⚡ Quick Answer
Class 10 Chemistry Chapter 2 covers Acids, Bases and Salts. Key topics include: definitions and examples of acids and bases, the pH scale (0-14), chemical properties and reactions, and important salts like common salt, baking soda, and washing soda. This chapter carries 5 marks in the CBSE Class 10 Board Exam.
📋 TL;DR — Chapter 2 at a Glance
- Acids are sour, turn blue litmus red, and release H⁺ ions in water. Examples: HCl, H₂SO₄, citric acid.
- Bases are bitter, turn red litmus blue, and release OH⁻ ions. Examples: NaOH, Ca(OH)₂, KOH.
- pH scale measures acidity (0-6), neutrality (7), and alkalinity (8-14). Universal indicator gives different colours.
- Salts are formed by neutralization. Important salts: NaCl (common salt), NaHCO₃ (baking soda), Na₂CO₃·10H₂O (washing soda).
Introduction to Acids, Bases and Salts
Chapter 2 of the NCERT Class 10 Science textbook introduces you to the fascinating world of Acids, Bases and Salts — three fundamental categories of chemical compounds that surround us every day. From the citric acid in your oranges to the baking soda in your kitchen, from the hydrochloric acid in your stomach to the soap you use to wash your hands, this chapter explains the chemistry behind substances we encounter constantly.
For the CBSE Class 10 Board Exam, this chapter is critical. It typically carries 5 marks and includes questions on definitions, chemical equations, the pH scale, and practical applications of salts. Understanding the concepts clearly here will also help you in Class 11 Chemistry, where these topics are explored in much greater depth.
What Are Acids? Definition, Types and Examples
Definition of Acids
An acid is a substance that tastes sour, turns blue litmus paper red, and releases hydrogen ions (H⁺) when dissolved in water. The word “acid” comes from the Latin acidus, meaning sour. All acids contain hydrogen as an essential constituent.
According to the Arrhenius theory, acids are substances that dissociate in water to produce hydrogen ions (H⁺). For example:
HCl → H⁺ + Cl⁻
H₂SO₄ → 2H⁺ + SO₄²⁻
Types of Acids
Physical Properties of Acids
- Sour taste (e.g., lemon, vinegar)
- Turn blue litmus paper red
- Conduct electricity in aqueous solution (electrolytes)
- React with metals to produce hydrogen gas
- Strong acids are corrosive and cause burns on skin
What Are Bases? Definition, Types and Examples
Definition of Bases
A base is a substance that tastes bitter, feels soapy to touch, turns red litmus paper blue, and releases hydroxide ions (OH⁻) when dissolved in water. According to the Arrhenius theory, bases are substances that dissociate in water to produce hydroxide ions.
NaOH → Na⁺ + OH⁻
Ca(OH)₂ → Ca²⁺ + 2OH⁻
Types of Bases
Physical Properties of Bases
- Bitter taste (e.g., soap, baking soda)
- Turn red litmus paper blue
- Feel soapy or slippery to touch
- Conduct electricity in aqueous solution
- Strong bases are caustic and cause skin burns
The pH Scale: Measuring Acidity and Alkalinity
The pH scale is a measure of the acidity or alkalinity of a solution. It ranges from 0 to 14, where pH stands for “potential of hydrogen.” The concentration of hydrogen ions [H⁺] determines the pH value.
Importance of pH in Daily Life
- Human body: Our blood has a pH of 7.4. Even a small change can be fatal. Antacids neutralize excess stomach acid (pH 1.5-3.5) to prevent acidity.
- Agriculture: Plants need soil pH between 6.5-7.5 for optimal growth. Acidic soil is treated with lime (CaO).
- Water treatment: Drinking water should have pH 6.5-8.5. Acidic water corrodes pipes; alkaline water tastes bitter.
- Tooth decay: Bacteria produce acids (pH < 5.5) that damage tooth enamel. Toothpaste is basic to neutralize this.
Chemical Properties and Reactions
1. Reaction of Acids with Metals
Acids react with active metals (above hydrogen in the reactivity series) to produce salt and hydrogen gas:
Zn + 2HCl → ZnCl₂ + H₂↑
Mg + H₂SO₄ → MgSO₄ + H₂↑
2. Reaction of Acids with Metal Carbonates and Hydrogen Carbonates
Acids react with metal carbonates and hydrogen carbonates to produce salt, water, and carbon dioxide gas. This is the basis of the lime water test for CO₂:
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
3. Reaction of Acids with Bases (Neutralization)
When an acid reacts with a base, they neutralize each other to form salt and water. This reaction is called neutralization and is exothermic:
NaOH + HCl → NaCl + H₂O + Heat
Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O + Heat
4. Reaction of Bases with Metals
Not all metals react with bases. Only amphoteric metals like aluminium and zinc react with strong bases to produce salt and hydrogen:
2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂↑
Zn + 2NaOH → Na₂ZnO₂ + H₂↑
5. Reaction of Bases with Non-Metal Oxides
Non-metal oxides are acidic in nature. They react with bases to form salt and water:
CO₂ + 2NaOH → Na₂CO₃ + H₂O
SO₂ + 2NaOH → Na₂SO₃ + H₂O
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g). This earns you full marks in the exam.Important Salts: Preparation, Properties and Uses
1. Common Salt (Sodium Chloride — NaCl)
- Source: Obtained from seawater and rock salt mines.
