NCERT Class 10 Science Chapter 2 – Acids, Bases and Salts

NCERT Class 10 Science Chapter 2 explains the properties, reactions, and applications of acids, bases, and salts in everyday life. In NCERT Class 10 Science Chapter 2, students understand how substances behave in aqueous solutions and how their strength is measured using the pH scale.

The chapter begins by identifying acids as substances that turn blue litmus red and bases as substances that turn red litmus blue. Natural indicators like litmus (from lichens) and synthetic indicators like phenolphthalein and methyl orange help in identifying acidic and basic substances.

A key concept in NCERT Class 10 Science Chapter 2 is that acids produce H⁺ (hydrogen ions) in water, while bases produce OH⁻ (hydroxide ions). The acidic or basic nature of a solution depends on the presence of these ions. Hydrogen ions combine with water to form H₃O⁺ (hydronium ions).

The chapter explains important chemical reactions:

• Acid + Metal → Salt + Hydrogen gas
• Acid + Metal carbonate → Salt + Water + Carbon dioxide
• Neutralisation reaction: Acid + Base → Salt + Water

Another major topic in NCERT Class 10 Science Chapter 2 is the pH scale (0–14). A pH value less than 7 indicates acidity, 7 is neutral, and above 7 indicates basic nature. The chapter connects pH with daily life, including stomach acidity, soil pH, tooth decay, and blood pH (~7.4).

The chapter also discusses important salts and industrial processes. The chlor-alkali process involves electrolysis of brine (NaCl solution) to produce sodium hydroxide (NaOH), chlorine (Cl₂), and hydrogen (H₂). Students also learn about washing soda (Na₂CO₃·10H₂O), baking soda (NaHCO₃), and bleaching powder (CaOCl₂).

Another important concept in NCERT Class 10 Science Chapter 2 is water of crystallisation, such as in CuSO₄·5H₂O, which gives crystals their colour and shape.

For board exams, NCERT Class 10 Science Chapter 2 is highly important as it builds the conceptual base for advanced chemistry topics like ionic compounds and electrochemistry. Students should verify concepts from the official NCERT website at for authentic textbooks and syllabus updates.

For structured preparation of NCERT Class 9–12 for UPSC, BPSC and State PCS examinations, strengthen your basics with our complete NCERT Book Notes PDF for Class 9-12, available inside the NCERT foundation course level-2.

Access Complete NCERT Book Notes PDF

Introduction

• Acids are substances that are sour in taste and turn blue litmus red, while bases are bitter in taste and turn red litmus blue
• If a person suffers from acidity due to overeating, a mild base such as baking soda solution is used because acids and bases neutralise each other
• Litmus is a natural indicator extracted from lichens (Thallophyta) and is used to test whether a substance is acidic or basic
• Other natural indicators include turmeric, which turns reddish-brown in basic medium and becomes yellow again after washing
• Synthetic indicators such as methyl orange and phenolphthalein are also used to detect acids and bases
• Acids and bases show their properties in aqueous solutions, and their reactions form the basis of many daily life applications
• In this chapter, we will study the chemical reactions of acids and bases, how they cancel each other’s effects, and their importance in everyday life

2.1 Understanding the Chemical Properties of Acids and Bases

• Acids and bases show their chemical properties through characteristic reactions with metals, metal carbonates, metal hydrogencarbonates, and each other
• In laboratory experiments, acids such as hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃) are commonly used to study reactions
• When acids react with metals, they produce salt and hydrogen gas, for example:
  Zn + 2HCl → ZnCl₂ + H₂
• When acids react with metal carbonates or hydrogencarbonates, they produce salt, water, and carbon dioxide gas, which turns lime water milky
• Acids and bases react with each other in a neutralisation reaction to form salt and water, for example:
  HCl + NaOH → NaCl + H₂O
• Metal oxides react with acids to form salt and water, showing that metal oxides are basic in nature
• Non-metallic oxides react with bases to form salt and water, showing that non-metallic oxides are acidic in nature
• These reactions help us understand the chemical behaviour and classification of substances as acidic or basic

