A common liquid solution that becomes red in an acid is litmus paper.

Here's why:

* Litmus paper is a type of pH indicator. It's made from a natural dye extracted from lichens.

* In a neutral solution, litmus paper is purple.

* In an acidic solution, litmus paper turns red.

* In a basic (alkaline) solution, litmus paper turns blue.

Therefore, the color change from purple to red is a clear indication that the solution has become acidic.