Using Blue and Red Litmus Paper

Introduction

Litmus paper is simple, but it is also easy to use badly. Dipping a whole strip into a bottle wastes the sample and can contaminate it; testing with dirty equipment can give a false result. This page explains how to use red and blue litmus paper properly and how to read the colour changes so that you can classify a solution with confidence.

Core explanation

The two key rules. Remember just two changes:

- Blue litmus turns red in an acid . - Red litmus turns blue in an alkali .

Everything else follows from these rules.

The results table.

Solution Blue litmus Red litmus --- --- --- Acidic Turns red Stays red Alkaline Stays blue Turns blue Neutral Stays blue Stays red

Notice that in each row only one paper can possibly change. An acid cannot change red paper, because red paper is already the acid colour. An alkali cannot change blue paper, because it is already the alkali colour.

Good technique. A careful test uses a small amount of sample and clean equipment:

- Place the strips on a clean white tile so the colours are easy to see. - Dip a clean glass rod into the solution and touch it onto the paper, so only one drop is used. - Rinse and dry the rod between samples to prevent contamination . - Look at the colour straight away, where the drop has spread. - Hold unused strips by their ends, since chemicals or sweat on fingers can affect the paper.

Testing gases. Litmus paper can also test gases, but only when it is damp , because the gas has to dissolve in water on the paper before it can act as an acid or alkali. Damp blue litmus turns red in acidic gases such as hydrogen chloride; damp red litmus turns blue in ammonia.

Bleaching. Some substances, such as chlorine and bleach solutions, can remove the colour from litmus altogether, leaving white paper. Chlorine often turns damp blue litmus red first and then bleaches it. A white result is a clue that the substance is a bleach, not an ordinary acid or alkali.

Coloured samples. Strongly coloured liquids, such as blackcurrant juice or ink, can stain the paper and hide the change. Look at the edge of the wet patch, where the sample's own colour is weakest.

Step-by-step reasoning

To classify a solution with both papers:

1. Place one blue and one red strip on a white tile. 2. Use a clean glass rod to add one drop of solution to each. 3. If blue turns red, the solution is acidic. 4. If red turns blue, the solution is alkaline. 5. If neither changes, the solution is neutral.

Visual explanation

Picture a white tile with three pairs of strips side by side. In the first pair the blue strip has a red spot. In the second pair the red strip has a blue spot. In the third pair neither strip shows a spot. Left to right: acid, alkali, neutral.

Real-world analogy

Using litmus is like a two-question quiz with yes-or-no answers. Blue paper asks "Are you an acid?" and red paper asks "Are you an alkali?" Two "no" answers mean the solution is neutral; one "yes" tells you exactly which group it belongs to.

Real-world example

Technicians who maintain car batteries and some industrial cleaning systems use litmus or similar test papers for a quick check of whether a spill is acidic or alkaline. Knowing this tells them which neutralising material to use to make the spill safe.

Why?

Why must litmus be damp to test a gas? Acidic and alkaline behaviour depends on ions in water. A dry gas cannot form these ions on dry paper, so no change appears. The moisture lets the gas dissolve and react with the dye.

Common misconception

"Red litmus turns red in acid." Red litmus is already red, so it cannot visibly change in acid. Always report which paper changed colour and what the change was, not just the final colour.

Worked example

Question: A drop of solution X leaves blue litmus blue and turns red litmus blue. A drop of solution Y turns blue litmus red and leaves red litmus red. Classify X and Y.

Reasoning: For X, the only change is red to blue, showing an alkali. For Y, the only change is blue to red, showing an acid.

Answer: X is alkaline; Y is acidic.

Quick check

1. Which litmus paper changes colour in an alkaline solution, and to what colour? Answer: Red litmus paper, which turns blue.

Exam focus

Examiners want the full change: "blue litmus turns red" or "red litmus turns blue". Remember that gases must be tested with damp litmus paper. Mention using a clean glass rod and a white tile when describing a method.

Advanced insight

The colour of litmus depends on the balance between two forms of its dye molecules. Near neutral, both forms are present and the colour is purple. Because this change happens over a range of acidity close to neutral, litmus gives a clear yes-or-no answer for most everyday acids and alkalis.

Summary

Blue litmus turns red in acid; red litmus turns blue in alkali; in a neutral solution neither changes. Use a clean glass rod, a single drop and a white tile. Damp litmus paper is needed for testing gases, and bleaches can remove the colour completely. Always state which paper changed and to what colour.

Practice questions

1. What happens to blue litmus paper in lemon juice? Answer: It turns red, because lemon juice is acidic. 2. Why should a glass rod be rinsed between testing different solutions? Answer: To avoid contamination, which could carry acid or alkali from one sample into the next and give a false result. 3. Explain why damp red litmus paper turns blue in ammonia gas but dry red litmus paper does not. Answer: Ammonia must dissolve in water to form an alkaline solution; dry paper has no water for it to dissolve in. 4. A solution turns blue litmus paper white. What does this suggest? Answer: The solution is a bleach, such as chlorine solution, that removes the dye's colour.