Chemist’s pure water which is formulated as H2O exists only in the theory. In the laboratory too, it has to be prepared from the water from the tap. The reason isits high polarity and very dielectric constant which makes it is a potent solvent. It can dissolve ionic inorganic salts, polar organic compounds, acidic, basic and polar gases. It also carries suspended matter like clay particles which are in the form of colloids. Some of these are loosely suspended and separate out with time by sedimentation and other are stable and has to be destabilized by using defloucculants to settle them out. For example, the river water continuously interacts with the rocks and soil during its flow and leaches many inorganic salts of alkali metals, alkaline earth metals in appreciable amounts and many other metal ions in trace amounts. It can absorb the lower molecular weight organic acids like formic acid and acetic acids. Gases are trapped in two ways by water. First category are gases which are acidic like carbondioxide and oxides of nitrogen, sulphur dioxide etc reacts with water and render it acidic and basic like ammonia react with it to form basic solutions. Another way by which water traps the molecules of non polar gases like oxygen, methane etc is that there are cavities in the grid of water molecules where these gases are trapped through weak Van der Wall forces.
Not only that, when water freezes, in the lattice it creates, gaseous hydrocarbons are trapped if certain conditions like availability of these gases at the time of freezing, pressure, temperature. If the conditions fall into the favorable envelope, gases are trapped. These are called gas hydrates. In fact, it has been estimated that vast amounts of methane is trapped world over in this form. Technology is being developed to tap the resource in a safe, environmental friendly and economically feasible way.
Chemist has to remove all these impurities to achieve a tentative pure water. The degree of purification depends upon the kind of experiment. For example, for the ion chromatography where presence of ions is determined at ppb or ppm level, the water used in this work has to be free from the ions under determination in a sample. This water is prepared with sophisticated water purifier called ultra purifier which removes suspended matter, kills bacteria and removes every ion present to negligible amounts. For ordinary work on a gross level determations, onetime distilled water will do the work.
This also does not last long as it is continuously absorb the gases from the environment if it is not sealed. If your laboratory is located near a highway with high volume of vehicles then due to the emission of the acidic nitrogen oxides, this water immediately turns acidic. Thus even during the conducting of experiment, special care using isolating techniques has to be taken if the correct results are to be achieved. In the analytical work, the procedures specially mention the freshly distilled water to be used.
So whenever we speak of water, it is not the pure H2O but a mixture of different salts and gases dissolved in the chemistry water. Some of these ions are required for human bodies but these should be within limits.
So the chemical composition of water is very important to know it’s potability for drinking purposes. Generally, in most of the cases treatments are necessary to make it fit for drinking.