Litmus paper

Litmus paper was paper used in chemistry experiments. Inactive chemicals caused the paper to turn from red to blue.

Ian Chesterton used litmus paper in his science class at Coal Hill School to teach his students about chemical changes. However, the lesson failed to interest Susan Foreman, who found it too obvious. (TV: "An Unearthly Child")

A miniaturised Ian Chesterton and Barbara Wright sat on a book of litmus papers while in Smithers' laboratory. (TV: Planet of Giants)