gives up that electron more readily.The other three invert real trends: atomic radius increases down the group, electronegativity decreases, and nuclear charge increases as protons are added.
- practice — not a real PYQ
Which one of the following metals will displace iron from an aqueous solution of iron(II) sulphate?
- (a)Copper
- (b)Zinc
- (c)Silver
- (d)Gold
Answerb — Zinc stands above iron in the activity series, so it passes into solution as zinc ions and turns out iron as the metal, the pale green solution losing its colour.Copper, silver and gold lie below iron in the series, so none of them can push iron out of its salt; the reverse happens, as when an iron nail is dipped in copper sulphate solution.