Sec 1 Science Separation Techniques notes

Magnetic attraction pulls out magnetic materials such as iron, steel, nickel and cobalt. It is used to recover iron from mixed scrap. Copper and aluminium are metals, but they are not picked up by an ordinary magnet, so “it is a metal” is not a reason to choose a magnet.

1. 1Fold and fit

   Fold filter paper into a cone and sit it in a filter funnel. Put the funnel in a conical flask.

2. 2Pour

   Pour the mixture down a glass rod into the paper. Do not overfill the paper.

3. 3Name the products

   The solid left on the paper is the residue. The liquid in the flask is the filtrate.

Filtration separates an insoluble solid from a liquid. Salt dissolved in water passes through the paper with the water. Sand in salty water stays on the paper; the salt does not.

Evaporation heats a solution until the solvent turns to vapour and escapes. The dissolved solid stays in the dish. Use it when you want the solid and do not need the solvent. Heating gently, or leaving the solution to evaporate slowly, can give crystals.

Distillation is evaporation followed by condensation. The solution is heated in a flask. Vapour passes into a condenser, where cold water cools it back to liquid. The collected liquid is the distillate. The dissolved solid is left in the flask. Cold water enters the condenser at the lower opening so the jacket stays full of cold water.

Want the solid only

Evaporate. The solvent is lost to the air.

Want the liquid

Distil. The solvent is condensed and collected.

Sea water

Distillation can separate water from dissolved salt. The distillate is water; salt stays behind.

1. 1Draw the start line

   Use a pencil line near the bottom of the paper. Ink from a pen would run and confuse the result.

2. 2Spot and dry

   Put a small spot of the mixture on the line and let it dry. Repeat if you need a stronger spot.

3. 3Stand it in solvent

   The solvent level must be below the spot, or the mixture washes straight into the solvent.

4. 4Let it run

   Solvent climbs the paper. Substances that travel more easily move further from the start line.

A single spot that does not split may be one substance. Several spots mean a mixture. Food testing uses the same idea to see whether a colouring is a single dye or a mixture of dyes.

• Filtration removes insoluble solids such as sand and grit.
• Distillation of sea water (desalination) collects water and leaves the salt behind.
• Some plants also force water through very fine membranes (reverse osmosis). You only need the idea: dissolved salts are held back and water passes through.
• Used water can be treated and filtered again so it can be reused. That is one reason a small country can have a more reliable supply.

Treatment uses energy and equipment. Using less water — shorter showers, turning off a running tap, fixing a leak — means less has to be separated and cleaned. That is the link between this chapter and taking care of a limited resource.