What is it that turns blue litmus paper red? Litmus paper is a commonly used indicator in chemical experiments to determine whether a substance is acidic or basic. It comes in two forms - blue litmus paper and red litmus paper. While red litmus paper remains unchanged in the presence of both acidic and basic substances, blue litmus paper undergoes a fascinating transformation when exposed to an acidic solution. Acid-Base Indicators: Acid-base indicators are substances that change color depending on the pH of the solution they are in. They are usually weak acids or bases themselves and exhibit different colors in their protonated and deprotonated forms. Litmus paper is derived from a natural dye obtained from lichens and acts as a pH indicator due to the presence of a pigment known as litmus. The Role of Litmus in Acid-Base Reactions: When blue litmus paper encounters an acidic solution, it turns red. This color change occurs due to a chemical reaction between the litmus pigment and the hydrogen ions (H+) present in the acid. The litmus pigment is a weak acid itself and acts as an indicator by undergoing a reversible reaction. In the presence of acidic conditions, the litmus pigment donates a proton (H+) to the hydrogen ions in the solution, resulting in a change in its structure and color. The Acid-Base Reaction: The reaction between the litmus pigment and the acid can be represented as follows: Litmus pigment (blue) + Acid → Litmus pigment (red) The blue litmus paper contains the litmus pigment in its deprotonated (blue) form. However, when an acidic substance is added, the litmus pigment accepts a proton from the acid, causing it to become protonated and turn red. This change in color is a visual representation of the presence of an acidic solution. Other Applications of Litmus Paper: Apart from its use in identifying acidic solutions, litmus paper also finds applications in various other fields. It is commonly used in laboratories, schools, and industries to test the pH of liquids, such as water, soil, and even urine. The ease of use and immediate results make litmus paper a convenient choice for quick pH measurements. In conclusion, blue litmus paper turns red in the presence of an acidic solution due to a chemical reaction between the litmus pigment and hydrogen ions. This color change serves as a valuable indicator in determining the pH of a substance and finds extensive use in various scientific and industrial applications.

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