Introduction and Acid-Base Indicators

Acids and Bases — An Introduction

Think for a moment — have you ever tasted a sour mango, lemon, or tamarind? What causes the sour taste? Acids.

And if soap ever touches your tongue, it tastes bitter — that's the taste of a base.

Basic Properties of Acids and Bases

Property	Acid	Base
Taste	Sour	Bitter
Touch	—	Soapy / slippery
Blue litmus	Turns red	No change
Red litmus	No change	Turns blue

Common Examples

Acidic substances (in daily life):

• Lemon, tamarind — citric acid
• Vinegar — acetic acid ($CH_3COOH$)
• Curd / yoghurt — lactic acid
• Apple — malic acid
• Stomach — hydrochloric acid ($HCl$)
• Ant sting — formic acid

Basic substances (in daily life):

• Soap — various bases
• Caustic soda (food industry) — $NaOH$
• Milk of magnesia (antacid) — $Mg(OH)_2$
• Baking soda at home — $NaHCO_3$
• Window cleaner — ammonia ($NH_3$)

A Curious Experience

When you have acidity trouble, you take baking soda. Why? Because baking soda is basic — it neutralises the excess $HCl$ in the stomach.

This is the central principle of this chapter — acids and bases nullify each other's effects.

Caution: Never identify acids or bases in the laboratory by tasting them — this is very dangerous. Always use indicators.

Acid-Base Indicators

Indicators = substances that show the presence of an acid or base — usually by changing colour or odour.

Why Are Indicators Necessary?

Identifying acids and bases by taste in the laboratory is dangerous (some are highly corrosive). So we use indicators.

Three Types of Indicators

1. Natural Indicators

• Litmus — most famous
• Turmeric — yellow on cloth
• Red cabbage juice
• Petals of certain flowers — Hydrangea, Petunia, Geranium

2. Synthetic Indicators

• Methyl orange
• Phenolphthalein
• Universal indicator

3. Olfactory Indicators

• Cloth strips dipped in onion juice
• Vanilla essence
• Clove oil

Litmus — The Most Famous Indicator

Source of litmus: A purple-coloured dye extracted from lichen, a plant of the Thallophyta division.

In neutral state: Litmus is purple.

Colour changes with acids and bases:

Indicator	In acid	In base
Blue litmus	Red	Unchanged (blue)
Red litmus	Unchanged (red)	Blue

Memory rule: Acid turns blue litmus red; base turns red litmus blue.

[Board Important] The litmus colour change is asked almost every year.

Activity 2.1 — Testing Different Indicators

Collect the following solutions in the lab — $HCl, H_2SO_4, HNO_3, CH_3COOH$ (acids), and $NaOH, Ca(OH)_2, KOH, Mg(OH)_2, NH_4OH$ (bases).

Put a drop of red/blue litmus, phenolphthalein, and methyl orange on each solution and observe the colour change.

Colours of the Four Indicators — A Table

Indicator	Neutral	In acid	In base
Red litmus	Red	Red (unchanged)	Blue
Blue litmus	Blue	Red	Blue (unchanged)
Phenolphthalein	Colourless	Colourless (unchanged)	Pink / magenta
Methyl orange	Orange	Red / pink	Yellow

Key Points

Phenolphthalein:

• The lab's most popular indicator.
• Shows colour only in a base — pink.
• Stays colourless in an acid.
• Widely used in neutralisation titration.

Methyl orange:

• Red/pink in acid.
• Yellow in base.
• Orange when neutral.
• Especially useful for weak bases.

A Curious Fact — The Turmeric Stain

When soap (basic) is rubbed on a turmeric stain on white cloth, the stain turns reddish-brown. Washing with plenty of water makes it yellow again. Turmeric is a natural base indicator.

[Board Important] The colour changes of methyl orange and phenolphthalein are commonly tested.

Activities 2.2 and 2.3 — Olfactory Indicators

Some substances change odour in acidic or basic environments. These are called olfactory indicators.