2.1.1 Acids and Bases in the Laboratory

• In laboratories, commonly used acids are hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃)
• Common laboratory bases include sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide [Ca(OH)₂], and ammonium hydroxide (NH₄OH)
• Acids change the colour of blue litmus to red, while bases change red litmus to blue
• Phenolphthalein remains colourless in acidic solution but turns pink in basic solution
• Methyl orange turns red in acidic medium and yellow in basic medium
• Acids and bases must be handled carefully in the laboratory because they can be corrosive and harmful
• Dilution of acids should always be done by adding acid to water slowly, not water to acid, to avoid excessive heat and splashing

2.1.2 How do Acids and Bases React with Metals?

• When acids react with metals, they generally produce a salt and hydrogen gas, which can be tested by the ‘pop’ sound when a burning matchstick is brought near it
• For example, when zinc reacts with dilute hydrochloric acid, it forms zinc chloride and hydrogen gas:
  Zn + 2HCl → ZnCl₂ + H₂
• Similarly, zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas
• Nitric acid (HNO₃) is an exception because it usually does not produce hydrogen gas; being a strong oxidising agent, it forms nitrogen oxides (NO or NO₂) instead
• However, very dilute nitric acid reacts with magnesium or manganese to produce hydrogen gas
• Strong bases like sodium hydroxide (NaOH) also react with certain metals such as zinc and aluminium to produce hydrogen gas and salts
• These reactions show that both acids and some bases can liberate hydrogen gas when reacting with suitable metals

2.1.3 How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

• Metal carbonates and metal hydrogencarbonates react with acids to produce salt, water, and carbon dioxide gas
• For example, when calcium carbonate (CaCO₃) reacts with dilute hydrochloric acid, it forms calcium chloride, water, and carbon dioxide:
  CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
• Similarly, sodium hydrogencarbonate (NaHCO₃) reacts with hydrochloric acid to produce sodium chloride, water, and carbon dioxide
• The carbon dioxide gas evolved turns lime water milky due to formation of calcium carbonate
• On passing excess carbon dioxide through lime water, the milkiness disappears due to formation of calcium hydrogencarbonate, which is soluble in water
• These reactions are commonly used to confirm the presence of carbon dioxide gas in laboratory experiments

2.1.4 How do Acids and Bases React with Each Other?

• When an acid reacts with a base, they neutralise each other and form salt and water; this reaction is called a neutralisation reaction
• For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), it forms sodium chloride and water:
  HCl + NaOH → NaCl + H₂O
• During neutralisation, the H⁺ ions from acid combine with OH⁻ ions from base to form water (H₂O)
• The remaining ions combine to form a salt, which may be neutral, acidic, or basic depending on the nature of acid and base
• Neutralisation reactions are generally exothermic, meaning heat is released
• Neutralisation is used in everyday life, such as treating indigestion with antacids, which contain mild bases

2.1.5 Reaction of Metallic Oxides with Acids

• Metallic oxides are generally basic in nature because they react with acids to form salt and water
• For example, copper(II) oxide (CuO) reacts with hydrochloric acid (HCl) to form copper chloride and water:
  CuO + 2HCl → CuCl₂ + H₂O
• In this reaction, the black copper(II) oxide dissolves to form a blue-green solution of copper chloride
• The formation of salt and water confirms the basic nature of metallic oxides
• These reactions are similar to neutralisation reactions, where a base reacts with an acid
• This property helps in classifying metallic oxides as basic oxides

2.1.6 Reaction of Non-metallic Oxides with Bases

• Non-metallic oxides are generally acidic in nature because they react with bases to form salt and water
• For example, carbon dioxide (CO₂) reacts with calcium hydroxide [Ca(OH)₂] to form calcium carbonate and water:
  CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
• In this reaction, lime water turns milky due to formation of calcium carbonate (CaCO₃)
• If excess carbon dioxide is passed through lime water, the milkiness disappears because calcium hydrogencarbonate [Ca(HCO₃)₂] is formed, which is soluble in water
• This reaction confirms that carbon dioxide is an acidic oxide
• The acidic nature of non-metallic oxides explains why they react with bases in a manner similar to acids

