PRINTER FRIENDLY PRACTICAL PAGE
AKA: Separating salt and water
Salt is separated from water using a liebig condensor.
Using standard distillation equipment (see Fig 1), we can separate a saline solution into salt and water.
The salt solution should always be made up using deionised (distilled) water.
The distillation process involves two changes of physical state; When the salt solution is heated, water evaporates, rises in the flask and travels into the liebig condenser where it condenses back into a liquid. The condenser houses an inner glass tube where the condensation takes place, around which is another containing running cold water. This cools the water vapour and turns it into a liquid.
When all the water in the flask has evaporated, you should be left with only salt crystals.
Simple distillation only works with solutions containing dissolved solids. This is because the boiling point of the dissolved solids are much greater than that of the water so they cannot evaporate at the temperature point as water.
To separate liquids with different boiling points we use a more complex system, called fractional distillation (see Fig 2). Much of the equipment used is similar to that we use for simple distillation apart from a tall glass tube called a fractioning column. This is a tube which contains glass beads, shelves or stepped sections to increase the surface area, enabling condensation and evaporation to happen more productively.
Mixtures of alcohol and water can be separated using a fractioning column, as can synthetic crude oil.
As the mixture is heated, less dense liquids with lower boiling points will evaporate at a lower temperature than dense heavy liquids. Each liquid that 'comes off' is called a 'fraction' and as the temperature of the mixture increases, so these fractions will become more and more dense.
The fractioning column is effective because as the mixture is heated, several of the liquids may evaporate at the same point however the liquid with the higher boiling point will condense on the glass beads or shelves inside the column and drip back into the flask. The liquids with the lower boiling points will remain as a gas and travel into the condenser.
Cold running water should always enter the liebig condenser at the bottom connector for maximum efficiency.
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