Activity 2.2 — Testing with Onion

Procedure:

1. Place finely chopped onions in a plastic bag along with strips of clean cloth.
2. Tie the bag tightly and leave it overnight in the fridge.
3. The cloth strips now carry the smell of onion.
4. Put a few drops of dilute $HCl$ on one strip and dilute $NaOH$ on another.
5. Smell each strip.

Observation:

• Strip with dilute $HCl$: the onion smell remains.
• Strip with dilute $NaOH$: the onion smell changes (or fades away).

So onion changes odour in the presence of a base.

Activity 2.3 — Vanilla and Cloves

Procedure: Take dilute $HCl$ and dilute $NaOH$ in two separate test tubes. Add a few drops of dilute vanilla essence to each. Repeat the test with clove oil.

Observations:

Substance	Smell in $HCl$	Smell in $NaOH$	Conclusion
Onion	Unchanged	Changes	Indicator for base
Vanilla	Unchanged	Changes / fades	Indicator for base
Clove oil	Changes / fades	Unchanged	Indicator for acid

Lesson

• Onion and vanilla — odour changes in the presence of a base.
• Clove oil — odour changes in the presence of an acid.

[Board Important] Three examples of olfactory indicators and which medium they work in — a 3-mark question.

Universal Indicator

The earlier indicators only tell us whether a substance is an acid or a base — not how strong it is.

The universal indicator is a mixture — of several indicators — that shows different colours at different pH values.

Universal Indicator Colours and pH

pH	Colour	Nature
1-3	Red	Strong acid
4-5	Orange / yellow	Weak acid
6	Pale green-yellow	Very weak acid
7	Green	Neutral
8-9	Blue-green	Weak base
10-11	Blue	Moderate base
12-14	Violet / purple	Strong base

Three Forms

1. Paper: universal indicator paper — most common. 2. Solution: for adding to a test tube. 3. Digital pH meter: for precise measurement.

What Has These Colours?

pH ~	Substance
1	Stomach acid
2	Lemon, vinegar
4	Tomato, beer
5	Coffee
6	Milk, urine
7	Pure water
8	Sea water
9	Baking soda solution
11	Ammonia
13-14	Concentrated $NaOH$ solution

(The pH scale will be studied in detail in Section 5.)

[Board Important] What is the universal indicator? State its use. — A common 2-mark question.

🧠 Memory Capsule

A one-glance recap to revisit just before the board exam — every key idea about indicators in one place.

1. Acids and Bases — Basic Identification

Property	Acid	Base
Taste	Sour	Bitter
Blue litmus	Red	Unchanged
Red litmus	Unchanged	Blue

2. Acids in Daily Life

• Lemon → citric acid
• Vinegar → acetic acid ($CH_3COOH$)
• Curd → lactic acid
• Stomach → hydrochloric acid ($HCl$)
• Ant sting → formic acid

3. Bases in Daily Life

• Soap — basic
• Antacid (milk of magnesia) — $Mg(OH)_2$
• Baking soda — $NaHCO_3$
• Caustic soda — $NaOH$

4. Four Indicator Colours

Indicator	Acid	Base
Red litmus	Red	Blue
Blue litmus	Red	Blue
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow

5. Olfactory Indicators

Substance	Where odour changes
Onion	Base
Vanilla	Base
Clove oil	Acid

6. Universal Indicator — pH Colour Code

• Red = strong acid
• Green = neutral
• Violet = strong base

7. Source of Litmus

Litmus = purple dye extracted from lichen (Thallophyta).

8. Repeatedly Asked Board Questions

1. What are indicators? Give three examples.
2. What colour does phenolphthalein show in acids and bases?
3. What are olfactory indicators? Give examples.
4. Given only red litmus paper, how do you identify three solutions (acid, base, water)?
5. From which plant is litmus extracted?

The Bottom Line: Acid sour-red; base bitter-blue; neutral-green.

Solved Examples

Example 1: NCERT — Identifying Three Solutions with Only Red Litmus

You are given three test tubes — one contains distilled water and the other two contain an acidic solution and a basic solution. If you have only red litmus paper, how will you identify the contents of each tube?

