Litmus paper provides a simple, rapid, and cost-effective method for distinguishing between acidic and basic substances. This fundamental chemical indicator allows for a quick classification of an unknown substance based on its hydrogen ion concentration. Understanding how to correctly use this treated paper and interpret its color changes is a basic skill for working with chemistry, providing a clear, qualitative answer about a substance’s acidity or alkalinity.
Understanding the Litmus Test
Litmus paper is absorbent paper treated with a natural, water-soluble dye extracted from lichens. The dye is a complex mixture of compounds sensitive to changes in the concentration of hydrogen ions (\(\text{H}^+\)) in a solution, which is the basis for its function as a general acid-base indicator.
The paper is commercially available in two types: red litmus and blue litmus. Red litmus paper contains the dye in its acidic form, while blue litmus paper contains the dye in its basic, or alkaline, form. The active component is a weak acid that undergoes a reversible chemical reaction. Exposure to a base causes the dye to react and shift its structure, resulting in the color change.
Performing the Litmus Test
Testing an unknown substance requires the use of both red and blue strips to obtain a conclusive result. For testing a liquid, use a clean stirring rod to transfer a single drop of the solution onto a small piece of the litmus paper. This method prevents the contamination of the entire chemical sample by the litmus dye.
Do not fully submerge the litmus paper into the bulk solution, as the dye may bleed into and contaminate the liquid. If the substance is a solid, a small amount should first be dissolved in distilled water. Alternatively, the litmus paper can be slightly moistened with distilled water before touching the solid. Using distilled water is necessary because tap water may interfere with the test’s accuracy.
Decoding the Color Changes
The observation of color change provides a direct indication of the substance’s nature.
Acidic Result
An acidic result is confirmed when the blue litmus paper turns red, but the red litmus paper shows no color change. The presence of excess hydrogen ions causes the blue, basic form of the dye to shift its structure to the red, acidic form.
Basic Result
A basic or alkaline result is confirmed when the red litmus paper turns blue, and the blue litmus paper remains blue. The hydroxide ions (\(\text{OH}^-\)) in the basic solution react with the red litmus dye, leading to a structural change that results in the blue color.
Neutral Result
If a substance is neutral, neither the red nor the blue litmus paper will change color, remaining red and blue, respectively. Litmus paper is a general indicator, confirming if a substance is below a \(\text{pH}\) of \(4.5\) (red) or above a \(\text{pH}\) of \(8.3\) (blue), but it does not provide a specific \(\text{pH}\) value.