2.2 What do all Acids and all Bases have in Common?

• All acids produce hydrogen ions (H⁺) in aqueous solution, and all bases produce hydroxide ions (OH⁻) in aqueous solution
• The acidic or basic nature of a substance is due to the presence of these ions in water, not in dry form
• For example, hydrochloric acid (HCl) produces H⁺ ions in water, while sodium hydroxide (NaOH) produces OH⁻ ions
• Hydrogen ions do not exist freely in solution; they combine with water molecules to form hydronium ions (H₃O⁺)
• The formation of ions explains why acids and bases show their characteristic properties only in aqueous solutions
• The strength of acids and bases depends on the concentration of H⁺ or OH⁻ ions present in the solution

2.2.1 What happens to an Acid or a Base in a Water Solution?

• Acids produce H⁺ ions only in the presence of water, which is why dry hydrogen chloride gas does not show acidic properties
• When hydrogen chloride (HCl) gas dissolves in water, it forms hydronium ions (H₃O⁺):
  HCl + H₂O → H₃O⁺ + Cl⁻
• Similarly, bases produce OH⁻ ions in aqueous solution, for example:
  NaOH → Na⁺ + OH⁻
• The presence of H₃O⁺ ions is responsible for the acidic nature, while OH⁻ ions are responsible for the basic nature
• Since water helps in ion formation, acids and bases show their characteristic behaviour only in aqueous medium
• Solutions of acids and bases conduct electricity because they contain free ions, making them electrolytes

2.3 Strength of Acids and Bases: pH Scale

• The strength of an acid or base is measured using the pH scale, where pH means ‘potential of hydrogen’
• The pH scale ranges from 0 to 14, indicating how acidic or basic a solution is
• A solution with pH less than 7 is acidic, pH equal to 7 is neutral, and pH greater than 7 is basic
• The lower the pH value, the stronger the acid, and the higher the pH value, the stronger the base
• The pH value is related to the concentration of hydrogen ions (H⁺) in the solution
• Universal indicator is used to determine the pH of a solution by showing different colours at different pH values
• The pH of our body fluids is important; for example, blood has a pH around 7.4, and even small changes can be harmful
• In cases of indigestion, excess acid in the stomach is neutralised by antacids, which are mild bases such as magnesium hydroxide

2.4 More about Salts

• A salt is formed when an acid reacts with a base in a neutralisation reaction
• Salts consist of a positive ion (cation) from the base and a negative ion (anion) from the acid
• Salts can be neutral, acidic, or basic depending on the strength of the acid and base used in their formation
• The nature of a salt solution depends on the pH of the solution, which may be less than, equal to, or greater than 7
• Salts are widely used in daily life, including common salt (NaCl), which is essential for food and industrial processes
• Some salts undergo hydrolysis in water, affecting the pH of the solution
• Important chemicals such as washing soda, baking soda, and bleaching powder are prepared from salts

2.4.1 Family of Salts

• Salts formed from the same acid but different bases belong to the same family of salts
• For example, chlorides such as sodium chloride (NaCl), potassium chloride (KCl) and calcium chloride (CaCl₂) belong to the chloride family, derived from hydrochloric acid (HCl)
• Similarly, salts like sodium sulphate (Na₂SO₄) and copper sulphate (CuSO₄) belong to the sulphate family, derived from sulphuric acid (H₂SO₄)
• Salts can also be classified as acidic salts, basic salts, or neutral salts based on the strength of the parent acid and base
• A salt formed from a strong acid and strong base is generally neutral, such as NaCl
• A salt formed from a strong acid and weak base is acidic, while one formed from a strong base and weak acid is basic

2.4.2 pH of Salts

• The pH of a salt solution depends on the strength of the acid and base from which the salt is formed
• A salt formed from a strong acid and strong base, such as sodium chloride (NaCl), has a neutral pH of 7
• A salt formed from a strong acid and weak base, such as ammonium chloride (NH₄Cl), produces an acidic solution (pH < 7)
• A salt formed from a strong base and weak acid, such as sodium carbonate (Na₂CO₃), produces a basic solution (pH > 7)
• The change in pH occurs due to hydrolysis of salts in water
• Testing the pH of salt solutions helps determine whether they are acidic, basic, or neutral in nature