Solution:

Step-by-step procedure:

1. First test: Put the red litmus paper in each tube.
2. Observation:

• The tube where red litmus turns blue → that is a base (identified).
• The two tubes where red litmus is unchanged — could be water or acid.

1. Second test: Take the now-blue litmus from the basic tube (or first dip the red litmus in the base to convert it to blue).
2. Put this blue litmus in the remaining two tubes:

• Where it turns red → that is an acid.
• Where it stays unchanged (blue) → that is water.

Answer:

• Base = turns red litmus blue.
• Acid = turns blue litmus (made by reaction with base) red.
• Water = no change.

[NCERT textbook question — frequently asked]

Example 2: Definition and Examples of Indicators

What are acid-base indicators? Name three types and give one example of each.

Solution:

Definition: Acid-base indicators are substances that show the acidic or basic nature of a solution by changing colour or odour.

Three types:

1. Natural indicators:

• Litmus — extracted from lichen, purple in colour.
• Turmeric — yellow, turns reddish-brown in a base.
• Red cabbage juice.

2. Synthetic indicators:

• Phenolphthalein — colourless in acid, pink in base.
• Methyl orange — red in acid, yellow in base.

3. Olfactory indicators:

• Onion — odour changes in a base.
• Vanilla — odour changes in a base.
• Clove oil — odour changes in an acid.

[Board 3-mark question]

Example 3: Phenolphthalein Colour

What colour does phenolphthalein show in (a) dilute $HCl$ (b) dilute $NaOH$ (c) distilled water?

Solution:

Medium	Colour of phenolphthalein
(a) dilute $HCl$ (acid)	Colourless
(b) dilute $NaOH$ (base)	Pink / magenta
(c) distilled water (neutral)	Colourless

Explanation:

• Phenolphthalein is a weak acid.
• In acidic or neutral medium, it remains un-ionised — colourless.
• In a base, it ionises into the pink $A^-$ ion.

Practical use:

• Identifying the endpoint in neutralisation titration.
• For example, in a $NaOH$ + $HCl$ titration, the pink colour disappears at the endpoint.

[Repeatedly asked in board]

Example 4: Colour of Methyl Orange

What colour does methyl orange show in different media? Make a table.

Solution:

Medium	Colour of methyl orange
Acid ($HCl$, $H_2SO_4$, …)	Red / pink
Base ($NaOH$, $KOH$, …)	Yellow
Neutral (water)	Orange

Explanation:

• Methyl orange changes colour between pH 3.1 (red) and pH 4.4 (yellow).
• It is more useful in titrations of weak bases.

Comparison — Methyl Orange vs. Phenolphthalein:

Property	Methyl orange	Phenolphthalein
pH range	3.1 - 4.4	8.3 - 10
In acid	Red	Colourless
In base	Yellow	Pink
Indicates	End of strong acid	End of strong base

[Board Important — 2 marks]

Example 5: NCERT — Olfactory Indicators

From vanilla, onion, and clove, which can be used as olfactory indicators?

Solution:

Definition of olfactory indicator: A substance whose odour changes in an acidic or basic medium.

Analysis (based on Activity 2.3):

Substance	Smell in acid	Smell in base	Olfactory indicator?
Vanilla	Unchanged	Changes	Yes ✓
Onion	Unchanged	Changes	Yes ✓
Clove oil	Changes	Unchanged	Yes ✓

Answer: Vanilla, onion, and clove — all three are olfactory indicators.

Specifics:

• Vanilla and onion → odour changes in the presence of a base (so they indicate bases).
• Clove oil → odour changes in the presence of an acid (so it indicates acids).

[NCERT textbook question]

Example 6: The Mystery of the Turmeric Stain

Why does the colour of a turmeric stain on white cloth turn reddish-brown when soap is rubbed on it? And why does it turn yellow again when washed with water?

Solution:

1. Nature of turmeric: Turmeric is a natural indicator. It stays yellow in acidic/neutral medium.
2. Nature of soap: Soap is basic.
3. On rubbing soap on the stain: turmeric meets a base → colour turns reddish-brown.
4. On washing with water: soap washes away → turmeric returns to neutral medium → colour goes back to yellow.

