How do you get dissolved substances out of water?

Answer 1

This is a good question, because getting materials out of solution is a key step in many industrial processes and thus, to life as we know it. For example, we would not enjoy pure crystalline sugar, nor salt, nor cream of tartar, nor any number of other pure foods, industrial chemicals, or drugs, without methods for "breaking" solutions to remove whatever is dissolved within.

Since breaking solutions is so fundamental to our way of life, it is not surprising that many methods and procedures have been developed to accomplish the task -- some dating back to antiquity.

Here are a few:

(1) Solvent Removal by Vaporization: in this procedure, the solvent is removed, often by boiling it off, leaving the desired solute behind in the boiling vessel. This will work, for example, with both sugar- and salt-water. In the case of sugar-water, care must be taken not to get the temperature up too high, as this will destroy the sugar. With salt, the water can be boiled away as vigorously as desired.

(2) Solvent Concentration by Evaporation and Crystallization: in this procedure, solvent is removed gently, and at some point the solution becomes so concentrated that the solute begins to "crystallize out'. Sometimes the crystallization is stimulated or helped along through the introduction of seed crystals. This procedure is good for purifying sugar or making rock candy, and is also heavily used in thousands of industrial processes for chemical manufacturing.

(3) Salting Out: in this procedure, a solute having a greater affinity for the solvent than the product does is introduced into the solution. If enough of the new solute is added, the product will begin to crystallize, leaving the new solute and the solvent behind. Salting out is also widely used in industry.

(4) Reverse Osmosis: a solution can be forced through a membrane having tiny molecularly-sized pores. The pores are too small for the product to pass through, but large enough for the solvent to pass. This can be thought of as molecular filtration and is commonly used industrially.

(5) Solvent Extraction: in this procedure a solution is vigorously mixed with a new, different solvent for the product, but which happens to be immiscible with the original solvent. The resulting mixture is then allowed to separate into layers, one of which contains the product, while the other is now exhausted of the product and can be discarded or recycled.

(6) Preparative Chromatography: in this procedure, a solution is made to pass through a tube stuffed with material (called adsorbent) that has a greater attraction for the product than for the solvent and a different attraction for the contaminants. The product will move more slowly through the tube than the solvent, due to the attraction of the product by the absorbent. Eventually, with continued addition of solvent, the product will be flushed out at the bottom of the tube, in a highly purified state. Widely used.

There are other methods of getting products out of solution, and indeed, the subject is a very broad area of academic research and practical application. The area of concern, broadly speaking, is "separation technology", which is an application of the field of Chemical Engineering.

Answer 2

The main way to get dissolved substances out of water is by evaporation. Another way is to get a solvent that is soluble in water also, and change the polarity to precipitate out the solid, and then filter to regain the solid on the filter paper.