Change of volume: water to water vapour

Class practical

Apparatus and Materials

• Syringe kit (large glass syringe, rubber cap, hypodermic syringe with short needle)
• Tripods, gauzes and heat-resistant mats, 2
• Bunsen burners, 2
• Beakers, deep, 2
• Brine

Health & Safety and Technical Notes

Gloves, even just rubber washing-up gloves, will give some protection from the high temperature when extracting the large syringe from the hot water.

Read our standard health & safety guidance

The glass syringe has a rubber cap fitted over its end when the piston is pushed down to zero volume (as shown in the diagram). As it takes a long time to heat the syringe, it is recommended that it be heated before the arrival of the class by immersing the whole thing in a deep container of water brought nearly to boiling. The second large beaker, containing brine, should also be brought to boiling before the lesson begins.

It is essential that the large syringe be internally dry before use; the caps must be tight fitting, otherwise they will be blown off.

Salt water getting into the very narrow space between piston and barrel may cause the syringe to jam. A smear of silicone grease prevents this. Vaseline contains water droplets and is unsuitable.

Procedure

1. Partially fill the hypodermic syringe with water and then remove the large syringe from the hot water. If helpful, hold in a clamp attached to a retort stand.
2. Invert the hypodermic and eject any air in it.
3. Inject 0.1 ml of water through the rubber cap into the syringe. The cap seals on removal of the needle.
4. Immerse the large syringe in the boiling brine. The water will turn to steam and the volume change can be observed.
5. After all the water has turned to vapour, remove the syringe from the beaker. The syringe will cool down and the water vapour condenses back to water.

Teaching Notes

• Twisting the piston as the volume changes may be helpful, though students will doubtless call this cheating unless the decrease in volume on condensation is also shown.
• Precise results will not be obtained from this experiment. Its accuracy will show an order of magnitude: 0.1 ml of water becoming at least 100 ml of steam. (The recognised value is a change of 1 to 1650.)
• Everyday use of the word steam confuses students. Steamed up windows are in fact covered in water droplets; the water vapour condenses into water on the cold window so the steam is water not a gas (vapour).

This experiment was safety-tested in August 2006