Class 10 Science Chapter 1 Notes: Acids Bases and Salts

Class 10 Science Chapter 1 Acids Bases and Salts Notes

🔹 1. Indicators

Indicators are substances that show whether a solution is acidic or basic by a change in colour or smell.

Types of Indicators

(a) Natural Indicators

Obtained from natural sources.

• Litmus

  • Acid → Blue litmus turns red

  • Base → Red litmus turns blue

• Turmeric

  • Acid → No change (yellow)

  • Base → Reddish brown

• Red Cabbage Juice

  • Acid → Reddish

  • Base → Greenish yellow

(b) Olfactory Indicators

Substances that change smell in acids or bases.
Used for visually impaired students.

• Onion → Smell disappears with base

• Vanilla → Smell disappears with base

(c) Synthetic Indicators

Prepared in laboratory.

• Phenolphthalein

  • Acid → Colourless

  • Base → Pink

• Methyl Orange

  • Acid → Red

  • Base → Yellow

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🔸 Indicator Colour Change Table

Indicator	Original	Acid	Base
Red litmus	Red	No change	Blue
Blue litmus	Blue	Red	No change
Turmeric	Yellow	No change	Reddish brown
Red cabbage	Purple	Reddish	Greenish
Phenolphthalein	Colourless	Colourless	Pink
Methyl orange	Orange	Red	Yellow
Onion	—	No change	Smell vanishes
Vanilla	—	No change	Smell vanishes

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🔹 2. Acids

Acids are substances that:

• Are sour in taste

• Turn blue litmus red

• Release H⁺ (or H₃O⁺) ions in aqueous solution

Examples

HCl, H₂SO₄, HNO₃, CH₃COOH

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Properties of Acids

• Sour taste

• Conduct electricity in aqueous solution

• Release hydrogen ions (H⁺)

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Types of Acids

(a) Natural / Organic Acids

Obtained from plants or animals.

Acid	Source
Acetic acid	Vinegar
Citric acid	Lemon
Lactic acid	Curd
Oxalic acid	Tomato
Tartaric acid	Tamarind

(b) Mineral / Inorganic Acids

Prepared from minerals.

Examples:
HCl, H₂SO₄, HNO₃, H₂CO₃

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🔹 3. Chemical Properties of Acids

(i) Reaction with Metals

Metal + Acid → Salt + H₂↑

✔ Hydrogen gas burns with pop sound

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(ii) Reaction with Metal Carbonates

Metal carbonate + Acid → Salt + CO₂ + H₂O

(iii) Reaction with Metal Hydrogen Carbonates

Metal bicarbonate + Acid → Salt + CO₂ + H₂O

✔ CO₂ test: Turns lime water milky

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Common Property of Acids

All acids produce H⁺ ions in aqueous solution.

Example:

HCl + H₂O → H₃O⁺ + Cl⁻

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Strong and Weak Acids

• Strong acids: Completely ionised

  • HCl, H₂SO₄, HNO₃

• Weak acids: Partially ionised

  • CH₃COOH, H₂CO₃

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🔹 4. Bases

Bases are substances that:

• Are bitter and soapy

• Turn red litmus blue

• Release OH⁻ ions in water

Examples

NaOH, KOH, Ca(OH)₂

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Properties of Bases

• Bitter taste

• Soapy touch

• Conduct electricity

• Release hydroxide ions (OH⁻)

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Types of Bases

• Alkalis: Water-soluble bases

  • NaOH, KOH, Ca(OH)₂

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🔹 5. Chemical Properties of Bases

(i) Reaction with Metals

Base + Metal → Salt + H₂

(ii) Reaction with Non-metal Oxides

Base + Non-metal oxide → Salt + Water

(Non-metal oxides are acidic)

(iii) Neutralisation Reaction

Acid + Base → Salt + Water

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Common Property of Bases

Basic nature is due to OH⁻ ions in aqueous solution.

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🔹 6. Dilution of Acids and Bases

• Dilution is exothermic

• Always add acid/base to water

• Never add water to acid (safety rule)

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🔹 7. Strength of Acids and Bases – pH Scale

• pH = –log[H⁺]

• Acidic → pH < 7

• Neutral → pH = 7

• Basic → pH > 7

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Universal Indicator

• Shows strength + nature of solution

• pH range: 1 to 14

• Available as paper strips and solution

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🔹 8. Role of pH in Everyday Life

(i) Digestive System

• Stomach contains HCl

• Excess acid → indigestion

• Antacids like Mg(OH)₂ neutralise acid

(ii) Tooth Decay

• pH < 5.5 causes tooth decay

• Toothpaste is basic

(iii) Soil pH

• Proper pH needed for healthy crops

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🔹 9. Salts

Salts are formed by neutralisation of acid and base.
Example: Sodium chloride (NaCl)

Properties of Salts

• Crystalline solids

• Usually soluble in water

• Conduct electricity in molten state

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Family of Salts

Salts with same radical belong to same family.

Example:

• NaCl, CaCl₂ → Chloride family

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Types of Salts

(i) Neutral Salts

Strong acid + Strong base
pH ≈ 7
Example: NaCl, Na₂SO₄

(ii) Acidic Salts

Strong acid + Weak base
pH < 7
Example: NH₄Cl

(iii) Basic Salts

Weak acid + Strong base
pH > 7
Example: Na₂CO₃, CH₃COONa

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🔹 10. Important Chemical Compounds

(a) Common Salt (NaCl)

Used in food and chemical industries.

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(b) Sodium Hydroxide (NaOH)

Prepared by Chlor-alkali process

Uses:

• Soap, paper, detergents

• Degreasing metals

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(c) Bleaching Powder (CaOCl₂)

Prepared by passing chlorine over slaked lime.

