Volume change on dissolving salt in water

Class practical

This is a thought-provoking experiment with a surprising result.

Apparatus and Materials

• Volumetric flask, one-litre
• Common salt

Health & Safety and Technical Notes

Read our standard health & safety guidance

When sodium chloride dissolves in water to make a saturated solution there is a 2.5 per cent reduction in volume. One would never notice that in a beaker. Even in an ordinary flask it would be barely perceptible. However, if a volumetric flask is available from the chemistry laboratory, the volume change will be noticeable in the narrow neck. It is essential to remove all air bubbles from the salt that is to be dissolved. Therefore, it must be thoroughly wetted at the start of the demonstration.

The solubility of salt does not change much with temperature, so there is little profit in using hot water.

The salt should be in small crystals and not in rocks or very fine powder.

Procedure

1. Place 300 to 400 g of salt in the flask.
2. Pour in enough water to cover the dry salt, and swirl the water around in the flask to wet the salt and let air bubbles float up to the top. (This will not be enough water to dissolve more than a little of the salt; students will still see a lot of salt crystals.)
3. As soon as the air bubbles seem to have gone, fill the flask to the mark with water.
4. Label the water level clearly, with an OHP pen or some other marker. Point out that most of the salt is still there, as a solid unable to dissolve.
5. Shake the flask to hurry the dissolving until as much salt as will dissolve has done so.
6. Point out the consequent small contraction. Ask students why they think this has happened.

Teaching Notes

This gives students a lesson in the need always to be on the lookout for unexpected results. It also provides students with an opportunity to use their imagination to think of possible explanations. Both of these are much more important than knowing the real reason for the effect: that the sodium and chlorine atoms in their crystalline array take up more room than when they are separated. Indeed, there is no harm in not giving the answer now but, instead, undertaking to return to the problem later.

This experiment was safety-tested in June 2004