Litmus paper is a straightforward tool for distinguishing between acidic and basic substances. This simple indicator, a strip of paper treated with a natural dye from lichens, reacts to changes in a substance’s acidity or alkalinity. Dating back to 1300 AD, litmus paper has been a widely used method for chemical analysis. Understanding how to use and interpret its color changes provides fundamental comprehension of chemical properties.
Understanding Litmus Paper Color Changes
Litmus paper contains a blend of natural dyes from lichens, sensitive to the concentration of hydrogen ions. These dyes undergo a structural change when exposed to different pH levels, leading to a visible color shift. When blue litmus paper encounters an acidic substance, it turns red. Conversely, red litmus paper changes to blue when immersed in a basic, or alkaline, solution.
Some litmus paper begins as a neutral purple color, and this type will turn red in the presence of an acid and blue when exposed to a base. If blue litmus paper remains blue, the substance is either basic or neutral. If red litmus paper stays red, the substance is acidic or neutral. The color transition typically occurs within a pH range of approximately 4.5 to 8.3.
Using Litmus Paper Correctly
To accurately test liquids, begin by pouring a small sample into a clean container. Hold one end of a litmus paper strip and briefly dip the other end into the liquid. The color change occurs almost immediately upon contact, so prolonged submersion is unnecessary. For solid substances, first dissolve them in distilled water to create a solution.
When testing gases, litmus paper must first be dampened with distilled water. Once moistened, the paper can be exposed to the gas, allowing the gas to dissolve into the water on the paper and react with the dye. Unlike liquids, gases tend to react more uniformly across the entire surface of the paper. Always use a fresh strip of litmus paper for each new test. Avoid touching the reactive part of the paper with your fingers, as oils or residues could interfere with the outcome.
What Litmus Paper Cannot Tell You
While litmus paper is a convenient indicator, it has certain limitations. It offers a qualitative result, meaning it indicates only if a substance is acidic, basic, or neutral. It does not provide a precise numerical pH value. For specific pH readings, tools like pH meters or universal indicator strips are necessary.
Highly colored solutions can obscure the litmus paper’s color change, making accurate interpretation difficult. Very weak acids or bases may produce unclear reactions, leading to ambiguous results. Litmus paper can also react to certain chemicals, such as chlorine gas, by bleaching rather than indicating pH.