Use it the night before, not instead of the lessons.
| What differs between the constituents? | Technique |
| One is magnetic, the other is not | magnetic attraction |
| Different particle sizes — an insoluble solid in a liquid | filtration |
| A dissolved solid, and you want the solid | evaporation |
| Different boiling points, and you want the liquid | distillation |
| They dissolve to different extents in a solvent | chromatography |
The principle behind it: the constituents of a mixture retain their physical properties, and the differences between those properties are what let us separate them. Other factors in the choice: cost · reliability · environmental impact.
| Evaporation | Distillation |
| You collect the solid | You collect the liquid |
| Liquid escapes as vapour and is lost | Vapour is condensed and collected |
| Dish, gauze, flame | Flask, thermometer, condenser with cold water running through, beaker |
| Salt from seawater | Pure water from seawater |
The deciding question is never "what is in the mixture?" but "what do I want to collect?"
| Technique | Applications |
| Magnetic attraction | Removing iron from scrap metal in a scrapyard so it can be reused or recycled. Left behind: glass mirrors, rubber tyres, plastic bumpers. |
| Filtration | Tea strainer (retains tea leaves) · siltation pond (sand and rocks of different sizes filter groundwater) · your nose (nostril hairs and mucus trap large particles) · waterworks · Gardens by the Bay (aquatic plants filter rainwater). |
| Evaporation | Salt production in salt pans under the sun · preserving food — herbs, spices, dried shrimp, ikan bilis · vacuum drying (air removed so water evaporates faster; preferred because normal drying spoils flavour). |
| Distillation | Perfumes · distilled water · food flavourings (citrus oil, herb and spice extracts) · drinking water from seawater · oil refineries on Pulau Bukom and Jurong Island making fuels from crude oil. |
| Chromatography | Identifying colours in dyes · identifying coloured substances in food · detecting water pollutants · SFA food testing by gas chromatography (a gas as the solvent). |
1. Salt solution in the distillation flask is heated until it boils. Boiling chips smoothen the boiling.
2. Water changes into steam, which passes through the condenser. The condenser has cold water running through it continuously.
3. The steam cools and condenses into pure water — the distillate. The salt remains in the flask.
The thermometer at the neck reads the boiling point of whatever is vaporising — 100 °C while water comes across.
Distillation in nature: the water cycle (evaporation then condensation). A solar still uses the same processes to collect water where supply is irregular.
The Four National Taps: water from local catchment · imported water · NEWater · desalinated water.
Reverse osmosis: pre-treated seawater is pumped through partially permeable membranes at high pressure. Very small pores let water particles through but not salt particles, microorganisms or chemical contaminants.
Water stress: in 2015 the WRI ranked Singapore among the most water-stressed countries, and one of eight most likely to face supply disruptions by 2040.
Conserving water — reduce wastage and reuse water. Also protect resources by not throwing litter or releasing animals into waterways and reservoirs.