What Is a Litmus Test?

Litmus paper is a quick, inexpensive way to gauge the acidity or alkalinity of a liquid or water‑soluble gas. The paper is treated with dyes that change color when they encounter acids or bases. The pH scale, ranging from 0 (most acidic) to 14 (most alkaline), uses 7 as the neutral midpoint. A value below 7 indicates acid; above 7 indicates base (or "alkaline").

Using Litmus Papers in Water

1. Prepare an Aqueous Sample

Pour the liquid into a clean glass or beaker. The test only works with aqueous solutions; non‑aqueous samples will not produce reliable results.

2. Dip the Paper

Hold a strip of red litmus paper at one end, dip it briefly into the liquid, and withdraw it. Color change occurs instantly—no waiting time is needed. Repeat with a blue litmus strip.

3. Interpret the Results

• Acidic solution: red paper remains red; blue paper turns red.
• Alkaline solution: red paper turns blue; blue paper stays blue.
• Neutral solution: neither paper changes color.

Using Litmus Papers with Gases

1. Prepare the Papers

Dampen a red and a blue strip with water. Always work in a fume hood and wear appropriate PPE (gloves, lab coat, eye protection).

2. Expose to Gas

Open the gas container under the hood, allowing the gas to contact both strips. Gases tend to coat the paper uniformly, producing a consistent color change.

3. Read the Results

• Acidic gas: blue strip turns red; red strip remains unchanged.
• Alkaline gas: red strip turns blue; blue strip stays blue.
• Neutral gas: no color change on either strip.

Materials Needed

• Red litmus paper
• Blue litmus paper
• Glass or beaker
• Fume hood
• Safety gear: gloves, lab coat, eye protection

Safety Notice

Minors must be supervised at all times when handling chemicals or performing tests involving gases.