PRELIMINARY STAGE 2025
Contest 15
Fijai SHS: 37 points (Winner, qualified to the one-eighth stage)
Nsutaman Catholic SHS: 36 points
Oda SHS: 26 points
ROUND 5 - RIDDLE
RIDDLE
I am a common laboratory and industrial process.
I rely on a porous medium to act as a barrier.
I can be performed by gravity, pressure, or vacuum.
My purpose is to separate solids.
ANSWER: Filtration
EXPLANATION:
Filtration separates an insoluble solid from a liquid by passing the mixture through a porous medium (such as filter paper) under gravity, pressure or vacuum.
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PRACTICE QUESTIONS
1. RIDDLE:
I separate a liquid from a solution.
I involve boiling and then condensing.
A Liebig condenser is my tool.
I make pure water from seawater.
My fractional form separates crude oil.
I rely on differences in boiling point.
Who am I?
ANSWER: DISTILLATION
EXPLANATION:
Distillation boils a liquid and condenses the vapour, leaving the solute behind.
2. RIDDLE:
I separate the dyes in ink.
I use a mobile phase and a stationary phase.
My paper version shows spots.
Each spot has an Rf value.
The components travel at different speeds.
Column, paper and thin-layer are my types.
Who am I?
ANSWER: CHROMATOGRAPHY
EXPLANATION:
Chromatography separates components by how strongly they stick to the stationary phase.
3. RIDDLE:
I remove a solvent by turning it into vapour.
I leave a solid solute behind.
Salt is obtained from seawater by me in salt pans.
I can happen below the boiling point.
I cool your skin when sweat undergoes me.
I am often done in a dish over a water bath.
Who am I?
ANSWER: EVAPORATION
EXPLANATION:
Evaporation removes the solvent and leaves the dissolved solid.
4. RIDDLE:
I produce a pure solid from a solution.
I happen as a hot saturated solution cools.
I form regular shapes with flat faces.
Copper(II) sulfate forms blue solids with flat faces by me.
I purify impure solids.
I follow evaporation to the point of saturation.
Who am I?
ANSWER: CRYSTALLISATION
EXPLANATION:
Crystallisation forms pure crystals as a saturated solution cools.
5. RIDDLE:
I separate miscible liquids with close boiling points.
I use a column packed with glass beads.
I split crude oil into fractions.
The liquid with the lowest boiling point comes off first.
I separate liquid air into nitrogen and oxygen.
I separate ethanol from water.
Who am I?
ANSWER: FRACTIONAL DISTILLATION
EXPLANATION:
A fractionating column gives many repeated boil–condense cycles.
6. RIDDLE:
I use a separating funnel.
I shake a mixture with a second, immiscible liquid.
The wanted substance moves into the liquid it dissolves in better.
I remove iodine from water using hexane.
I am used to obtain oils from seeds.
I leave two liquid layers to be run off.
Who am I?
ANSWER: SOLVENT EXTRACTION
EXPLANATION:
Solvent extraction relies on different solubilities in two immiscible liquids.