Understanding Litmus Paper and Acidic Solutions

Litmus paper is a type of indicator used to test whether a solution is acidic or basic. It is made from lichens and is impregnated onto paper. There are two main types: red litmus paper and blue litmus paper. Each type changes color depending on the pH of the solution it contacts.

How Blue Litmus Paper Reacts to Acids

The question asks what color blue litmus paper turns on contact with an acidic solution. Acidic solutions have a pH less than 7. Litmus paper is specifically designed to show a clear color change when exposed to substances with different pH levels.

• Blue litmus paper is commonly used to detect acids.
• When blue litmus paper is dipped into or comes into contact with an acidic solution, it undergoes a color change.
• The blue color changes to red.

This color change indicates that the solution is acidic. The stronger the acid, the faster and more pronounced the color change typically is, though litmus paper primarily indicates whether a solution is acidic or basic, not the exact strength of the acid or base.

Comparing Reactions: Red vs. Blue Litmus Paper

To fully understand how litmus paper works, it is helpful to know how red litmus paper reacts as well:

• Red litmus paper: Used to detect bases. It turns blue when it comes into contact with a basic (alkaline) solution (pH > 7). It remains red in acidic or neutral solutions.
• Blue litmus paper: Used to detect acids. It turns red when it comes into contact with an acidic solution (pH < 7). It remains blue in basic or neutral solutions.

Neutral solutions (pH = 7) generally do not cause either red or blue litmus paper to change color significantly.

Based on this information, blue litmus paper turns red on contact with an acidic solution. The other options (yellow, brown, green) are not the characteristic color changes for litmus paper in acidic solutions.

Revision Table: Litmus Test Summary

Additional Information on Acid-Base Indicators

Litmus is just one example of an acid-base indicator. Many other substances also change color depending on the pH. These indicators are weak acids or weak bases themselves that have different colors in their ionized and non-ionized forms. The equilibrium between these forms shifts depending on the pH of the surrounding solution, causing the visible color change.

• Examples of other indicators: Phenolphthalein (colorless in acidic, pink in basic), Methyl Orange (red in acidic, yellow in basic).
• Universal Indicator: A mixture of different indicators that shows a range of colors across a wide pH scale, allowing for a more precise estimation of pH.
• pH Meter: An electronic device that provides a numerical reading of the pH, offering the most accurate measurement.

Understanding acid-base indicators like litmus paper is fundamental in chemistry for quickly identifying the nature of solutions.