Why do acids turn litmus paper red?

Acids turn litmus paper red due to a chemical reaction that occurs between the acid and the dye in the paper.

Litmus paper is a type of pH indicator, which changes colour in response to an acid or a base. The term 'pH' stands for 'potential of hydrogen', and it measures the concentration of hydrogen ions in a solution. Acids are substances that increase the concentration of hydrogen ions, while bases decrease this concentration.

The litmus paper contains a dye called litmus, which is extracted from certain lichens. This dye is a mixture of different substances, but the most important ones for our purposes are azolitmin and erythrolitmin. These substances are sensitive to pH and change colour when they come into contact with an acid or a base.

When an acid is applied to litmus paper, the increased concentration of hydrogen ions triggers a reaction with the azolitmin and erythrolitmin. This reaction causes the dye to change its structure, which in turn changes the way it absorbs and reflects light. As a result, the litmus paper turns red.

It's important to note that the exact shade of red can vary depending on the strength of the acid. A strong acid, which has a high concentration of hydrogen ions, will cause a more intense colour change than a weak acid. This is why litmus paper can be used not only to identify acids and bases, but also to give a rough indication of their strength.

In conclusion, the reason why acids turn litmus paper red is due to a chemical reaction between the acid and the dye in the paper. This reaction changes the structure of the dye, altering its colour to red.