Why Turmeric Can Spot a Base but Never an Acid
Learn which kitchen substances are acidic or basic, how to make natural indicators from cabbage and China rose, and why an antacid settles an upset stomach.
What makes a substance acidic, basic or neutral?
Acidic substances taste sour, basic substances taste bitter and feel soapy, and neutral substances are neither. You can classify almost everything in a kitchen this way — and you never test by tasting in a laboratory, which is exactly why indicators exist.
This page covers everything in the CBSE Class 7 Science chapter on acidic, basic and neutral substances: classifying everyday materials, making and using natural indicators, predicting colour changes, and understanding neutralisation.
This page covers everything in the CBSE Class 7 Science chapter on acidic, basic and neutral substances: classifying everyday materials, making and using natural indicators, predicting colour changes, and understanding neutralisation.
Which everyday substances are acids and which are bases?
Sour things around the house are acidic, and each contains a named acid:
- lemon juice and orange juice — citric acid
- tamarind water — tartaric acid
- curd — lactic acid
- vinegar — acetic acid
Bitter, soapy things are basic: baking soda solution, lime water, soap solution, washing soda and milk of magnesia.
Neutral substances include tap water, sugar solution and common salt solution.
For example, when milk sets into curd, bacteria produce lactic acid — which is exactly why curd tastes sour while fresh milk does not.
The useful boundary case is milk itself: it is very close to neutral, which is why it does not taste sour until it turns and lactic acid forms.
- lemon juice and orange juice — citric acid
- tamarind water — tartaric acid
- curd — lactic acid
- vinegar — acetic acid
Bitter, soapy things are basic: baking soda solution, lime water, soap solution, washing soda and milk of magnesia.
Neutral substances include tap water, sugar solution and common salt solution.
For example, when milk sets into curd, bacteria produce lactic acid — which is exactly why curd tastes sour while fresh milk does not.
The useful boundary case is milk itself: it is very close to neutral, which is why it does not taste sour until it turns and lactic acid forms.
How do you make and use a natural indicator?
An indicator is a substance that changes colour to tell you whether something is acidic or basic, and many come straight from plants.
Litmus is obtained from lichens, and is supplied as blue or red paper or as a purple solution. Turmeric paper is made by rubbing turmeric paste onto paper and drying it. China rose (gudhal) or red rose petals soaked in warm water give a pink extract. Purple cabbage and beetroot water also work.
To test a substance, put a drop on the indicator paper, or add a few drops of the extract to the liquid, and watch the colour.
Some indicators work by smell instead of colour. Onion juice on a cloth loses its sharp smell when a basic solution is added, but keeps it with an acid — an olfactory indicator, useful for students who find colours hard to distinguish.
In real life, the pink stain from a China rose garland changing shade on soapy hands is this same chemistry happening by accident.
Litmus is obtained from lichens, and is supplied as blue or red paper or as a purple solution. Turmeric paper is made by rubbing turmeric paste onto paper and drying it. China rose (gudhal) or red rose petals soaked in warm water give a pink extract. Purple cabbage and beetroot water also work.
To test a substance, put a drop on the indicator paper, or add a few drops of the extract to the liquid, and watch the colour.
Some indicators work by smell instead of colour. Onion juice on a cloth loses its sharp smell when a basic solution is added, but keeps it with an acid — an olfactory indicator, useful for students who find colours hard to distinguish.
In real life, the pink stain from a China rose garland changing shade on soapy hands is this same chemistry happening by accident.
What colour change does each indicator show?
Learning the colour table properly is what most questions test.
Blue litmus turns red in an acid, and stays blue in a base or a neutral substance.
Red litmus turns blue in a base, and stays red in an acid or a neutral substance.
Turmeric paper turns red in a base, and stays yellow in an acid or a neutral substance.
China rose extract turns dark pink (magenta) in an acid and green in a base.
Now the question in the title. Turmeric only has one change: yellow to red with a base. Faced with an acid it stays yellow — and it also stays yellow with a neutral substance. Since both give the same result, turmeric paper cannot tell an acid from a neutral substance.
The same limitation applies to a single litmus paper. Blue litmus staying blue could mean basic or neutral, so you need both papers to identify all three cases with certainty.
Blue litmus turns red in an acid, and stays blue in a base or a neutral substance.
Red litmus turns blue in a base, and stays red in an acid or a neutral substance.
Turmeric paper turns red in a base, and stays yellow in an acid or a neutral substance.
China rose extract turns dark pink (magenta) in an acid and green in a base.