Principle: This confirms that turmeric is a natural base indicator.

Daily-life applications:

• We don't apply baking soda/lemon-soda on turmeric stains in the kitchen.
• After washing, the turmeric stain returns to its yellow colour.

[Board Important — daily-life question]

Example 7: Comparative Numerical with Indicators

A student has red litmus, blue litmus, phenolphthalein, and methyl orange. They tested an unknown solution —

• Blue litmus: no change
• Phenolphthalein: pink
• Methyl orange: yellow

What is the nature of the solution?

Solution:

Analysis:

1. Blue litmus unchanged → the solution is not acidic.
2. Phenolphthalein pink → the solution is basic.
3. Methyl orange yellow → the solution is basic.

Conclusion: The solution is basic.

Verification: If red litmus were tested, it would turn blue (since red → blue indicates a base).

[Lab-based question — common in boards]

Example 8: Acids/Bases in Daily Life

Classify the following as acidic or basic — (a) vinegar (b) soap (c) milk of magnesia (d) lemon juice (e) tomato (f) lime water

Solution:

Substance	Acid/Base?	Acid/base present
(a) Vinegar	Acid	Acetic acid ($CH_3COOH$)
(b) Soap	Base	Various bases
(c) Milk of magnesia	Base	$Mg(OH)_2$
(d) Lemon juice	Acid	Citric acid
(e) Tomato	Acid	Oxalic acid
(f) Lime water	Base	$Ca(OH)_2$

Rule — How to tell?

• Acid: sour taste, turns blue litmus red.
• Base: bitter taste, slippery touch, turns red litmus blue.

Example 9: The Source of Litmus

From which plant is litmus extracted? What is its nature?

Solution:

1. Source: Litmus is extracted from a plant called lichen. This plant belongs to the Thallophyta division.
2. Colour: Litmus is a purple-coloured dye.
3. Neutral state: Pure litmus is purple (neither red nor blue).
4. Two types of litmus paper:

• Red litmus: prepared by treating with acid.
• Blue litmus: prepared by treating with base.

1. Uses:

• The most common indicator in the lab.
• Quick, cheap, and reliable.

Interesting fact: Lichen is a 'symbiotic' plant — it consists of a fungus and an alga living together.

Example 10: A Caution Question

Why should one never identify acids or bases in the laboratory by tasting? What should be done instead?

Solution:

Why not taste?

1. Highly corrosive: Acids and bases can destroy cells.
2. Risk of burns: Sharp burns on skin, tongue, mouth.
3. Toxicity: Some acids/bases are deadly (like $HCl$, $H_2SO_4$, $NaOH$).
4. No visible clue: Most acids and bases are colourless — taste tells you nothing about them.

So what should be done?

• Use indicators — litmus, phenolphthalein, methyl orange.
• pH paper for an accurate pH value.
• Digital pH meter for even more precision.

Precautions:

• Wear goggles.
• Use rubber gloves.
• Follow lab safety rules.

[Safety-based question]

Example 11: Ant Sting

Why does an ant sting cause a burning sensation? What is the home remedy?

Solution:

1. Cause of burning: An ant's sting contains formic acid (HCOOH). It causes burning on the skin.
2. Home remedy: Apply a baking soda ($NaHCO_3$) solution.
3. How does it work?

• Baking soda is basic.
• It neutralises the formic acid (acidic).
• The acid's effect is reduced.

1. Reaction:

$$NaHCO_3 + HCOOH \rightarrow HCOONa + H_2O + CO_2\uparrow$$

5. Comparison: If you applied lemon (acidic), the pain would worsen! Only basic remedies work.

Daily-life applications:

• Bee sting — also acidic; treated with bases.
• Wasp sting — basic; treated with lemon/vinegar (acid).

Example 12: Home Remedy for Acidity

After a meal, you have stomach burning (acidity). You have two choices — lemon water or baking soda. Which is the correct remedy? Explain.

Solution:

Cause of acidity: Excess $HCl$ in the stomach.

