IMO Practice Test — Acids, Bases and Salts
12 Questions • 15 min • Olympiad level
15:00
Question 1 of 12
Equal volumes of HCl of pH 1 and NaOH of pH 13 are mixed. The resulting solution is approximately:
strongly acidic
strongly basic
neutral (pH 7)
pH 14
Explanation: pH 1 acid has [H+] = 0.1 and pH 13 base has [OH-] = 0.1; equal volumes neutralise exactly, giving a neutral solution.
Question 2 of 12
A metal M reacts with both dilute HCl and with NaOH solution to give hydrogen. M is most likely:
copper
zinc
gold
silver
Explanation: Amphoteric metals like zinc (and aluminium) react with both acids and strong alkalis to release hydrogen.
Question 3 of 12
A solution of pH 5 has how many times more H+ ions than a solution of pH 8?
3 times
30 times
300 times
1000 times
Explanation: Each pH unit is a tenfold change; a difference of 3 units means 10x10x10 = 1000 times more H+ ions.
Question 4 of 12
Heating washing soda crystals (Na2CO3·10H2O) strongly gives:
sodium hydrogencarbonate
anhydrous sodium carbonate
sodium hydroxide
carbon dioxide only
Explanation: Heating drives off the water of crystallisation, leaving anhydrous Na2CO3.
Question 5 of 12
Which salt, when dissolved in water, gives a solution with pH greater than 7?
NH₄Cl
NaCl
Na₂CO₃
Na₂SO₄
Explanation: Na2CO3 is the salt of a weak acid and a strong base, so its solution is basic (pH greater than 7).
Question 6 of 12
On adding dilute HCl to a solution, a colourless, odourless gas is evolved that turns lime water milky. The solution most likely contains:
a sulphate
a carbonate
a chloride
a nitrate
Explanation: Carbonates react with acid to release CO2, which turns lime water milky.
Question 7 of 12
A dry acid does not show acidic behaviour because:
it has no oxygen
it cannot ionise to give H+ ions
it is too concentrated
it reacts with glass
Explanation: Without water the acid cannot ionise, so no H+ ions form and there is no acidic character.
Question 8 of 12
Bleaching powder slowly loses its bleaching power on standing in air because it reacts with:
nitrogen
carbon dioxide and moisture
argon
sunlight only
Explanation: CO2 and moisture in air react with bleaching powder, releasing chlorine and reducing its available chlorine over time.
Question 9 of 12
Which pair will both turn moist red litmus blue?
CO₂ and SO₂
NH₃ and NaOH solution
HCl and CO₂
vinegar and lemon juice
Explanation: Ammonia and sodium hydroxide are basic and turn red litmus blue; the other options are acidic.
Question 10 of 12
When CaSO4·½H2O is mixed with water, the change that occurs is:
it dissolves completely
it sets into a hard mass of CaSO₄·2H₂O
it gives off chlorine
it turns into quicklime
Explanation: POP takes up water and sets into gypsum, CaSO4.2H2O, a hard solid.
Question 11 of 12
A student adds a few drops of water to white anhydrous copper sulphate. The expected observation is:
no change
it turns blue with a hissing/warming
it turns black
a gas is released
Explanation: Anhydrous CuSO4 takes up water (water of crystallisation) and turns blue, releasing some heat; this is a test for water.
Question 12 of 12
Two acids of equal concentration, HCl and CH3COOH, are taken. Which statement is correct?
both have the same pH
HCl has a lower pH than CH₃COOH
CH₃COOH has a lower pH than HCl
both are neutral
Explanation: HCl ionises fully (strong acid) and gives more H+ ions, so it has a lower pH than the weak acetic acid.