IMO Practice Test — Chemical Reactions and Equations
12 Questions • 15 min • Olympiad level
15:00
Question 1 of 12
medium
Assertion: 2Mg + O2 → 2MgO is a combination reaction. Reason: Two reactants form a single product. Choose the correct option.
Both true, reason explains assertion
Both true, reason does not explain assertion
Assertion true, reason false
Assertion false, reason true
Explanation: Both statements are true and the reason (two reactants → one product) is exactly why it is a combination reaction.
Question 2 of 12
hard
When 2Pb(NO3)2 is heated strongly, the total number of gas molecules (NO2 + O2) produced per formula unit balance is:
3
4
5
6
Explanation: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ gives 4 + 1 = 5 gas molecules.
Question 3 of 12
hard
A reaction is both a combination and a redox reaction. The best example is:
CaCO3 → CaO + CO2
C + O2 → CO2
Na2SO4 + BaCl2 → BaSO4 + 2NaCl
HCl + NaOH → NaCl + H2O
Explanation: C + O₂ → CO₂ forms one product (combination) and carbon gains oxygen (oxidation), so it is also redox.
Question 4 of 12
medium
In the balanced equation for the reaction of aluminium with chlorine, 2Al + 3Cl2 → 2AlCl3, the total number of atoms on the product side per balanced equation is:
6
8
2
5
Explanation: 2AlCl₃ contains 2 Al + 6 Cl = 8 atoms.
Question 5 of 12
medium
Assertion: Iron sheets are coated with zinc to prevent rusting. Reason: Zinc is more reactive than iron and corrodes preferentially. Choose the correct option.
Both true, reason explains assertion
Both true, reason does not explain assertion
Assertion true, reason false
Assertion false, reason true
Explanation: Galvanisation works because zinc, being more reactive, corrodes in place of the iron and also blocks air and moisture.
Question 6 of 12
hard
Three nails are placed: (i) in dry air, (ii) in boiled water with oil on top, (iii) in ordinary tap water open to air. Which nail rusts most?
(i)
(ii)
(iii)
all equally
Explanation: Only nail (iii) has both oxygen and water available, so it rusts most; (i) lacks water and (ii) lacks dissolved oxygen.
Question 7 of 12
medium
In ZnO + C → Zn + CO, which statement is correct?
Carbon is the oxidising agent
ZnO is the reducing agent
Carbon is oxidised and ZnO is reduced
No oxidation occurs
Explanation: Carbon gains oxygen (oxidised, reducing agent) and ZnO loses oxygen (reduced, oxidising agent).
Question 8 of 12
medium
A student adds dilute HCl to zinc and observes bubbles. The gas evolved, when tested, gives a 'pop' sound with a burning splint. The gas is:
oxygen
carbon dioxide
hydrogen
chlorine
Explanation: Zn + 2HCl → ZnCl₂ + H₂; hydrogen gas burns with a characteristic pop sound.
Question 9 of 12
hard
Which pair correctly matches reaction with type? (P) respiration in cells; (Q) digestion of food.
P: combination, Q: displacement
P: oxidation (exothermic), Q: decomposition
P: decomposition, Q: combination
P: displacement, Q: precipitation
Explanation: Respiration oxidises glucose releasing energy (exothermic oxidation); digestion breaks large food molecules into smaller ones (decomposition).
Question 10 of 12
hard
Assertion: Decomposition reactions are the opposite of combination reactions. Reason: Decomposition absorbs energy while combination often releases it. Choose the correct option.
Both true, reason explains assertion
Both true, reason does not explain assertion
Assertion true, reason false
Assertion false, reason true
Explanation: Both statements are true, but the energy comment is an added fact, not the reason decomposition is the reverse of combination.
Question 11 of 12
medium
Butter kept open develops a foul smell, but butter kept in a sealed nitrogen-filled tin does not. This best demonstrates:
rusting
neutralisation
the role of oxygen in rancidity
thermal decomposition
Explanation: Rancidity needs oxygen; removing it with nitrogen prevents the oxidation of fats, so no foul smell develops.
Question 12 of 12
hard
For the unbalanced reaction Fe2O3 + Al → Al2O3 + Fe (thermite), the correctly balanced coefficients (Fe2O3, Al, Al2O3, Fe) are:
1, 2, 1, 2
2, 2, 1, 3
1, 1, 1, 1
2, 1, 1, 2
Explanation: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe balances 2 Fe, 3 O and 2 Al on each side; it is also a displacement and redox reaction.