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A condenser is a device used in the laboratory to cool hot gases into liquids. It is usually a long, circular glass tube. Inside the tube there is another, smaller tube. The hot gas goes through the smaller, inside tube, while in the outside one cold water goes around. Water can be put in from the tap, and goes from the bottom to exit at the top. This makes sure the tube is always full of water, so that cooling is quick.
A condenser is used for example in distillation, where the hot vapour needs to be cooled down back to liquid to be collected. It is also used in reflux where it makes sure that the solvent used does not boil and go away.