Litmus paper is a simple yet effective chemical indicator, a small strip of absorbent paper treated with a natural, water-soluble dye extracted from lichens. Its primary function is to provide a quick visual test to determine whether a solution is acidic or alkaline. The paper’s color changes indicate the nature of the substance it contacts.
The Blue Indication
Red litmus paper turns blue when introduced to an alkaline solution. Alkaline substances (bases) have a pH greater than 7. These solutions contain a higher concentration of hydroxide ions (OH⁻), which are negatively charged molecules of oxygen and hydrogen. When an alkaline compound dissolves in water, it releases hydroxide ions, making the solution alkaline.
For example, ammonia gas turns red litmus paper blue because it forms hydroxide ions when dissolved in water. This increase in hydroxide ions shifts the chemical balance of the litmus dye, resulting in its characteristic blue appearance.
Understanding the pH Scale and Litmus Chemistry
The pH scale quantifies the acidity or alkalinity of an aqueous solution. It spans 0 to 14, with 7 representing a neutral state, such as pure water at 25 degrees Celsius. Values below 7 indicate increasing acidity, while values above 7 signify increasing alkalinity. Each whole number increment on the pH scale denotes a tenfold difference in the concentration of hydrogen ions within the solution.
Litmus paper functions due to litmus, a natural dye derived from lichens. This dye acts as a pH indicator, meaning its molecular structure undergoes a reversible chemical alteration in response to pH levels.
Red litmus paper contains a weak acid. When this acidic form encounters a basic environment, hydrogen ions from the dye react with the base. This generates a new chemical form, its conjugate base, which exhibits a blue color. The color transition for litmus typically occurs within a pH range of approximately 4.5 to 8.3.
Common Alkaline Substances
Many everyday substances turn red litmus paper blue due to their alkaline nature. Baking soda (sodium bicarbonate) is a common household item that is mildly alkaline (pH 8.3-8.4 when dissolved), its alkalinity stemming from bicarbonate ions producing hydroxide ions in water. Ammonia-based cleaning products are alkaline, as ammonia forms hydroxide ions when mixed with water. Soaps and detergents are generally alkaline, often containing compounds like sodium or potassium hydroxide. Antacids, like milk of magnesia (magnesium hydroxide), are alkaline substances designed to neutralize excess stomach acid.