Question
How do you test whether a given solution is an acid or a base using litmus paper? Describe the procedure and expected observations.
Solution — Step by Step
Litmus is a natural dye extracted from lichens. It acts as an acid-base indicator — a substance that changes colour depending on whether it is in an acidic or basic (alkaline) environment.
Litmus paper comes in two forms:
- Red litmus paper: Made by soaking paper in acidic litmus solution
- Blue litmus paper: Made by soaking paper in basic litmus solution
- Take a clean glass rod (or dropper)
- Dip it into the unknown solution
- Place a drop of the solution on a piece of red litmus paper
- Observe any colour change
- Repeat with a piece of blue litmus paper
- Record your observations
Alternatively, you can dip a small strip of litmus paper directly into the solution — but the glass rod method avoids contaminating your solution.
| Solution type | Effect on red litmus | Effect on blue litmus |
|---|---|---|
| Acidic (e.g., lemon juice, vinegar, HCl) | Remains red (no change) | Turns red |
| Basic (e.g., NaOH, soap water, baking soda) | Turns blue | Remains blue (no change) |
| Neutral (e.g., distilled water) | No change | No change |
The memory aid: “Acid turns blue to red; Base turns red to blue.”
Litmus contains a dye molecule that has different structures (and thus different colours) in acidic vs. basic environments.
In an acidic solution, H⁺ ions are present. These protonate the litmus molecule, giving it the red form.
In a basic solution, OH⁻ ions are present. These deprotonate the litmus molecule (if it was in its red/acid form), giving it the blue form.
This is a reversible change — if you neutralise an acidic litmus paper with a base, it turns blue again.
Why This Works
Litmus is a pH-sensitive dye — its structure changes with the concentration of H⁺ ions in solution. Acids increase H⁺ concentration; bases decrease it (equivalently, increase OH⁻). The colour change is a visual readout of which ion dominates.
This is why litmus is called an indicator — it indicates the nature of the solution. Unlike pH meters, it doesn’t give a precise pH value, but it tells you definitively: acid, base, or neutral.
Always test with both red and blue litmus paper to confirm. If red turns blue → definitely basic. If blue turns red → definitely acidic. If neither changes → neutral (or the solution is very dilute). Using only one paper can be confusing if there’s a subtle colour shift.
Alternative Method
Other natural indicators can also be used:
- Turmeric solution: Remains yellow in acid, turns red-brown in base
- Red cabbage juice: Pink in acid, green in base
- China rose petals (hibiscus): Dark pink in acid, green in base
These are excellent for home experiments when litmus paper isn’t available.
Common Mistake
Students sometimes say “blue litmus turns blue in a base” as if that’s an observation — but blue litmus stays blue in a base (no change!). The meaningful observation is the change. Red turning blue confirms a base. Blue staying blue in a base is not informative alone. Always describe the observation in terms of change or no-change.