IMO Practice Test — The s-Block Elements
14 Questions • 15 min • Olympiad level
15:00
Question 1 of 14
Caesium has the lowest ionisation enthalpy yet lithium is the strongest reducing agent in water. The key reason is:
Li has a higher melting point
Li+ has an exceptionally high hydration enthalpy
Cs is radioactive
Li has a larger atomic size
Explanation: Li+'s very large (exothermic) hydration enthalpy outweighs its high ionisation enthalpy, giving the most negative E°.
Question 2 of 14
Burning Li, Na and K in excess air gives, respectively:
oxide, peroxide, superoxide
superoxide, peroxide, oxide
oxide, oxide, oxide
peroxide, oxide, superoxide
Explanation: Li forms Li2O (oxide), Na forms Na2O2 (peroxide) and K forms KO2 (superoxide).
Question 3 of 14
Among Li+, Na+, K+, Cs+, the ion with the greatest ionic mobility in water is:
Li+
Na+
K+
Cs+
Explanation: Cs+ is the least hydrated (smallest hydrated radius), so it moves fastest; Li+, most hydrated, is slowest.
Question 4 of 14
The diagonal relationship Li–Mg arises because:
both have the same atomic number
the rise in charge (right) and rise in size (down) give similar charge/radius ratios
both are radioactive
both belong to the same group
Explanation: Going one group right raises charge and one period down raises size; these cancel, matching the charge-to-radius ratios.
Question 5 of 14
A white solid X liberates CO2 with dilute acid and, on strong heating, leaves a residue that gives an alkaline solution turning red litmus blue. X is most likely:
NaCl
CaCO3
CaSO4
BaSO4
Explanation: CaCO3 gives CO2 with acid and on heating gives CaO, whose solution Ca(OH)2 is alkaline.
Question 6 of 14
Solubility of the hydroxides of Group 2 (Be to Ba) down the group:
decreases
increases
stays constant
is always zero
Explanation: Hydroxide solubility increases down Group 2 because lattice energy falls faster than hydration energy; Ba(OH)2 is fairly soluble.
Question 7 of 14
Solubility of the sulphates of Group 2 down the group:
increases
decreases
stays constant
first rises then falls
Explanation: Sulphate solubility decreases down Group 2 (BeSO4 soluble, BaSO4 insoluble) as hydration energy falls faster than lattice energy.
Question 8 of 14
Which pair of compounds best illustrates the Be–Al diagonal relationship?
Li3N and Mg3N2
BeO and Al2O3
NaCl and KCl
CaO and BaO
Explanation: Both BeO and Al2O3 are amphoteric, a classic Be–Al diagonal similarity.
Question 9 of 14
A metal burns with a brick-red flame and its carbonate decomposes on heating to a basic oxide. The metal is:
Na
K
Ca
Ba
Explanation: Calcium gives a brick-red flame; CaCO3 → CaO + CO2, and CaO is basic.
Question 10 of 14
On passing CO2 through lime water it turns milky, but excess CO2 clears it. The clearing is due to formation of:
CaO
Ca(HCO3)2
CaCl2
CaC2
Explanation: Excess CO2 converts insoluble CaCO3 to soluble calcium hydrogencarbonate, Ca(HCO3)2, so the milkiness disappears.
Question 11 of 14
Why does the second ionisation enthalpy of Group 1 jump enormously above the first?
the second electron comes from a noble-gas core
the atom becomes larger
shielding increases sharply
the nucleus loses charge
Explanation: After losing the ns1 electron, the +1 ion has a stable noble-gas configuration, so removing a second electron is extremely difficult.
Question 12 of 14
Lithium and magnesium both form nitrides on direct reaction with N2. The respective formulae are:
LiN and MgN
Li3N and Mg3N2
Li2N and Mg2N
LiN3 and Mg2N3
Explanation: Balancing charges (Li+, Mg2+ with N3-) gives Li3N and Mg3N2.
Question 13 of 14
Which property correctly increases down both Group 1 and Group 2?
ionisation enthalpy
electronegativity
atomic radius
hydration enthalpy of the cation
Explanation: Atomic radius increases down both groups; ionisation enthalpy, electronegativity and cation hydration enthalpy all decrease.
Question 14 of 14
Plaster of Paris sets to a hard mass because:
it loses CO2
it absorbs water and reverts to gypsum
it melts and recrystallises
it reacts with oxygen
Explanation: On mixing with water, CaSO4·½H2O reabsorbs water to re-form interlocking crystals of gypsum (CaSO4·2H2O).