What Is a Chemical Indicator?
A chemical indicator is a substance that undergoes a distinct, observable change, usually a color change, in the presence of specific chemical conditions. This visible change allows scientists to visually determine the state of a chemical system, such as identifying whether a solution is acidic or basic, without the need for complex instruments.
Section 2: How Do Indicators Work?
The color change in most indicators is due to a reversible chemical reaction. pH indicators, for example, are typically weak acids or bases. They exist in a chemical equilibrium between two forms that have different colors. When the pH of the solution changes, the equilibrium shifts to favor either the acidic form or the basic form, causing the dominant color to change accordingly.
Section 3: A Practical Example
A classic example is litmus, which is often used on paper strips. Litmus is a natural dye that turns red when dipped into an acidic solution (pH below 7) and blue when placed in a basic or alkaline solution (pH above 7). If the paper does not change color, the solution is neutral. This provides a simple and immediate test for the general acidity of a substance.
Section 4: Importance and Applications
Chemical indicators are essential in many laboratory procedures, most notably in titrations. During an acid-base titration, an indicator like phenolphthalein is added to a solution of unknown concentration. A solution of known concentration is then added until the indicator abruptly changes color, signaling that the reaction has reached its endpoint. This allows for the precise calculation of the unknown concentration.