Swimmers often say that sea water gives them more buoyancy than fresh water. In the Dead Sea, the buoyancy is so great that bathers can lean back and read a newspaper. The important factor is the amount of salt dissolved in the water. Salty water is more dense than fresh water. To demonstrate this, ask pupils to mix up some very salty water. The solubility of common salt does not vary much with temperature, so heating the water is of little help. Next, weigh equal volumes of fresh water and salty water and compare the values. If pupils are familiar with the idea of density, they can calculate the density of each sample. Pupils can recover and weigh the dissolved salt by leaving samples in a dish in a warm place until they evaporate. Ordinary hens' eggs sink in fresh water - boiling an egg for tea would otherwise be much more difficult. By applying their knowledge of the density of salt water, it is possible to create the illusion of a floating egg. The egg can be suspended in water at any depth chosen by the pupils. The trick is to prepare a salt water solution whose density exceeds that of the egg. Trial and error will soon produce a suitable concentration. Pupils can pour different depths of this solution into several tall narrow transparent containers. They need to add an egg and check that it floats. The hard bit is to add fresh water on top carefully enough to avoid it mixing with the denser salt solution below. By varying the proportions of salt water to fresh water, the egg can be made to float at different levels. Once set up, the water looks the same throughout the container.
For those whose pupils would be more interested in smashing the eggs, there is an alternative. Use a small sealable container, such as a film cassette case, partly filled with sand. By altering the amount of sand, the container can be made to float just like the egg. Adding dye to one water layer improves the effect.
Ray Oliver teaches science at St Albans Girls' School, Hertfordshire