Mixtures & separating techniques

Year 7 answers

Fluency

Pure: one kind of particle. Mixture: two or more substances not chemically joined. Solution: homogeneous mixture (solute dissolved in solvent). Suspension: heterogeneous mixture with undissolved solid particles.
(a) Heterogeneous — milk under a microscope shows fat droplets. (b) Homogeneous. (c) Heterogeneous. (d) Homogeneous. (e) Heterogeneous.
Filtration: particle size. Distillation: boiling point.
(a) Magnetic separation. (b) Evaporation or crystallisation. (c) Chromatography. (d) Distillation.
Residue is the solid trapped in the filter paper; filtrate is the liquid that passes through.
Total mass $= 15 + 85 = 100$ g. Concentration $= 15/100 \times 100 = 15\%$ .
Filtration: tea strainer, coffee filter. Evaporation: drying wet clothes, salt from salt pans.
Heating turns the solvent to vapour; cooling (condenser) turns the vapour back to liquid so it can be collected separately.
Evaporation just removes the solvent (keeps the solid behind). Distillation collects both the solid residue and the pure solvent.
Muesli, salad, fruit cocktail, a bowl of soup with chunks, oil-and-vinegar dressing.

Reasoning

Sugar is dissolved — its particles are separated into individual molecules among the water molecules, small enough to pass through the filter paper along with the water.
At least some pigments must dissolve in the solvent so they can be carried up the paper. Otherwise nothing moves and no separation occurs.
Not safe. Filtration removes undissolved solids but not dissolved chemicals, viruses, or most bacteria. Boiling or chemical treatment is still required.
(i) Add water to dissolve the salt. (ii) Use a magnet to remove iron filings. (iii) Filter to separate sand (residue) from salt solution (filtrate). (iv) Evaporate the filtrate to recover salt.
A pure substance has only one type of particle, so chromatography shows one spot. A mixture contains several pigments, each travelling a different distance — producing several spots.
Ethanol collects first because it has the lower boiling point ( $78$ °C); it vaporises and condenses before water does.

Reasoning

(i) Filter the mixture — insoluble pill solids stay as residue. (ii) Evaporate the filtrate — water leaves, dissolved sugar and colouring remain as solid. (iii) If pure sugar is needed, further chromatography or recrystallisation could separate the dye from the sugar.
$3.5\%$ of $2.0$ kg $= 0.035 \times 2000 = 70$ g.
Spot each suspect pen and the stain on a single chromatography sheet. Run in a solvent and compare the patterns. A suspect pen matching the stain’s distances and colours is a likely source; any pen whose pattern differs can be eliminated.
Crude oil is heated and rises through a tall column. Lighter hydrocarbons with low boiling points rise highest before condensing; heavier ones condense at lower levels. Each fraction is collected where it condenses, separating the mixture by boiling point.

Reasoning

A solar still has a shallow tray of sea water with a sloped transparent cover. Sunlight heats the water, some evaporates, condenses on the cool cover, and runs down into a separate collection channel. The process is evaporation + condensation — the core steps of distillation — powered by solar energy instead of a flame.
(i) Magnet → remove iron filings. (ii) Add water → dissolve salt, leave sand + sawdust. (iii) Sawdust floats; pour off water with sawdust (decantation) and filter sawdust. (iv) Filter remaining mixture to separate sand (residue) from salt water (filtrate). (v) Evaporate filtrate to recover salt.
At low temperature, gas particles slow; nitrogen and oxygen condense at different points ( $-196$ °C and $-183$ °C). Fractional distillation separates them by their different boiling points. At room temperature, both are gases mixed together — no difference in state to exploit.
Blood cells are roughly the same size as plasma particles at filter scales and can clog paper, but they are denser than plasma. Centrifugation spins the sample; denser red and white cells settle to the bottom, plasma stays at the top. It exploits density rather than particle size.