Analysis of the two options:

(a) Lemon water:

• Lemon = citric acid (acidic).
• Stomach already has too much acid — adding more will worsen the problem.
• ❌ Not the right remedy.

(b) Baking soda:

• Baking soda = $NaHCO_3$ (basic).
• The base will neutralise the excess stomach acid.
• ✓ Correct remedy.
• Reaction: $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$

Conclusion: Baking soda is the correct remedy because the acid is neutralised by a base.

[Board frequently-asked daily-life question]

Example 13: Comparative Question — Acid vs. Base

Compare five physical/chemical properties of acids and bases.

Solution:

#	Property	Acid	Base
1	Taste	Sour	Bitter
2	Touch	Normal	Slippery (soap-like)
3	Effect on blue litmus	Turns red	Unchanged
4	Effect on red litmus	Unchanged	Turns blue
5	Phenolphthalein	Colourless	Pink
6	Methyl orange	Red/pink	Yellow
7	Reaction with metals	Releases $H_2$ gas	Usually no reaction (exceptions: Zn, Al)
8	Ions in water	Give $H^+$	Give $OH^-$
9	pH	< 7	> 7
10	Neutralisation	With base → salt + water	With acid → salt + water

Daily examples:

• Acids: lemon, vinegar, tamarind
• Bases: soap, baking soda, antacid

[Board 5-mark question]

Example 14: A Question About Soap

Why is soap basic? Why does it feel slippery on the hand?

Solution:

1. How soap forms: Soaps are sodium/potassium salts of long-chain fatty acids. They are made by treating fatty acids with bases — so they retain a basic character.
2. Why is soap basic?

• The aqueous solution of soap releases $OH^-$ ions.
• So it is basic.

1. Why does it feel slippery?

• Basic solutions react with the oily layer on the skin.
• This reaction 'saponifies' the skin oil.
• The new product gives a slippery feel.

1. Applications:

• In washing clothes — removes dirt.
• In hand-washing — removes germs, dust, and oil.

1. Caution: Leaving soap on the skin too long can cause irritation (itching, dryness) — because of its basic nature.

Example 15: Universal Indicator

What is the universal indicator? How is it different from ordinary litmus? Give its pH-colour code.

Solution:

Universal indicator: A mixture — of several indicators — that shows different colours at different pH values.

Difference from litmus:

Aspect	Litmus	Universal indicator
What does it tell?	Acid/base presence	Also the strength
Number of colours	2 (red/blue)	7+ (one per pH value)
Accuracy	Approximate	Precise pH value

pH-Colour Code:

pH	Colour	Nature
1-2	Red	Strong acid
3-5	Orange / yellow	Moderate acid
6	Pale green-yellow	Weak acid
7	Green	Neutral
8-9	Blue-green	Weak base
10-12	Blue	Moderate base
13-14	Violet	Strong base

Uses: as pH paper, solution, or digital meter.

[Board 3-mark question]

Example 16: An Experiment — Identifying an Unknown Solution

In an experiment, the following tests were performed on five unknown solutions ($A, B, C, D, E$):

Solution	Red litmus	Phenolphthalein	Methyl orange
A	Unchanged	Colourless	Red
B	Blue	Pink	Yellow
C	Unchanged	Colourless	Orange
D	Unchanged	Colourless	Red
E	Blue	Pink	Yellow

State the nature of each.

Solution:

Analysis:

• A: Red litmus unchanged (not basic), methyl orange red (acidic), phenolphthalein colourless (acid or neutral) → Acidic.
• B: Red litmus blue (basic), phenolphthalein pink (basic), methyl orange yellow (basic) → Basic.
• C: Red litmus unchanged (acid or neutral), phenolphthalein colourless (acid or neutral), methyl orange orange (neutral) → Neutral (water).
• D: Same as A → Acidic.
• E: Same as B → Basic.

Final classification:

Solution	Nature
A	Acidic
B	Basic
C	Neutral (pure water)
D	Acidic
E	Basic

Lesson: Using two or more indicators makes the identification of a solution precise.

[Board HOTS — 5 marks]