Chapter 14 of 23

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Tamil Nadu (TNBSE)Class 8 Science★ 5-8 marks in the TN Class 8 annual exam through book-back MCQs, fill-ups, true/false and short answers

Acids and Bases

Samacheer Kalvi (TN State Board) Class 8 Science — Chemistry Chapter 14, Acids and Bases. Properties of acids and bases, alkalis, natural and chemical examples (formic acid in a red ant's sting, citric acid, HCl, NaOH), indicators (litmus, turmeric, phenolphthalein), neutralization, antacids and everyday uses, and the pH scale. Notes with the actual book-back questions, common mistakes and quick revision.

⏱ 16 min readEasy difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1List the properties of acids and bases
2Define an alkali and the OH⁻ ion rule
3Name natural and chemical acids and bases
4Use indicators (litmus, turmeric, phenolphthalein)
5Explain neutralization and everyday antacids

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Why this chapter matters

Acids and Bases explains sour acids, bitter bases, alkalis and neutralization — including everyday cases like formic acid in an ant's sting and antacids for indigestion. Alkali (OH⁻) and the antacid example are directly tested book-back content in the TN Class 8 exam.

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Chapter 9 — Matter Around Us

Substances and solutions.

Acids and Bases — Class 8 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 8 Science, Chemistry — Chapter 14. Sour acids, bitter bases, alkalis and how they neutralize.

1. About this chapter

This chapter covers acids, bases, alkalis, indicators, neutralization, everyday acids and bases, and the pH scale.

2. Acids

Acids taste sour, turn blue litmus red, and conduct electricity in solution.
Examples: hydrochloric acid (HCl), sulphuric acid, citric acid (in lemon), acetic acid (in vinegar), and formic acid — the acid in a red ant's sting that causes the burning pain.

3. Bases and alkalis

Bases taste bitter, feel soapy, and turn red litmus blue.
A base that dissolves in water is called an alkali, and it gives hydroxide (OH⁻) ions. All alkalis are bases, but not all bases are alkalis.
Examples: sodium hydroxide (NaOH), magnesium oxide, calcium hydroxide.

4. Indicators, neutralization and pH

Indicators show whether a substance is acidic or basic: litmus (red/blue), turmeric (yellow → red in a base) and phenolphthalein (colourless → pink in a base).
Neutralization: acid + base → salt + water (e.g. HCl + NaOH → NaCl + H₂O).
Everyday uses: magnesium oxide / milk of magnesia is an antacid that neutralizes excess stomach acid; toothpaste is basic and neutralizes mouth acids.
The pH scale (0–14) measures acidity: pH < 7 acidic, pH = 7 neutral, pH > 7 basic.

5. Worked examples

Example 1. Which acid is present in the sting of a red ant? Formic acid.

Example 2. What is an alkali? A base that dissolves in water and gives OH⁻ ions.

Example 3. What colour does turmeric turn in a base? Red.

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

The acid present in the sting of a red ant is — (a) citric acid / (b) formic acid. Ans: (b) formic acid.
A base that is soluble in water is called — (a) an alkali / (b) a salt. Ans: (a) an alkali.
Turmeric solution turns ____ when a base is added — (a) red / (b) green. Ans: (a) red.
The substance used to relieve acidity in the stomach (antacid) is — (a) magnesium oxide / milk of magnesia / (b) vinegar. Ans: (a) magnesium oxide.
The reaction between an acid and a base is called — (a) oxidation / (b) neutralization. Ans: (b) neutralization.

II. Fill in the blanks 6. Acids turn blue litmus red. 7. An alkali gives hydroxide (OH⁻) ions in water. 8. Toothpaste is basic in nature.

III. True or False 9. All alkalis are bases, but all bases are not alkalis. — True. 10. Acids conduct electricity in solution. — True.

7. Common mistakes

Mistake: Saying every base is an alkali. Fix: Only a water-soluble base is an alkali; all alkalis are bases, not all bases are alkalis.
Mistake: Naming the wrong acid for a red ant's sting. Fix: It is formic acid.
Mistake: Saying turmeric turns blue in a base. Fix: Turmeric turns red in a base.

