Rainwater becomes a solution of bicarbonate and carbonate when it dissolves carbon dioxide ((CO_{2})) from the atmosphere to form carbonic acid ((H_{2}CO_{3})), which then dissociates into bicarbonate ((HCO_{3}^{-})) and carbonate ((CO_{3}^{2-})) ions. Carbonate and bicarbonate are important components of the Earth's carbon cycle and determine water's pH and hardness, with bicarbonate being dominant at a neutral pH and carbonate dominating at high pH levels. How rainwater forms bicarbonate and carbonate Dissolving (CO_{2}): When rainwater falls, it absorbs carbon dioxide from the atmosphere.Forming carbonic acid: This dissolved (CO_{2}) reacts with water to create unstable carbonic acid ((H_{2}CO_{3})).Dissociating into ions: The carbonic acid then releases hydrogen ions ((H^{+})), forming bicarbonate ions ((HCO_{3}^{-})).Further dissociation: If the solution becomes more alkaline (higher pH), the bicarbonate can lose another hydrogen ion to form carbonate ions ((CO_{3}^{2-})). The role of water's chemistry pH and ion concentration: At a pH of about (6.3), the concentration of dissolved (CO_{2}) and bicarbonate are equal. As pH increases, bicarbonate becomes the dominant ion, and at a pH above (10.3), the carbonate ion becomes dominant.Buffering capacity: Because of this reaction, rainwater naturally contains bicarbonate and acts as a buffer, keeping most natural water bodies within a pH range of (6) to (9).Mineral dissolution: Rainwater's acidity also allows it to dissolve minerals like calcium carbonate from rocks, which increases the concentration of both calcium and bicarbonate ions in the water, leading to hard water.

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Wood ash contains a high concentration of calcium carbonate, along with potassium carbonate and magnesium carbonate, which are responsible for its strong alkaline properties and ability to neutralize acids (raise pH).

Fresh ash may contain highly reactive calcium oxide (CaO) and magnesium oxide (MgO), which react with water to form hydroxides (like Ca(OH)₂) or with carbon dioxide to form carbonates, all of which are basic.

Typical mineral content by weight includes:

Calcium (Ca): 7–33%Potassium (K): 3–10%Magnesium (Mg): 1–2%Phosphorus (P): 0.3–1.4%