- Preparation: Evaporation of seawater in salt pans.
- Uses: Food preservative, seasoning, manufacture of chlorine, caustic soda, and baking soda.
2. Baking Soda (Sodium Hydrogen Carbonate — NaHCO₃)
- Preparation: Solvay process: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl
- Properties: Mild, non-corrosive base. Releases CO₂ when heated or mixed with acid.
- Uses: Baking (makes cakes fluffy), antacid (relieves acidity), fire extinguisher (produces CO₂).
3. Washing Soda (Sodium Carbonate Decahydrate — Na₂CO₃·10H₂O)
- Preparation: Heating baking soda: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Then recrystallizing in water.
- Properties: Strong base, removes hardness of water, efflorescent (loses water of crystallization).
- Uses: Laundry (removes dirt), glass manufacture, paper industry, water softening.
4. Bleaching Powder (Calcium Oxychloride — CaOCl₂)
- Preparation: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O (Chlorine gas passed through dry slaked lime)
- Properties: Strong oxidizing agent. Releases chlorine on reaction with dilute acid.
- Uses: Bleaching cotton and linen, disinfecting water, making wool unshrinkable.
5. Plaster of Paris (Calcium Sulphate Hemihydrate — CaSO₄·½H₂O)
- Preparation: Gypsum (CaSO₄·2H₂O) heated to 373K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
- Properties: Sets into hard mass on mixing with water (exothermic setting).
- Uses: Plastering walls, making toys, statues, casts for fractures, dental moulds.
Water of Crystallization and Hydrated Salts
Water of crystallization is the fixed number of water molecules present in one formula unit of a salt. These water molecules are loosely bonded and can be removed by heating.
Frequently Asked Questions (NCERT Based)
What is the difference between an acid and a base?
An acid is a substance that tastes sour, turns blue litmus red, and releases H⁺ ions in water. A base tastes bitter, feels soapy, turns red litmus blue, and releases OH⁻ ions in water. Acids have pH less than 7, while bases have pH greater than 7. Acids react with metals to produce hydrogen gas, while bases react with non-metal oxides to form salt and water.
Why does an aqueous solution of acid conduct electricity?
An aqueous solution of acid conducts electricity because acids dissociate in water to produce hydrogen ions (H⁺) and anions. These ions are free to move in the solution and carry electric current. For example, HCl dissociates into H⁺ and Cl⁻ ions, making the solution an electrolyte. Dry HCl gas does not conduct electricity because it lacks free ions.
What is a neutralization reaction? Give two examples.
A neutralization reaction is a reaction between an acid and a base that produces salt and water, with the evolution of heat. The acid and base neutralize each other’s properties. Examples: (1) NaOH + HCl → NaCl + H₂O, and (2) Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O. These reactions are exothermic and are used in antacids to treat acidity.
Why does dry HCl gas not change the colour of dry litmus paper?
Dry HCl gas does not change the colour of dry litmus paper because acids produce H⁺ ions only in the presence of water. Dry HCl gas exists as covalent molecules (HCl) and does not dissociate into H⁺ ions. Without H⁺ ions, the acidic property is not exhibited. When water is present, HCl dissociates: HCl + H₂O → H₃O⁺ + Cl⁻, and the litmus changes colour.
What is the common name of CaOCl₂ and what are its uses?
The common name of CaOCl₂ is Bleaching Powder (chemical name: Calcium Oxychloride). It is prepared by passing chlorine gas through dry slaked lime. Its uses include: (1) bleaching cotton and linen in textile industry, (2) disinfecting drinking water and swimming pools, (3) making wool unshrinkable, and (4) as an oxidizing agent in chemical industries.
What is water of crystallization? Give two examples.
Water of crystallization is the fixed number of water molecules chemically combined in a definite ratio with one formula unit of a salt. These water molecules are part of the crystal structure and can be removed by heating. Examples: (1) Blue vitriol (CuSO₄·5H₂O) contains 5 water molecules, and (2) Washing soda (Na₂CO₃·10H₂O) contains 10 water molecules. When heated, these salts lose water and become anhydrous (white powder).
Related Class 10 Chemistry Notes
Explore more NCERT-aligned study material for your Board Exam preparation:
- 👉 Adulteration in Food — Chemistry Investigatory Project
- 👉 Growing Crystals Science Project — Learn Science Fair Experiment.
Final Thoughts
Chapter 2 — Acids, Bases and Salts — is one of the most scoring chapters in the Class 10 Science Board Exam. The concepts are straightforward, the reactions follow clear patterns, and the questions are highly predictable. Focus on: (1) definitions with examples, (2) chemical equations with state symbols, (3) the pH scale and its applications, and (4) preparation and uses of important salts.
Practice writing every reaction at least three times. Use the memory tricks and tables in this guide for quick revision the night before the exam. And remember — understanding why a reaction happens is just as important as memorizing what happens.
If you found these notes helpful, bookmark this page and share it with your classmates. Good luck with your Board Exams!
EduNotes Editorial Team
Experienced CBSE educators and subject matter experts with 12+ years of teaching Chemistry at the secondary school level. Our notes are aligned with the latest NCERT syllabus and CBSE Board Exam pattern. Every concept is verified against official NCERT textbooks before publication.
Last Updated: | Aligned with: NCERT Class 10 Science (2026-27 Edition) & CBSE Board Exam Pattern
