Mixtures, solutions & separation
The idea
Most of what you handle daily — sea water, soil, air, salad dressing — is not one pure substance but several mingling together. In a mixture the components keep their own identities and properties, are not chemically bonded, and can occur in any proportions. A solution is the smoothest kind of mixture: one substance, the solute, spreads particle by particle through another, the solvent, until the result looks like a single clear substance, the way salt disappears into water.
Because the components keep their properties, you can pull a mixture apart by attacking a property only one component has. Filtration catches particles too large to pass through paper; evaporation removes a liquid with a low boiling point and strands the dissolved solid; a magnet picks out iron; distillation evaporates a liquid and then condenses it back to keep it. The misconception to drop is that dissolved salt is gone or has become new stuff — its particles are merely scattered among the water particles, and evaporating the water hands back the very same salt.
Worked example
A beach experiment leaves you with a jar of water containing loose sand and dissolved salt. Plan the steps to recover dry sand and dry salt separately, naming the property difference each step exploits.
- Deal with the sand first: it never dissolved, so its grains are huge compared with water particles. Pour the jar through filter paper — sand is trapped while salty water passes through. This step exploits the difference in particle size and solubility. Let the sand dry.
- Notice why filtering cannot catch the salt: its dissolved particles are spread one by one among the water particles and slip through any filter, so a different property is needed.
- Heat the salty water gently until the water evaporates away. Water leaves as vapor near 100 °C while salt stays put — it would need over 800 °C just to melt — leaving dry salt crystals behind. This step exploits the gap between their boiling points.
- Review the plan: each separation targeted a property only one component had — grain size for the sand, then easy evaporation for the water. To keep the water as well, you would trap and condense the steam, which is distillation.
Answer. Filter the mixture to recover the sand, then evaporate the water to leave the salt — filtration uses particle size, evaporation uses the difference in boiling points.
Check your understanding
- Why can filter paper trap sand but never trap dissolved salt, no matter how fine the paper is?
- How would you separate a mixture of iron filings, salt, and sand, and in what order would you run the steps?
- What evidence convinces you that salt dissolved in water still exists and has not chemically changed into something new?
- Where does a separation plan begin when you face a brand-new mixture — what do you need to know about its components first?
Build the foundations first
Mixtures, solutions & separation builds on these concepts. If any feel shaky, start there.