Water might be the most varied chemical compound on the planet. Plain old H2O is the same anywhere, of course. But brewing with pure water is not only expensive, but leads to poor beer - flat, stale and downright incorrect. That's
because the dozen or more elements or compounds found in natural sources contribute greatly to the final product.
Two of the major elements are the minerals calcium (Ca) and magnesium (Mg). These two add the 'hardness' in hard water. Undesirable when they cause your glasses to spot in the dishwasher, they're indispensable when making a fine beer. Not only do they add a desirable mouthfeel of their own, but they aid many of the bio-chemical processes taking place during brewing.
Calcium, for example, helps
produce an acid that balances the alkaline phosphates found in malts.
Control of that pH (a measure of acidity/alkalinity) is vital for the
activity of enzymes that take part in the brewing process.
Magnesium is essential because
it's used by yeast in the production of enzymes required for
fermentation. But, as luck would have it, magnesium can compete with calcium and so
its concentration has to be carefully controlled for proper results.
Also, above about 20 mgs/l (milligrams per liter), it can make the beer bitter or sour.
Not all naturally occurring or artificially added components are desirable for brewing beer...at least not good beer. Chlorine (Cl),
for example helps keep bacteria from building up in commercial water
supplies. But it adds a bitter taste to beer and can contribute to killing yeast. Fortunately, it's volatile and easily removed by boiling or
Sodium (Na) contributes a salty taste (it's half of table salt, Sodium Chloride or NaCl), but at a too high
concentration it can kill yeast. Most natural sources contain a reasonable amount, but control of salinity at sites near a sea-river
conjunction is important.
Even trace elements, such as Zinc (Zn) and Copper (Cu) play an important role in many brewing
processes, since they figure prominently in yeast metabolism. It's the yeast that turns malt sugar into alcohol and carbon dioxide during
fermentation. High levels can contribute to haze, though.
More complex compounds play a part, as well.
Sulfates, SO4, give a dry, sharp flavor and can compliment hops. British ales make frequent
use of this feature. But in too high concentration it can be excessively bitter.
Carbonates, CO3, promote the extraction of tannins from hops and grains. Barley is a grain and
goes into making malt sugar, used in fermentation. They help promote darker colors in some beers and provide alkalinity to balance the acids.
Levels of these elements and compounds vary naturally throughout the world.
Pilsen, for example, has very soft water and produces a very mild lager in most cases. The
lagers from Munich, by contrast, are delightful in part because of the hard water used by breu meisters there for centuries. Dortmund, home to
a famous lager style, has very hard water with high levels of most minerals found in water.
Ales from Burton-on-Trent in England similarly benefit from the hard water in that locale. The
high carbonate levels in Dublin, where some excellent stouts are produced, require balancing with acidic dark malts.
There are over 800
compounds in beer (some studies suggest over 1,000 and the list is
growing with the growth in popularity of microbrews), but a dozen or so found in water are significant factors in
the final product. Not to mention the H2O itself. Beer is 90% water.
Learn Bartending at Home
Review of Beer Brewing Made Easy
The Country and London Brewer - 1736: Of the Nature of several Waters and their use in Brewing. And first of Well-waters.