How Litmus Paper Works

Introduction

Most high school students are introduced to litmus paper in their general science or biology class. It is a small, red or blue strip of paper. Litmus has been treated with a fungi that sensitizes the paper to the acidic or basic qualities of a substance. By dipping the litmus paper into a sample, students can get a very rough estimation as to whether a substance is an acid or a base.

Red or Blue

The natural shade of litmus paper is blue, originating from the algae/fungi that is comes from. Red litmus paper is made by infusing sulfuric acid into the mix and letting it dry. Blue litmus will turn red in the presence of an acid, whereas red litmus will turn blue when in contact with a base. Neutral substances such as water have no effect on the paper at all. Purple litmus has recently been introduced; this variety turns red with the presence of acids and blue when there is a base present.

Bleaching

Although most substances turn litmus paper blue or red, depending on their acidic content, there are certain chemicals that will turn litmus paper white. Chlorine is the most common of these. It will turn red litmus paper white upon contact. It turns blue litmus paper red because it is an acid; however, students who do not pay close attention to the change may miss it, as chlorine then bleaches the paper white. Other bleaching agents include hydrogen peroxide and borax.

Resources

• Fun Science Gallery