The question of whether “lime” is a base represents a common confusion rooted in language, as the single word describes two fundamentally different substances. The chemical answer depends entirely on context, as the term applies both to a highly acidic culinary fruit and a series of highly alkaline chemical compounds. One substance is a biological product used to add a tart flavor to foods and beverages, while the other is an industrial material derived from rock and used to neutralize acidity in soil and water. Understanding the chemistry of both types of lime is necessary to resolve this ambiguity.
Defining the Two Types of Lime
The term “lime” refers to two distinct categories: a botanical fruit and a chemical compound derived from limestone. Chemical lime, used in construction and agriculture, begins as calcium carbonate (\(\text{CaCO}_3\)), or limestone. Heating this rock to high temperatures in a process called calcination yields quicklime, or calcium oxide (\(\text{CaO}\)).
A subsequent reaction with water produces slaked lime, or calcium hydroxide (\(\text{Ca(OH)}_2\)), and both compounds are strongly alkaline bases. The other substance is the small, green citrus fruit, defined by its characteristic sour taste. This taste results from a high concentration of organic acids, primarily citric acid.
The Chemical Basis of Alkalinity
Quicklime and slaked lime are characterized as bases because of their behavior in an aqueous solution. A substance is considered a base if it dissociates in water to increase the concentration of hydroxide ions (\(\text{OH}^-\)). Quicklime (\(\text{CaO}\)) reacts vigorously with water to form the more stable slaked lime (\(\text{Ca(OH)}_2\)). When dissolved, this compound releases hydroxide ions into the solution.
The strength of this alkalinity is measured on the \(\text{pH}\) scale, which ranges from 0 to 14. A neutral solution, like pure water, has a \(\text{pH}\) of 7, while bases occupy the range from 7 to 14. Since slaked lime is a strong base, its saturated aqueous solution, often called limewater, exhibits a \(\text{pH}\) value near the upper end of the scale. This property makes it an effective neutralizer for acids, as the released hydroxide ions readily react with and consume hydrogen ions (\(\text{H}^+\)).
Why Citrus Lime is Acidic
In stark contrast to its chemical namesake, the culinary lime fruit is acidic due to its high concentration of organic acids. The dominant acid found in the fruit is citric acid, which gives the juice its sharp, tart flavor. Chemically, a substance is classified as an acid if it releases hydrogen ions (\(\text{H}^+\)) into a solution.
The concentration of these hydrogen ions determines the \(\text{pH}\) level of the fruit’s juice, placing it on the acidic end of the scale. Fresh lime juice typically has a \(\text{pH}\) that ranges between 2.0 and 2.8. This low \(\text{pH}\) value indicates a high presence of hydrogen ions. The inherent acidity of the fruit is why it is employed in cooking and mixology, where its sourness is used to balance flavors and act as a natural preservative.
Practical Uses of Alkaline Lime
The alkaline nature of chemical lime, particularly calcium oxide and calcium hydroxide, is harnessed across a wide range of industrial and environmental applications. One primary use is in agriculture, where it is often referred to as agricultural lime. Farmers apply various forms of lime to neutralize acidic soil, a process called liming. This action raises the soil’s \(\text{pH}\) level, which improves the availability and uptake of essential plant nutrients like phosphorus and nitrogen.
Environmental management also relies on the basic properties of lime for wastewater treatment. Slaked lime is introduced into acidic industrial wastewater to neutralize contaminants and precipitate heavy metals, making the water safer for discharge. The construction industry uses lime in the production of mortar and cement. When slaked lime is mixed with sand and water, it forms a workable paste that slowly hardens as it reacts with carbon dioxide in the air.