The grape and the grain may live side-by-side, but few think of the most common link between the two, that of water. The Bavarian Beer Purity Law, the Reinheitsgebot of 1516, names three ingredients essential to beer, with yeast as the unknown magic that brought the goodness of God into the brewer’s cask. Of those three ingredients – barley, hops, and water – the latter one is the least contemplated by most people consuming the beverage. In fact, craft beer drinkers commonly poke fun at mainstream beer-makers for producing Light Lagers, saying that they taste like nothing more than over-priced water.

Water is essential to all things living on the planet. According to Wikipedia, “Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in ice and groundwater.” We, as human beings, have developed a highly effective olfactory system, which can not only smell such dangers as fire and toxic gas, but also such things as the potability of water, necessary to keep us healthy and free of illness.

Brewers have always focused on water, although it is doubtful they understood the dramatic effect it had in the creation of specific styles. The low sulfate, low carbonate water of Pilzn, the birthplace of the Pilsner style of beer, gave a softness to the mouthfeel that rounded out the malts and accentuated the profile of Saaz hops.

The water at Burton-on-Trent had a complex combination of ion flavors from dissolved salts in the local water. This imparted a crisp edge and complexity that enhanced these hoppy Ales. In Russian Imperial Stouts, alkaline water tends to balance the acidic profile inherent in roasted grains within the grist.

Since beer is usually 85-95 percent water, it is the most important foundation for any brewer to build upon. Pure water, Dihydrogen monoxide or H2O, is odorless and tasteless. Purity means that the water is free of toxins, pollutants and microbes, but not free of naturally occurring minerals. Minerals in spring and mineral water give it interest and flavor when brewing.

Deionized or distilled water is not a good choice for brewing. Ions in water are necessary for making beer, and water that has been deionized or distilled lacks precious ions that aid fermentation. You also need to be sure your water is not sodium-free.

Off flavors in water will show-up in your beer, so it is often beneficial to have an analysis done of your tap water unless you already have access to a detailed report from your local water company. Chlorophenols are common in city water, so using bottled water is a good alternative. For consistency, choose a brand of bottled water you like, and stay with it. Brewing is both a science and an art, so the scientist side needs to have a control factor that remains constant.

Electrical charges are important because "like dissolves like". Water has partial positive charges at each of the hydrogen atoms and partial negative charges at the oxygen atom. Molecules with positive charges dissolve easily in water, but those with negative charges, such as hop oils, are called hydrophobic and need to be modified in order to work. This process of modifying the molecules is called isomerization, and occurs in the boil.

For those that have difficulty being modified, ions step in to help. Not only do ions help dissolve some parts of beer, but they also add specific tastes that give special character to beer. Water used in Burtonized beer carries calcium, sulfates, magnesium, sodium, and chloride. Ions also help yeast convert the sugars in beer to ethanol, or alcohol.

Some of these ions are:

Calcium – Required by yeast at proper levels. If levels used are too high, it can remove essential yeast nutrients and cause hazy beer.

Carbonate – Used in high levels, it removes over-the-top bitter hops flavors.

Chloride – Increases bitterness, stabilizes the beer, and helps to clear it. If levels are too high, it can inhibit yeast flocculation.

Magnesium – Essential for yeast metabolism. Used excessively, it imparts a strong bitter flavor.

Nitrite – Toxic to yeast.

Potassium – Gives beer a salty taste. If used excessively, it can inhibit yeast metabolism.

Silicate – Can cause hard scaling on brewing equipment.

Sodium – Used at normal levels, it increases the flavor in beer. If used with sulfate, it imparts harshness.

Sulfate – Imparts a dry, sharp, full flavor to beer.

Tin – Causes haze and metallic flavors.

Zinc – Although it is an essential yeast nutrient, it is poisonous to yeast at high levels.

Cheers!

A great and understandable guide for brewers and would-be brewers:
Brew Chem 101: The Basics of Homebrewing Chemistry

Essential understanding about brewing and how it works:
Principles of Brewing Science: A Study of Serious Brewing Issues