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Acidity (pH) is a measure of the activity of hydrogen ions in an aqueous solution (solution in which the solvent is water). It is also termed basicity: the same number quantifies either of the opposing qualities (in the manner that a single number quantifies both "length" or "shortness"). Acidity is roughly the negative of the base-10 logarithm of the molar concentration (moles per liter) of positive hydrogen ions. A pH of less than 7 is acidic and more than 7 is basic. pH is popularly thought to range between 0 and 14, but this range merely reflects typical values: actually there are no limits.
Carbon dioxide in water lowers its pH by reacting with it to form carbonic acid which can break down into bicarbonate or carbonate, losing protons, thus decreasing pH.