Is Lemon Juice Acidic or Basic?
The Question
Determine whether lemon juice is acidic or basic using the litmus test. Describe what you observe.
The Litmus Test
Litmus is the most widely used indicator in chemistry. It is extracted from lichens (a plant-like organism).
Litmus comes in two forms:
- Blue litmus paper — indicates acids (turns red in acids)
- Red litmus paper — indicates bases (turns blue in bases)
The litmus test tells us immediately whether a substance is acidic, basic, or neutral.
| Substance type | Blue litmus | Red litmus |
|---|---|---|
| Acid | Turns red | No change |
| Base | No change | Turns blue |
| Neutral | No change | No change |
About Lemon Juice
Lemon juice is the liquid squeezed from lemons. You know from experience that it tastes sour — that sour taste is the first clue that it is an acid.
Lemon juice contains citric acid — a naturally occurring acid found in citrus fruits like lemons, oranges, and limes.
The Litmus Test for Lemon Juice
Test with Blue Litmus Paper
When blue litmus paper is dipped in lemon juice, the blue litmus paper turns red.
This is the classic indication of an acid.
Test with Red Litmus Paper
When red litmus paper is dipped in lemon juice, there is no colour change.
Red litmus paper stays red in acids. This confirms our finding — the substance is an acid, not a base.
Conclusion
Lemon juice is acidic.
It turns blue litmus red and has no effect on red litmus.
The acid responsible is citric acid.
Other Evidence That Lemon Juice Is an Acid
- Taste: Sour taste is characteristic of acids.
- China rose indicator: Lemon juice turns china rose indicator dark pink (magenta) — the colour change for acids.
- Turmeric indicator: Lemon juice does NOT change turmeric paper — it stays yellow (turmeric doesn’t react to acids).
Other Common Acidic Substances in Daily Life
| Substance | Acid present |
|---|---|
| Lemon, orange | Citric acid |
| Vinegar | Acetic acid |
| Curd (yoghurt) | Lactic acid |
| Tomato | Tartaric acid, citric acid |
| Tamarind | Tartaric acid |
| Ant sting | Formic acid |
| Our stomach | Hydrochloric acid |
Citrus fruits (lemon, orange, lime) are named after the citric acid they contain. “Citrus” comes from the Latin word for citron (a lemon-like fruit). The acid makes them sour and also helps preserve them from bacteria.
Common mistake: Getting confused about which litmus paper changes colour in an acid.
In an acid → Blue litmus turns Red. (Blue becomes Red = BR) In a base → Red litmus turns Blue. (Red becomes Blue = RB)
The litmus paper that DOESN’T match the substance is the one that changes. Acids and blue don’t get along — blue turns red. Bases and red don’t get along — red turns blue.
Try These Similar Problems
Problem 1: Vinegar is dipped in litmus solution (purple). What colour does the solution turn?
Vinegar is acidic (contains acetic acid). Litmus solution is purple. Acids turn litmus red.
The solution turns red.
Problem 2: How would you test if a colourless liquid is an acid or a base using litmus?
Use BOTH blue and red litmus paper for a complete test:
-
Dip blue litmus paper in the liquid.
- If it turns red → the liquid is an acid.
- If it stays blue → the liquid is a base or neutral.
-
Dip red litmus paper in the liquid.
- If it turns blue → the liquid is a base.
- If it stays red → the liquid is an acid or neutral.
If neither changes colour → the liquid is neutral.
Problem 3: Name the acid in lemon juice. What other fruit contains the same acid?
The acid in lemon juice is citric acid. Other fruits that contain citric acid: oranges, limes, grapefruits, and gooseberries (amla).
Exam tip: The litmus test question is almost guaranteed in Class 7 science exams. Know both the blue and red litmus reactions for acids AND bases. A common format is: “Describe an activity to test whether a given substance is acidic or basic.” Always describe using BOTH blue AND red litmus for a complete test.