Uses:

• Disinfecting water

• Bleaching cotton and paper

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(d) Baking Soda (NaHCO₃)

On heating:

2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

Uses:

• Baking powder

• Antacid

• Fire extinguisher

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(e) Washing Soda (Na₂CO₃·10H₂O)

Used in:

• Cleaning clothes

• Removing hardness of water

• Glass and soap industries

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🔹 11. Water of Crystallisation

Water molecules present in crystals of salts.

Example:

• CuSO₄·5H₂O (blue colour)

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✨ Quick Revision Points

• Acids give H⁺, bases give OH⁻

• pH shows strength of acid/base

• Salts can be acidic, basic or neutral

• NaCl, NaOH, CaOCl₂, NaHCO₃ are very important

✍️ Top 10 Short Question–Answers

Q1. What are indicators?
Ans: Indicators are substances that show whether a solution is acidic or basic by change in colour or smell.

Q2. Name any two natural indicators.
Ans: Litmus and turmeric.

Q3. What colour does blue litmus show in an acid?
Ans: Blue litmus turns red.

Q4. Define acids.
Ans: Acids are substances that release H⁺ (or H₃O⁺) ions in aqueous solution.

Q5. Name any two mineral acids.
Ans: Hydrochloric acid (HCl) and sulphuric acid (H₂SO₄).

Q6. What is a base?
Ans: A base is a substance that releases OH⁻ ions in aqueous solution.

Q7. What is neutralisation reaction?
Ans: A reaction between an acid and a base forming salt and water.

Q8. What is pH scale?
Ans: pH scale is used to measure the strength of acids and bases.

Q9. What is a salt?
Ans: Salts are formed by the neutralisation of an acid and a base.

Q10. What is water of crystallisation?
Ans: Water molecules present in the crystals of salts are called water of crystallisation.

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📝 Long Answer Questions

Q1. Explain different types of indicators with examples.

Ans:
Indicators help to identify acidic or basic nature of substances.

1. Natural Indicators:
   Obtained from natural sources like plants.
   Examples:

   • Litmus

   • Turmeric

   • Red cabbage juice

2. Olfactory Indicators:
   Change smell in acids or bases and are useful for visually impaired students.
   Examples:

   • Onion

   • Vanilla

3. Synthetic Indicators:
   Prepared in laboratory.
   Examples:

   • Phenolphthalein

   • Methyl orange

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Q2. Describe the properties and types of acids.

Ans:

Properties of Acids:

• Sour in taste

• Turn blue litmus red

• Conduct electricity in aqueous solution

• Release H⁺ ions

Types of Acids:

1. Natural / Organic Acids: Found in plants or animals
   Examples: Acetic acid, Citric acid

2. Mineral / Inorganic Acids: Prepared from minerals
   Examples: HCl, H₂SO₄, HNO₃

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Q3. Explain pH scale and its role in everyday life.

Ans:

pH Scale:

• Measures acidity or basicity of a solution

• Range: 1 to 14

• pH < 7 → Acidic

• pH = 7 → Neutral

• pH > 7 → Basic

Role of pH in Everyday Life:

• Digestive system: Excess acid causes indigestion

• Tooth decay: pH below 5.5 damages teeth

• Soil pH: Affects crop production

📝 Previous Years’ Questions (PYQs)

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🔹 1 Mark PYQs

1. Name one natural indicator.

2. What colour does phenolphthalein show in a basic solution?

3. Write the pH value of a neutral solution.

4. Name the acid present in lemon.

5. What is the nature of washing soda solution?

6. What ion is responsible for acidic nature of a solution?

7. Name the gas evolved when an acid reacts with a metal.

8. Write the chemical formula of baking soda.

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🔹 2 Marks PYQs

9. What is an indicator? Name any two indicators.

10. Define acids and bases.

11. What is neutralisation reaction? Write its general equation.

12. Why are acids diluted by adding acid to water and not vice versa?

13. What is pH scale? Write its range.

14. What happens to tooth enamel when pH falls below 5.5?

15. Write two properties of bases.

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🔹 3 Marks PYQs

16. Differentiate between acids and bases (any three points).

17. Explain the role of pH in our digestive system.

18. What is water of crystallisation? Give one example.

19. Write three uses of baking soda.

20. Explain why dry HCl gas does not change the colour of dry blue litmus paper.

21. Write chemical properties of acids with suitable reactions.

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🔹 5 Marks PYQs (Long Answer Type)

22. Explain different types of indicators with examples.

23. Describe the pH scale and its importance in everyday life.

24. Explain the preparation, properties and uses of washing soda.

25. What are salts? Explain types of salts with examples.

26. Describe the manufacture and uses of sodium hydroxide.

27. Explain bleaching powder: preparation, properties and uses.

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⭐ Very Important Repeated Board Questions

✔ Why should acids be diluted carefully?
✔ Why toothpaste is basic in nature?
✔ Identify the type of salt: NH₄Cl, Na₂CO₃
✔ Name the substance used to neutralise excess acid in stomach
✔ What happens when baking soda is heated? Write the equation

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❓ FAQs (Concept Clarity – 5)

Q1. Why are olfactory indicators useful?
Ans: They help visually impaired students to identify acids and bases.

Q2. Why acids show acidic nature only in aqueous solution?
Ans: Because they produce H⁺ ions only in water.

Q3. Why should acids be diluted carefully?
Ans: Dilution is exothermic and can cause splashing if done incorrectly.

Q4. Why toothpaste is basic?
Ans: It neutralises the acids produced in the mouth.

Q5. Why CuSO₄·5H₂O is blue in colour?
Ans: Due to presence of water of crystallisation.