8. Quick revision

Chemistry Ch 14 · acids, bases, alkalis, indicators, neutralization.
Acids: sour, blue litmus → red, conduct electricity (HCl, citric, formic in ant sting).
Bases: bitter, soapy, red litmus → blue; alkali = soluble base giving OH⁻.
Indicators: litmus, turmeric (yellow → red in base), phenolphthalein.
Neutralization: acid + base → salt + water; antacid = magnesium oxide; toothpaste basic.
pH scale 0–14: <7 acidic, 7 neutral, >7 basic.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Acids

sour; blue litmus → red; conduct electricity

HCl, citric, formic (ant sting).

Bases / alkali

bitter; red litmus → blue; alkali gives OH⁻

All alkalis are bases, not vice versa.

Neutralization

acid + base → salt + water

HCl + NaOH → NaCl + H₂O.

pH scale

0–14: <7 acidic, 7 neutral, >7 basic

Measures acidity.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Saying every base is an alkali

✓ Only a water-soluble base is an alkali; all alkalis are bases, not all bases are alkalis.

WATCH OUT

✗ Naming the wrong acid for a red ant's sting

✓ It is formic acid.

WATCH OUT

✗ Saying turmeric turns blue in a base

✓ Turmeric turns red in a base.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Choose the correct answer

Book-back MCQs on formic acid, alkali, indicators and neutralization.

Questions

Fill in the blanks

Litmus, OH⁻ ions and toothpaste.

Questions

True or False / Answer briefly

Alkali rule, conduction, antacids and indicators.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· MCQ

The acid present in the sting of a red ant is ____.

▶ Show solution

formic acid.

Q2EASY· MCQ

A base that is soluble in water is called ____.

▶ Show solution

an alkali.

Q3EASY· MCQ

Turmeric solution turns ____ when a base is added.

▶ Show solution

red.

Q4EASY· Fill in the blanks

An alkali gives ____ ions in water.

▶ Show solution

hydroxide (OH⁻).

Q5EASY· Answer briefly

What is neutralization? Give an example.

▶ Show solution

The reaction of an acid with a base to give salt and water, e.g. HCl + NaOH → NaCl + H₂O.

Q6MEDIUM· Application

Why is magnesium oxide (milk of magnesia) taken for stomach acidity?

▶ Show solution

It is a base (antacid) that neutralizes the excess hydrochloric acid in the stomach, relieving the acidity.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Chemistry Chapter 14 of Samacheer Kalvi Class 8 Science.
•Acids: sour, blue litmus → red, conduct electricity (HCl, citric, formic in ant sting).
•Bases: bitter, soapy, red litmus → blue; alkali = soluble base giving OH⁻.
•All alkalis are bases, not all bases are alkalis.
•Indicators: litmus, turmeric (yellow → red in base), phenolphthalein.
•Neutralization: acid + base → salt + water; antacid = magnesium oxide; toothpaste basic.
•pH scale 0–14: <7 acidic, 7 neutral, >7 basic.

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 5-8 marks across book-back MCQ, fill-ups, true/false and short answers

Question type	Marks each	Typical count	What it tests
MCQ / Fill	1	3-5	Properties, alkali, indicators
True / False	1	1-2	Alkali rule, conduction
Short Answer	2-3	1-2	Neutralization, antacids

Prep strategy

Learn the litmus colour changes for acids and bases
Remember formic acid is in a red ant's sting
Note the alkali rule (OH⁻; soluble base)
Link antacids and toothpaste to neutralization

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Health

Antacids neutralize excess stomach acid.

Daily life

Toothpaste (basic) neutralizes mouth acids; baking soda soothes stings.

Agriculture

Lime treats acidic soil.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Quote the litmus colour changes both ways
Name formic acid for a red ant's sting
State the alkali rule (all alkalis are bases, not vice versa)
Write a neutralization equation

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Test five household substances with turmeric or litmus and classify them.
Explain why pH matters in soil and the human stomach.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

TN Class 8 Annual ExamHigh

Foundation / NMMS ScienceMedium

School unit testsHigh

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

A base reacts with acids to give salt and water; an alkali is a base that also dissolves in water and gives hydroxide (OH⁻) ions. So all alkalis are bases, but not all bases are alkalis.

The sting injects formic acid; baking soda is a mild base that neutralizes the acid and eases the pain.

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