1Filtration
Filtration separates an insoluble solid from a liquid. Water passes through the tiny holes in filter paper; sand grains are too big.
filtration
Lesson 8.3 notes
Filtration and evaporation
Separating an insoluble solid from a liquid, and getting a dissolved solid back out of a solution.
2Residue and filtrate
The solid left in the filter paper is the residue. The liquid that passes through is the filtrate.
residuefiltrate
3Dissolved substances pass through
Dissolved particles are tiny, like water particles, so they pass through filter paper too. Filtering salt water doesn't remove the salt.
dissolved
4Evaporation
Evaporation gets a dissolved solid back. The water escapes as a gas and the solid is left behind as crystals.
evaporationcrystals
5Crystallisation
For big crystals, heat the solution to evaporate some water until it is saturated, then leave it to cool slowly. This is crystallisation.
crystallisation
6Salt from rock salt
To get pure salt from rock salt: dissolve the salt in water, filter out the sand, then evaporate the water.
rock salt
7Salt pans
In hot countries, sea water is left in shallow ponds. The Sun evaporates the water and the salt is left behind.
sea salt
Explore: Rock salt challenge
Get pure salt from rock salt. Choose the steps in the right order.
Common mistake: Filtering salt water does not remove the salt
Dissolved salt particles are small enough to pass through the holes in filter paper, along with the water. To get the salt back, evaporate the water.
Remember
- Filtration separates an insoluble solid (the residue) from a liquid (the filtrate).
- Evaporation gets a dissolved solid back from its solution.
- Rock salt: dissolve, filter, evaporate. For big crystals, cool a saturated solution slowly.
Going further at GCSE
GCSE: Mixtures and separation → GCSE: Making a soluble salt →