Dissolved substances in tap water and seawater

In this experiment, learners 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 will 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, learners 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 leaners are waiting for their water to evaporate. If several of the shortcuts suggested are used, these experiments could be done in 45 minutes. 

Learning objectives 

1. Set up and carry out an evaporation practical.
2. Compare the dissolved substances in tap water and seawater.
3. Describe what happens to the particles of a substance when it dissolves.
4. Explain why it’s important for water to contain dissolved substances. 

Equipment

Apparatus

• Eye protection: safety glasses to EN166F

For the teacher demonstration

• Round bottomed flask (250 cm3),x 2 
• Bung and delivery tube, x 2 
• Bunsen burner Tripod and gauze, x 2 
• Heat resistant mat, x 2 Stand and clamp, x 2
•  Beaker (250 cm3) or a small trough, x 2 
• Test tube, x 2 

For the class practical

• Glass watch glasses (approximately 7.5 cm diameter), x 3
• Beaker (100 cm3) 
• Bunsen burner 
• Tripod and gauze 
• Heat resistant mat 
• Tongs 

Chemicals

For the teacher demonstration

• Seawater, 400 cm³ (see note 3 below)

For the class practical

• Sewater (5 cm³)
• Distilled (or deionised) water (5 cm³)

Preparation

If the real thing isn’t 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.) 

Safety and hazards

Learners should be able to use a Bunsen burner with confidence and they should be reminded how to handle hot liquids in beakers. 

Allow enough time for the equipment to cool down before asking the students to put away their equipment. In case of a burn,  cool the burn by immediately irrigating with gently running water for at least 20 minutes and until pain is relieved and heat is no longer felt. 

The solid residue on the watch glass can be disposed of in the general waste bin. Remind learners never to eat or drink anything in the lab as they may be tempted to taste the salt. 

Method

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 use tongs to remove the watch glass (Do not touch the watch glass 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

Teacher demonstration

1. Fill a round bottomed flask right up to the top with tap water. Insert a bung carrying a delivery tube so that the tube itself becomes completely filled with water (see diagram). If this cannot easily be done, place the bung in the flask while the whole apparatus is immersed in a sink of water. 
2. Fill a collection test tube with water and place inverted into a beaker of water. Position the other end of the delivery tube under the test tube (see diagram). 
3. Repeat step 1 and 2 with a flask of seawater. 
4. Position each flask over a Bunsen burner using a clamp and stand. Heat each flask until bubbles of gas are 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.
5. Conduct a splint test to demonstrate the presence of oxygen.

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: rsc.li/4oojpdk  

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 is 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 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.