Matter, Mixtures, and Separation Techniques
Classify matter from evidence and choose a separation method by asking which physical property is different.
Matter, Mixtures, and Separation Techniques
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Classify the sample first; then match the separation method to a physical property
Pure substances have fixed composition. An element contains one type of atom, while a compound contains elements chemically combined in a fixed ratio. A mixture contains substances physically combined in variable proportions.
A homogeneous mixture is uniform throughout, like saltwater. A heterogeneous mixture is nonuniform or has distinct parts, like sand in water. Mixtures can be separated physically because their components retain useful properties such as particle size, solubility, boiling point, density, or magnetism.
A physical change changes form, state, or distribution without forming a new substance. A chemical change forms one or more new substances; useful evidence may include an unexpected precipitate, gas, color change, heat change, or light.
Ask what property differs
Filtration uses particle size; distillation uses boiling point; a separating funnel uses immiscibility and density; chromatography uses differing attractions; a magnet uses magnetism.
Recovery changes the method
Evaporation can recover a dissolved solid. Distillation is needed when you also want to collect the solvent.
Name the components, name the useful difference, then order the methods
Problem: Recover iron, sand, and salt from one dry mixture.
1. Magnetism: Use a magnet to remove the iron.
2. Solubility: Add water so the salt dissolves but the sand does not.
3. Particle size: Filter. Sand remains as residue; saltwater passes through as filtrate.
4. Volatility: Evaporate the filtrate to recover salt—or distill it if the water must also be collected.
Why it works
Each method works only because the components differ in a physical property. Stating that property prevents attractive but impossible choices such as filtering dissolved salt.
Five forms you should recognize
Problem: A clear liquid has the same appearance throughout, but evaporation leaves crystals.
Reason: More than one substance is present, and the components were uniformly distributed.
Answer: It is a homogeneous mixture, not a pure liquid.
Problem: Recover both solids from a sand-and-salt mixture.
Steps: Add water to dissolve only the salt. Filter to collect sand as residue. Dry the sand, then evaporate the filtrate to recover salt.
Problem: Obtain liquid water, not merely dry salt.
Steps: Heat the solution, direct the water vapor into a condenser, and collect the condensed liquid. The salt remains behind.
Answer: Use simple distillation.
Problem: Determine whether black ink contains more than one dye.
Reason: Dyes with different attractions to the solvent and paper travel different distances.
Answer: Paper chromatography can reveal separate colored spots.
Problem: Compare melting candle wax with burning wax vapor.
Reason: Melting changes only the state and can be reversed by cooling. Burning produces new substances such as carbon dioxide and water.
Answer: Melting is physical; burning is chemical.
Check before you commit
- Calling every clear sample a pure substance
- Using filtration for a dissolved solute
- Using evaporation when the solvent must be collected
- Calling dissolving a chemical reaction merely because the solute becomes invisible
- Choosing a method without identifying the relevant property
- Assuming one observation such as bubbling always proves a reaction without considering boiling or trapped gas
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Matter, Mixtures, and Separation Techniques FAQ
Can a compound be separated physically?
No. Its elements are chemically bonded and require chemical change for separation.
Why does dissolved salt pass through filter paper?
Its ions are dispersed among water particles and are much smaller than the filter pores.
When should I choose fractional distillation?
Use it for miscible liquids with different, often relatively close, boiling points; a fractionating column improves repeated separation.
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