Experiments on How to Separate Salt From Water

Various techniques can be used to separate water and salt.

Separating salt from water is one of the most fundamental experiments taught in chemistry. Through this experiment important differences in the way substances are combined can be taught, such as the distinction between soluble and insoluble and between solutions and mixtures. This experiment also shows how important industrial processes, such as distillation and desalinization, are carried out.

The Basic Evaporation Procedure

The key lesson that needs to be taught is that salt can be dissolved into water but will not evaporate. Weigh 10 measuring beakers and 10 grams of salt. Pour 500ml of warm water into a measuring jug and stir in the salt. Pour an equal amount of the solution into the 10 measuring beakers and boil over a flame until all the solution has evaporated. A white crust of salt should be left over on the bottom of each beaker. Weigh the beakers with the crust and subtract the weight of the beaker to work out how much salt has been left behind. Add up the total weight of salt. It should be 10 grams.

Distilling

Repeat the basic version of this experiment, but put the emphasis on collecting and weighing the water instead of the salt. It is better to boil the solution in a conical or round-bottomed flask than in a measuring beaker for this experiment. You will need a glass tube that fits into the the top of the flask. This is known as a fractioning column. Adjoining the column is another glass tube surrounded by a glass "jacket" through which cold water runs. This is a condenser. When the water is boiled, it flows up through the fractioning column and condenses in the condenser to trickle out as pure water. Collect this water in a measuring beaker and compare the amount collected to the amount that you started with.

Dissolving and Mixing

Repeat the basic version of the experiment, but this time mix in an additional 10 grams of very fine sand. Before pouring the water into the measuring jugs, pour it through a funnel with a folded disk of filter paper inside. Weigh the paper before starting. Allow the filter paper to dry entirely and weigh again with the sand still trapped inside it. The weight of the salt deposit and the weight of the filtered out sand should be the same as they were before the experiment.

Solar Distillation

Solar distillation offers one of the only ways in which salt can be separated from water in an environmentally friendly manner. Build a rectangular wooden box with one of the long sides higher than the other, creating a slanting opening in the top. Take a sheet of glass, the same size as the opening, and run a line of silicone sealant along the surface of the glass a couple of inches from one of the long edges. Cut a hole in one of the short sides of the box, adjacent to the low side of the box, and slide a length of guttering through the hole and all the way up the long side of the box. Fix it in place. Place the box in the sun. Place a trough of salty water in the box against the higher wall and place the sheet of glass over the top with the strip of sealant above the guttering. Place a collecting container at the end of the guttering. Leave for a few hours and taste the water collected in the container. It should be completely pure.

References

• Practical Chemistry: Separating Sand and Salt