2.4.3 Chemicals from Common Salt

• Common salt (sodium chloride, NaCl) is an important raw material used to manufacture several useful chemicals

• When a concentrated solution of sodium chloride (brine) is electrolysed, it produces sodium hydroxide (NaOH), chlorine gas (Cl₂) and hydrogen gas (H₂); this process is called the chlor-alkali process
• The reaction involved is:
  2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
• Chlorine gas is used in the manufacture of PVC, bleaching powder, disinfectants, and CFCs, and for water purification
• Hydrogen gas is used for the manufacture of ammonia (NH₃) and in the production of hydrochloric acid (HCl)
• Sodium hydroxide (caustic soda) is used in the manufacture of soap, paper, textiles, and detergents
• From common salt, other useful chemicals such as washing soda (Na₂CO₃·10H₂O), baking soda (NaHCO₃), and bleaching powder (CaOCl₂) are also prepared

2.4 Are the Crystals of Salts Really Dry?

• Some salts contain a fixed number of water molecules as part of their crystal structure; this water is called water of crystallisation
• Water of crystallisation gives crystals their definite shape and colour
• For example, copper sulphate crystals (CuSO₄·5H₂O) are blue due to the presence of water of crystallisation
• When copper sulphate crystals are heated, they lose water of crystallisation and become white anhydrous copper sulphate (CuSO₄)
• On adding water again, the white powder regains its blue colour, showing reversible hydration
• Another example is washing soda (Na₂CO₃·10H₂O), which contains 10 molecules of water of crystallisation
• This shows that salt crystals are not completely dry; they may contain chemically combined water molecules

Exam Oriented Facts

• Acids turn blue litmus red, while bases turn red litmus blue
• Litmus is extracted from lichens (Thallophyta)
• Phenolphthalein turns pink in basic medium and remains colourless in acidic medium
• Methyl orange turns red in acid and yellow in base
• Acid + Metal → Salt + Hydrogen gas (e.g., Zn + 2HCl → ZnCl₂ + H₂)
• Nitric acid (HNO₃) usually does not produce hydrogen gas because it is a strong oxidising agent
• Acid + Metal carbonate → Salt + Water + Carbon dioxide
• Neutralisation reaction: Acid + Base → Salt + Water
• pH scale range: 0 to 14; pH < 7 acidic, pH = 7 neutral, pH > 7 basic
• Blood pH ≈ 7.4
• Chlor-alkali process: Electrolysis of brine (NaCl solution) produces NaOH, Cl₂, and H₂
• Chemical equation of chlor-alkali process:
  2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
• Water of crystallisation example: CuSO₄·5H₂O (blue crystals)
• Washing soda: Na₂CO₃·10H₂O
• Baking soda: NaHCO₃
• Bleaching powder: CaOCl₂
• Baking soda: NaHCO₃
• Bleaching powder: CaOCl₂

Understanding NCERT Class 10 Science Chapter 2 – Acids, Bases and Salts strengthens clarity about chemical reactions, pH scale, and industrial applications.

NCERT Class 10 Science Chapter 2 directly connects with practical chemistry, environmental issues, and competitive exam preparation.

Continue reading NCERT Class 10 Science Chapter 3 – Metals and Non-metals to understand reactivity and extraction of metals.

FAQs

Q1. What is NCERT Class 10 Science Chapter 2 about?
It explains properties and reactions of acids, bases, salts, and the pH scale.

Q2. What is the pH range?
The pH scale ranges from 0 to 14.

Q3. What is the chlor-alkali process?
It is the electrolysis of brine to produce sodium hydroxide, chlorine, and hydrogen.

Q4. What is water of crystallisation?
It is the fixed number of water molecules present in the crystal structure of a salt.

Q5. Why is Chapter 2 important for exams?
It forms the foundation for chemical reactions, ionic compounds, and industrial chemistry topics.

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