Chapter 14 of 25

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Tamil Nadu (TNBSE)Class 9 Science★ 5-6 marks in assessments

Acids, Bases and Salts

Samacheer Kalvi (TN State Board) Class 9 Science — Chemistry Chapter 14, Acids, Bases and Salts. Acids, bases, and salts are encountered everywhere in nature and industry. Understanding their properties, pH values, indicators, and salt preparations explains digestion, soil chemistry, soap making, and food preservation. Notes, worked examples, common mistakes, and quick revision.

⏱ 19 min readMedium difficulty✓ Free · NCERT-aligned

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

1Define acids, bases, and salts based on chemical theories
2Differentiate properties of acids and bases
3Explain pH scale and solve simple pH calculations
4Understand neutralization and preparation of common salts

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Why this chapter matters

Acids, bases, and salts are encountered everywhere in nature and industry. Understanding their properties, pH values, indicators, and salt preparations explains digestion, soil chemistry, soap making, and food preservation.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Class 8 Science — Acids and Bases

Indicators and natural acid examples.

Acids, Bases and Salts — Class 9 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 9 Science, Chemistry — Chapter 14. Acids, bases, and salts are encountered everywhere in nature and industry. Understanding their properties, pH values, indicators, and salt preparations explains digestion, soil chemistry, soap making, and food preservation.

1. About this chapter

This chapter covers properties of acids and bases, indicators, pH scale, neutralization, and preparation and uses of important salts.

2. Acids and Bases

Acids: Sour taste, turn blue litmus red, release $H^{+}$ ions in aqueous solution (Arrhenius). Example: HCl, H₂SO₄.
Bases: Bitter taste, soapy feel, turn red litmus blue, release $O H^{-}$ ions. Alkaline bases soluble in water (NaOH, KOH).
Indicators: Litmus, Phenolphthalein (pink in base, colorless in acid), Methyl Orange (yellow in base, red in acid).

3. pH Scale

Measure of hydrogen ion concentration: $pH = - lo g_{10} [H^{+}]$ .
Acidic: pH < 7. Neutral: pH = 7. Basic: pH > 7.
Importance: Human body works within pH 7.0–7.8. Acid rain has pH < 5.6.

4. Salts

Formed by neutralization. Examples:
- Sodium Hydroxide (NaOH): Chlor-alkali process.
- Baking Soda (NaHCO₃): Used in cooking, fire extinguishers.
- Washing Soda (Na₂CO₃.10H₂O): Used in glass, soap, paper industries.
- Bleaching Powder (CaOCl₂): Used for disinfection.

Key formulas & results

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

pH definition

pH = -log10[H+]

H+ = concentration of hydrogen ions in moles/litre.

pH and pOH relation

pH + pOH = 14

At 25°C.

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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

✗ Diluting concentrated acid by adding water directly to the acid.

✓ Always dilute by adding concentrated acid slowly to water with constant stirring. Adding water to acid generates sudden, explosive heat causing splashing.

WATCH OUT

✗ Calling all bases alkalies.

✓ Only water-soluble bases are called alkalies (e.g. NaOH, KOH). All alkalies are bases, but not all bases are alkalies.

NCERT exercises (with solutions)

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

Multiple Choice Questions

Questions on pH values, indicators, and salt preparation.

Questions

Short Answer Questions

Neutralization, uses of washing soda, baking soda, and plaster of paris.

Questions

Practice problems

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

Q1EASY· Concept

What is neutralization? Write a balanced equation.

▶ Show solution

Reaction between an acid and a base to form salt and water. Equation: HCl + NaOH -> NaCl + H₂O.

Q2MEDIUM· Concept

State the chemical name, formula, and two uses of Bleaching Powder.

▶ Show solution

Chemical name: Calcium oxychloride, Formula: CaOCl₂. Uses: 1. Disinfecting drinking water. 2. Bleaching cotton in textile industries.

5-minute revision

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

•Acids release H⁺ (or H₃O⁺) in water. Bases release OH⁻.
•pH < 7 is acidic; pH > 7 is basic; pH = 7 is neutral.
•Neutralization: Acid + Base -> Salt + Water.
•Salts like Washing Soda (Na₂CO₃.10H₂O) and Baking Soda (NaHCO₃) have major domestic and industrial uses.

Tamil Nadu (TNBSE) marks blueprint

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

Typical chapter weightage: 5-6 marks in assessments

Question type	Marks each	Typical count	What it tests
MCQ	1	1-2	Base concepts and definitions
Short Answer	2-3	1-2	Descriptive and application points

Prep strategy

Understand core definitions and solve standard textbook problems.
Review common mistakes to avoid losing easy marks.

Where this shows up in the real world

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

Agriculture

Soil pH is monitored and adjusted (using lime or organic matter) to suit specific crop growth requirements.

Medicine

Antacids containing mild bases like magnesium hydroxide are used to neutralise excess stomach acidity.

Exam strategy

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

Write definitions precisely as defined in the textbook.
Draw neat, labeled diagrams for biology and physics chapters.

Going beyond the textbook

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

Read advanced reference materials to explore concepts beyond the school syllabus.

Where else this chapter is tested

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

Class 9 Annual ExamsHigh

NTSE Stage 1Medium

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Questions students ask

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

pH stands for 'potenz' (power) of hydrogen. Range: 0 to 14. pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic.

The fixed number of water molecules chemically combined in a formula unit of salt (e.g., copper sulphate pentahydrate, CuSO₄.5H₂O).

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