The compound with the formula CO₂. An odorless gas, carbon dioxide (CO₂) is widely distributed in nature and is a minor component of air. It is highly soluble in water and oil, especially under pressure. In water, it occurs as carbonic acid, a weak acid that can donate one or two hydrogen ions in neutralization reactions that produce bicarbonate (HCO₃^-) and carbonate (CO₃^-2) salts or ions. CO₂, being an acid in water, reacts instantly with NaOH or KOH in an alkaline water mud, forming carbonate and bicarbonate ions. Similarly, it reacts with Ca(OH)₂ (lime) to form insoluble calcium carbonate and water.