Acid changes the colour of blue litmus to __________ .

red

The correct answer is option 2. red.

Litmus is a natural dye extracted from lichens. It is used as a pH indicator in chemistry. Litmus paper comes in two colors: red litmus paper and blue litmus paper.

Blue Litmus Paper: Blue litmus paper is blue in its natural state. It turns red under acidic conditions.

When blue litmus paper is exposed to an acidic solution, the hydrogen ions (H⁺) from the acid react with the blue litmus, causing a change in color. The specific reaction involves the litmus dye molecule undergoing a structural change when it interacts with acidic conditions, resulting in a shift in its absorption spectrum and a visible change from blue to red.

So, if an acid is applied to blue litmus paper, it will turn red, indicating the presence of an acidic solution. This color change is a simple and effective way to test for the acidity of a solution.