its area. Place the watch-glass on the beaker. d Heat the water in the beaker until it boils, and then let it boil briskly. e When all the water on the watch-glass has evaporated, turn off the Bunsen and remove the watch-glass with tongs, as it will be hot. It can safely be placed on the bench, though. f Examine the watch-glass for traces of solid residue. g Repeat the steps c to f with distilled water sea water Demonstration a Fill a round-bottomed flask right up to the top with tap water, and insert a bung carrying a delivery tube (see diagram) so that the tube itself becomes completely filled with water. If this cannot easily be done, place the bung in the flask having the whole apparatus immersed in a sink of water. Repeat with a flask full of sea water. b Set up each flask as shown in the diagram. Ensure that the collection test tube is full of water in each case. c Heat each flask by means of a Bunsen burner. Bubbles of gas will be released from the water and travel into the test-tube. Continue until the contents of the flask are boiling. About half a test-tube full of gas will be collected in each case, all of which has been displaced from solution by heating. Teaching notes Distilled water should contain no dissolved solids, tap water some dissolved solids (those causing 'hardness' for example) and sea water more dissolved solids (sodium chloride and