Video: Giving the Color of Litmus Paper in a Basic Medium

What is the color of litmus paper in a basic medium? [A] Blue [B] Red [C] Pink/purple [D] Colorless [E] Yellow

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Video Transcript

What is the color of litmus paper in a basic medium? A) blue, B) red, C) pink slash purple, D) colorless, or E) yellow.

Litmus paper is a very useful tool that usually comes in two varieties. Blue litmus paper will turn pink when dipped into an acidic solution, while red litmus paper will turn blue when dipped into a basic one. The question asks, what is the color of litmus paper in a basic medium?

A medium is a phase or state or a mixture. Typically, when we say a basic medium, we mean a liquid which has a pH of greater than seven. A good example is a solution of sodium hydroxide. We’ve already looked at the two varieties of litmus paper and seen that when a piece of red litmus paper is put into a basic medium, it will turn blue. But what about a bit of blue litmus paper?

The key thing we need to understand here is that, in blue litmus paper and red litmus paper, the exact same dyes are used. It’s just that the basic form of these dyes is introduced to the paper to make blue litmus paper. And it’s the acidic forms of these dyes that make the red litmus paper red. So if we were to use a piece of blue litmus paper in a basic solution, it would just stay blue. So the color of litmus paper, regardless of whether it’s blue or red to start with, will be blue in a basic medium.

Litmus is actually a mixture of many dyes extracted from lichen. These dyes share a common chemical group, which means that below pH 4.5 they’re all red and above pH 8.3 they’re all blue. In between, it’s a bit of a mixture. The litmus test is usually better for strongly acidic solutions or strongly basic solutions. In between, you can get a bit of a mixed color. However, we can broadly say that when litmus is put into a basic medium, it’s going to be more blue than red. So our answer is blue.

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