Dissolved substances in tap water and seawater

In this experiment, students evaporate tap water, distilled water and seawater to reveal solid residues. They then observe as the teacher boils the three types of water to release their dissolved gases, which may be collected in a test tube and tested using a glowing splint.

This is a class practical and teacher demonstration. If there is sufficient apparatus, students could do the three evaporations in parallel. Alternatively, the three types of water could be distributed round the class and the results compared at the end. It takes quite a long time for the water in a beaker to boil, so this should be built into the planning.

The teacher demonstration could be set up while the students are waiting for their water to evaporate.

If several of the shortcuts suggested above are used, these experiments could be done in 45 minutes.

Equipment

Apparatus

• Eye protection

For the teacher demonstration

• Round-bottomed flask, 250 cm³, x2
• Bung and delivery tube, to fit flask (see diagram below), x2
• Tripod and gauze, x2
• Bunsen burner
• Heat resistant mat, x2
• Stand and clamp, x2
• Beaker, 250 cm³, or small trough, x2
• Test tube, x2

For the students’ experiments

• Hard glass watch-glasses, approximately 7.5 cm diameter, x3
• Beaker, 100 cm³
• Bunsen burner
• Tripod
• Gauze
• Heat resistant mat
• Tongs

Chemicals

For the teacher demonstration

• Seawater, 400 cm³ (see note 3 below)

For the students’ experiments

• Sewater, 5 cm³ (see note 3)
• Distilled (or deionised) water, 5 cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• If the real thing is not available, ‘seawater’ can be made up by dissolving about 35 g of sodium chloride in 1000 cm³ of tap water. This will provide enough for both parts of the experiment. (Note: seawater contains a complex mixture of salts, but this gives a suitable solution for this experiment, resembling seawater in having 3.5% salinity.)

Procedure

Students’ experiment

1. Set up a Bunsen burner on a heat resistant mat. Over it, place a tripod and gauze.
2. Half-fill a beaker with water and place it on the gauze.
3. Take a watch-glass and place enough tap water on it to cover half its area. Place the watch-glass on the beaker.
4. Heat the water in the beaker until it boils, and then let it boil briskly.
5. 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.
6. Examine the watch-glass for traces of solid residue.
7. Repeat the steps 3 to 6 with:
   • Distilled water
   • Seawater

Demonstration

1. 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 seawater.
2. Set up each flask as shown in the diagram. Ensure that the collection test tube is full of water in each case.
3. 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 seawater more dissolved solids (sodium chloride and other salts). A related experiment describes how to analyse the salts obtained from seawater.

The gas that comes out of both seawater and tap water is air (with a higher percentage of oxygen than normal air, as oxygen is more soluble than nitrogen). The best test available would be to show that a glowing splint continues to glow and does not immediately go out when placed in the gas.

The presence of dissolved oxygen in water is, of course, vital for fish to survive.

Further information

• Kidzone includes some elementary material on the water cycle.
• Lenntech provides an explanation of why oxygen dissolved in water is important.