IMO Practice Test — Is Matter Around Us Pure
14 Questions • 15 min • Olympiad level
15:00
Question 1 of 14
A solution contains 20 g of salt in 80 g of water. If 20 g more water is added, the new mass percentage of salt is:
20%
16.7%
25%
12.5%
Explanation: New solution mass = 20 + 80 + 20 = 120 g; mass % = (20 / 120) x 100 = 16.7%.
Question 2 of 14
At 25 C the solubility of a salt is 40 g per 100 g water. How much salt will a saturated solution made with 250 g of water contain?
40 g
80 g
100 g
250 g
Explanation: 40 g per 100 g water, so 250 g water holds (40/100) x 250 = 100 g.
Question 3 of 14
A mixture has sand, salt, iron filings and camphor. The number of these that can be removed by a single physical step each is:
1
2
3
4
Explanation: Magnet removes iron, sublimation removes camphor, water dissolves salt (then filter sand) — all four are separable physically.
Question 4 of 14
Which sequence correctly separates a mixture of iron filings, ammonium chloride and salt?
dissolve, filter, evaporate
magnet, sublimation, dissolve-filter-evaporate
distillation only
centrifugation, magnet
Explanation: Use a magnet for iron, sublimation for ammonium chloride, then dissolve and evaporate the salt.
Question 5 of 14
Two liquids with boiling points 60 C and 65 C are best separated by:
simple distillation
fractional distillation
a separating funnel
evaporation
Explanation: The boiling points differ by only 5 C, so a fractionating column (fractional distillation) is needed.
Question 6 of 14
Which one is NOT a sign of a chemical change?
change in colour
evolution of a gas
change of state on heating
formation of a precipitate
Explanation: A simple change of state (e.g. melting) forms no new substance, so it is physical, not chemical.
Question 7 of 14
A colloid in which a liquid is dispersed in another liquid is called an:
aerosol
emulsion
foam
gel
Explanation: Liquid-in-liquid colloids are emulsions; milk is a common example.
Question 8 of 14
16 g of oxygen combines with hydrogen to form water in a fixed ratio of 1:8 by mass (H:O). The mass of hydrogen needed is:
1 g
2 g
8 g
16 g
Explanation: H:O = 1:8 by mass, so for 16 g of O the hydrogen needed is 16/8 = 2 g.
Question 9 of 14
Brass (copper + zinc) is best described as:
a compound
an element
a homogeneous mixture (alloy)
a suspension
Explanation: Alloys like brass are homogeneous mixtures of metals, not compounds.
Question 10 of 14
A beam of light passes unseen through liquid X but is visible in liquid Y. Therefore:
X is a colloid, Y is a true solution
X is a true solution, Y is a colloid
both are colloids
both are true solutions
Explanation: The Tyndall effect appears only in colloids, so Y (where the beam is visible) is the colloid.
Question 11 of 14
150 mL of a 30% (by volume) alcohol solution contains how much alcohol?
30 mL
45 mL
50 mL
120 mL
Explanation: Volume of alcohol = (30/100) x 150 = 45 mL.
Question 12 of 14
Which of the following can be separated into simpler substances only by a chemical method?
air
sea water
water (H2O)
brass
Explanation: Water is a compound, separable into hydrogen and oxygen only by chemical means; the others are mixtures.
Question 13 of 14
When 50 g of a saturated solution at 30 C is cooled to 10 C, crystals appear. This shows that the solubility:
increases on cooling
decreases on cooling
stays constant
depends only on pressure
Explanation: Cooling lowers solubility, so excess solute crystallises out — the basis of crystallisation.
Question 14 of 14
Heating iron filings with sulphur and then testing the product with a magnet shows no attraction. This proves that:
iron evaporated
a new compound (FeS) formed
the magnet was weak
it is still a mixture
Explanation: The loss of magnetism shows the iron is now chemically combined as iron sulphide, a compound.