Now the question in the title. Turmeric only has one change: yellow to red with a base. Faced with an acid it stays yellow — and it also stays yellow with a neutral substance. Since both give the same result, turmeric paper cannot tell an acid from a neutral substance.
The same limitation applies to a single litmus paper. Blue litmus staying blue could mean basic or neutral, so you need both papers to identify all three cases with certainty.
What is neutralisation, and where do you see it?
Neutralisation is the reaction in which an acid and a base cancel each other's effect, producing salt and water, and releasing heat:
acid + base → salt + water + heat
The products are neither acidic nor basic, which is the whole point of the reaction.
It explains several everyday remedies:
- Acidity in the stomach comes from too much acid, so an antacid (a mild base such as milk of magnesia) neutralises it and the pain settles.
- An ant or bee sting injects an acid, so rubbing on baking soda — a base — relieves it.
- Acidic soil is treated by adding lime (a base), so crops grow properly.
- Acidic factory waste is neutralised with a base before release, so it does not harm fish and plants in rivers.
The heat released is real and measurable: mixing an acid and a base in a test tube warms it noticeably.
Note that neutralisation needs the right amount of base. Too much base leaves the mixture basic rather than neutral, which is why an antacid dose is measured rather than guessed.
acid + base → salt + water + heat
The products are neither acidic nor basic, which is the whole point of the reaction.
It explains several everyday remedies:
- Acidity in the stomach comes from too much acid, so an antacid (a mild base such as milk of magnesia) neutralises it and the pain settles.
- An ant or bee sting injects an acid, so rubbing on baking soda — a base — relieves it.
- Acidic soil is treated by adding lime (a base), so crops grow properly.
- Acidic factory waste is neutralised with a base before release, so it does not harm fish and plants in rivers.
The heat released is real and measurable: mixing an acid and a base in a test tube warms it noticeably.
Note that neutralisation needs the right amount of base. Too much base leaves the mixture basic rather than neutral, which is why an antacid dose is measured rather than guessed.
Exam tip
Exam tip: never identify a substance with just one litmus paper
Students write "blue litmus stayed blue, so the substance is basic" — but a neutral substance does exactly the same thing.
One paper can only rule things out. To identify all three possibilities you need both: if blue litmus turns red, it is acidic; if red litmus turns blue, it is basic; if neither paper changes, it is neutral.
Write your answers in that structure — state what each paper did, then the conclusion. And remember the direction of each change, since "litmus turned red" is meaningless unless you say which litmus you started with.
One paper can only rule things out. To identify all three possibilities you need both: if blue litmus turns red, it is acidic; if red litmus turns blue, it is basic; if neither paper changes, it is neutral.
Write your answers in that structure — state what each paper did, then the conclusion. And remember the direction of each change, since "litmus turned red" is meaningless unless you say which litmus you started with.
Did you know
Why is litmus taken from lichens rather than made in a factory?
Lichens naturally contain a purple dye whose colour depends on whether its surroundings are acidic or basic, so it works as an indicator with no processing beyond extraction.
That is why litmus comes in both blue and red forms: the same dye is simply prepared in slightly acidic or slightly basic conditions before it reaches the paper.
That is why litmus comes in both blue and red forms: the same dye is simply prepared in slightly acidic or slightly basic conditions before it reaches the paper.
Key takeaways
Acids, bases and indicators: quick revision
- Acidic substances taste sour, basic ones taste bitter and feel soapy, and neutral ones are neither — never test by tasting in a laboratory.
- Lemon has citric acid, tamarind tartaric acid, curd lactic acid and vinegar acetic acid; baking soda, lime water and soap solution are basic.
- Natural indicators include litmus from lichens, turmeric, China rose, purple cabbage and beetroot, plus olfactory indicators such as onion juice.
- Blue litmus turns red in acid, red litmus turns blue in base, and turmeric turns red only in base — so turmeric cannot separate an acid from a neutral substance.
- Neutralisation gives salt, water and heat, and explains antacids, baking soda on a sting, lime on acidic soil and treating factory waste.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Lemon has citric acid, tamarind tartaric acid, curd lactic acid and vinegar acetic acid; baking soda, lime water and soap solution are basic.
- Natural indicators include litmus from lichens, turmeric, China rose, purple cabbage and beetroot, plus olfactory indicators such as onion juice.
- Blue litmus turns red in acid, red litmus turns blue in base, and turmeric turns red only in base — so turmeric cannot separate an acid from a neutral substance.
- Neutralisation gives salt, water and heat, and explains antacids, baking soda on a sting, lime on acidic soil and treating factory waste.
You will remember all of this far better after answering five questions on it than after reading it twice.