Calcium Carbonate (CaCO₃) is a precipitate of bicarbonate. Bicarbonate is the critical component in the measurement of temporary water hardness (carbonate hardness). This part of water hardness is defined via the titration of all carbonates, bicarbonates, and hydroxides and is expressed in equivalent amounts of calcium carbonate. This measurement can sometimes be referred to as the alkalinity of water. A high carbonate level in brewing water is generally considered to be negative as it can impart a harsh bitterness in beer, especially when combined with assertive hopping rates. Therefore, removal or reduction is typically desired and can be accomplished through heating or by the addition of acid to water. Via these treatments bicarbonate can be converted to calcium carbonate, a substance that is non-soluble and will precipitate out, reducing overall carbonate levels. However, though calcium carbonate is often considered undesirable, when making dark beers the presence of carbonates can offset the acidity of dark malts. In these situations a higher carbonate level is considered to be a positive attribute of brewing water. Calcium carbonate is formed by the solvent action of carbon dioxide present in rain or surface water, which then reacts with minerals present in the earth such as calcite or dolomite. Munich, Dublin, and London are examples of classic water sources that exhibit high calcium carbonate levels. Porters and stout are both beer styles that tend to benefit from higher calcium carbonate levels in brewing water.