8E TEACHER GUIDE.pdf - swallowhillresources

In the drink8ESimple distillation involves separating a liquid from dissolvedsolids. The method is effectively the same as evaporation, exceptthat the vapour is collected and condensed to form a liquid.When using a Liebig condenser in distillation, ensure that thecooling water goes in at the bottom and out at the top. This ensuresthat the cooling jacket remains full of water. When demonstratingthe Liebig condenser to pupils it is worth setting it up the wrongway round to show them that if water is fed in at the top it just runsdown the lower surface and does not remain in contact with thecentral tube.8E When heating liquids in a flask, anti-bumping granules mustbe used. These help small bubbles of gas to form as the liquidboils. In the absence of anti-bumping granules, large bubbles ofsteam can form and shake the flask.Fractional distillation involves the separation of two or moreliquids with different boiling points. If the mixture is heated sothat one of the liquids boils, this liquid will turn into a gas andleave the boiling flask. If this gas is then channelled so that it canno longer return to the hot flask, it can be cooled. As a result,it will condense and turn back into a liquid, which can then becollected. If the original mixture contained several different liquids,the mixture can be heated further until the next liquid boils. Thegas produced can then be condensed and collected in a differentcontainer. This process of collecting different liquids from the samemixture is known as fractional distillation and is used to separatethe different chemicals found in crude oil into fractions. Note thatpure liquids are not formed during fractional distillation becauseliquids will evaporate when heated, whether or not they are boiling.So when the mixture in the flask reaches the boiling point of oneliquid, other liquids in the mixture will also be evaporating to someextent.Desalination can be carried out by evaporating sea water andthen condensing the vapour, but this is energy-intensive. Reverseosmosis forces water through a semipermeable membrane at highpressure. Only water molecules, or other molecules of similar size,can pass through the membrane.© Pearson Education Limited 2008 Exploring Science edition 139