An alkaline solution is a liquid mixture defined by its chemical properties, which stand opposite to those of an acid. A solution is considered alkaline, or basic, when it has a greater concentration of hydroxide ions (OH-) than hydrogen ions (H+). This chemical imbalance determines how the solution behaves, dictating its feel and reactivity. The compounds that form these solutions are often ionic salts of alkali or alkaline earth metals that dissolve readily in water.
Defining Alkalinity on the pH Scale
A solution’s alkalinity is quantitatively measured using the pH scale, which ranges from 0 to 14. Pure water, which is considered neutral, sits exactly in the middle at a pH of 7, indicating an equal balance of hydrogen and hydroxide ions. Any solution with a pH value greater than 7 is classified as alkaline, or basic.
The scale is logarithmic, meaning each whole number increase in pH represents a tenfold decrease in the hydrogen ion concentration, and a corresponding tenfold increase in alkalinity. Strong alkaline substances typically occupy the upper end of the scale, ranging from about pH 11 to 14.
The Role of Hydroxide Ions
The defining characteristic of an alkaline solution is its ability to produce hydroxide ions (OH-) when dissolved in water. These negatively charged ions are responsible for the chemical properties associated with alkalinity. An alkali, which is a water-soluble base, dissociates upon mixing with water, releasing OH- ions into the solvent. For instance, when sodium hydroxide (NaOH) dissolves, it separates into a sodium ion (Na+) and a hydroxide ion (OH-).
The OH- ions act as proton acceptors, reacting with hydrogen ions (H+) present in the solution to form water (H2O). This consumption of H+ ions causes the pH level to rise above 7. The higher the concentration of released hydroxide ions, the stronger the alkaline solution.
Physical and Chemical Properties
Alkaline solutions possess physical and chemical properties that differentiate them from neutral and acidic substances. A common physical trait is a slippery or soapy feel when touched, which occurs because the alkali reacts with the fatty acids and oils on the skin’s surface in a process called saponification. Concentrated alkaline solutions often have a corrosive nature, meaning they can cause chemical burns and damage organic tissue.
Chemically, they have the ability to neutralize acids in a reaction that yields water and a salt. This acid-neutralizing capacity is why alkaline substances are used in antacids and wastewater treatment. In the presence of a chemical indicator like litmus paper, an alkaline solution will cause the paper to change color from red to blue.
Common Alkaline Solutions
Many household and industrial products utilize alkaline solutions. Household ammonia, often used in cleaning products, is a moderately strong alkaline solution with a pH typically around 11. It is effective at breaking down organic materials like grease and stains.
A common, weaker alkaline substance is sodium bicarbonate, widely known as baking soda, which has a mild pH of approximately 8.3. This weak alkalinity allows it to safely neutralize mild acids, making it useful as a deodorizer and a leavening agent in baking. Conversely, sodium hydroxide, or lye, is an extremely strong alkali with a pH near 14, used in powerful drain cleaners and commercially in soap manufacturing.