10.3 Characteristic Properties of Acids

This topic explains the general properties of acids — how they taste, how they behave with litmus, and how they react with metals, metal carbonates, and bases.

Definition

• Taste sour
• Turn blue litmus red
• React with metals to produce hydrogen gas
• React with metal carbonates to produce carbon dioxide
• React with bases to form salt and water

Simple explanation: Acids have a set of characteristic properties that make them recognizable and useful.

Main Idea

The book lists five general properties of acids:

• Acids have sour taste.
• Acids change the colour of blue litmus paper to red.
• Acids react with most metals and corrode them, forming salts and hydrogen gas.
• Acids react with metal carbonates to form salts and water, with the liberation of carbon dioxide.
• Acids combine with bases to produce salt and water — this is called a neutralization reaction.

Simple explanation: Acids taste sour, turn blue litmus red, and react with metals, carbonates, and bases.

Core Concept

Summary Table

Detailed Explanation

Property 1: Sour Taste

Acids have sour taste.

• Lemon juice (citric acid) — sour
• Vinegar (acetic acid) — sour
• Tamarind (tartaric acid) — sour

Simple explanation: The sour taste of lemon, vinegar, and tamarind is due to acids.

Property 2: Effect on Litmus Paper

Acids change the colour of blue litmus paper to red.

• Blue litmus → red in acid
• Red litmus → no change in acid

Simple explanation: Blue litmus paper is used to test for acids — it turns red.

Property 3: Reaction with Metals

Acids react with most metals and corrode them. Acids combine with metals like zinc, magnesium, aluminium, and calcium to form their salts and hydrogen.

The hydrogen gas is liberated in the form of bubbles.

General equation:

Metal + Acid → Salt + Hydrogen

Examples from the book:

Zinc reacts with hydrochloric acid:

Zn + HCl → ZnCl₂ + H₂

Magnesium reacts with sulphuric acid:

Mg + H₂SO₄ → MgSO₄ + H₂

Simple explanation: Metals react with acids to produce salt and hydrogen gas (bubbles).

Property 4: Reaction with Metal Carbonates

Acids react with metal carbonates to form salts and water, with the liberation of carbon dioxide.

The liberated carbon dioxide forms bubbles in water.

General equation:

Metal carbonate + Acid → Salt + Water + Carbon dioxide

Examples from the book:

Hydrochloric acid reacts with sodium carbonate:

Na₂CO₃ + HCl → NaCl + H₂O + CO₂

Sulphuric acid reacts with calcium carbonate (limestone or marble):

CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Industrial use: This reaction is used in the industrial preparation of glass, paper, and soap.

Simple explanation: Acids react with carbonates to produce salt, water, and carbon dioxide gas (fizzing).

Property 5: Reaction with Bases (Neutralization)

Acids combine with bases to produce salt and water. This reaction is called a neutralization reaction.

General equation:

Acid + Base → Salt + Water

Examples from the book:

Hydrochloric acid neutralizes sodium hydroxide:

HCl + NaOH → NaCl + H₂O

Sulphuric acid combines with potassium hydroxide:

H₂SO₄ + KOH → K₂SO₄ + H₂O

Simple explanation: Acids and bases cancel each other out to form salt and water — this is neutralization.

Note on Acid Rain

The book mentions:

> "Normal rain is slightly acidic due to dissolved carbon dioxide."

This is because CO₂ in the air dissolves in rainwater to form carbonic acid (H₂CO₃), making normal rain slightly acidic (pH ~5.6).

Simple explanation: Even normal rain is slightly acidic because of CO₂ from the air.

Intermediate Level

Why Acids Have These Properties

Acids produce H⁺ ions in water. These H⁺ ions are responsible for the characteristic properties:

• Sour taste — H⁺ ions interact with taste receptors.
• Litmus change — H⁺ ions change the dye colour.
• Reaction with metals — H⁺ ions oxidize metals, producing H₂ gas.
• Reaction with carbonates — H⁺ ions react with CO₃²⁻ to form CO₂ and H₂O.
• Neutralization — H⁺ ions react with OH⁻ ions to form H₂O.

Simple explanation: It's the H⁺ ions that give acids all their characteristic properties.

Balancing the Reactions

Reaction with metals:

Zn + 2HCl → ZnCl₂ + H₂

Reaction with carbonates:

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Neutralization:

HCl + NaOH → NaCl + H₂O

Note: Always balance the equations to satisfy the law of conservation of mass.

Industrial Applications

Simple explanation: These reactions are used in many industries and everyday products.

Concept Assessment Exercise 10.2 (from the book)

1. Write equations showing the ionization of the following as Arrhenius acids.

(a) HI(aq) — hydroiodic acid

HI → H⁺ + I^-

(b) HNO₂(aq) — nitrous acid

HNO₂ ⇌ H⁺ + NO₂^-

2. Write balanced chemical equations for the following chemical reactions.

(i) Sulphuric acid + Magnesium hydroxide → Magnesium sulphate + Water

H₂SO₄ + Mg(OH)₂ → MgSO₄ + 2H₂O

(ii) Sulphuric acid + Sodium carbonate → Sodium sulphate + Water + Carbon dioxide

H₂SO₄ + Na₂CO₃ → Na₂SO₄ + H₂O + CO₂

(iii) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

2HCl + Mg → MgCl₂ + H₂

3. Ammonium hydroxide and nitric acid react and produce ammonium nitrate and water. Write the balanced chemical equation.

NH₄OH + HNO₃ → NH₄NO₃ + H₂O

Important Points

• - Acids have sour taste.
• - Acids turn blue litmus red.
• - Acids react with metals to form salt and hydrogen gas.
• - Acids react with metal carbonates to form salt, water, and carbon dioxide.
• - Acids react with bases to form salt and water (neutralization).
• - General equations:
• - Metal + Acid → Salt + Hydrogen
• - Metal carbonate + Acid → Salt + Water + CO₂
• - Acid + Base → Salt + Water
• - Examples:
• - Zn + HCl → ZnCl₂ + H₂
• - Mg + H₂SO₄ → MgSO₄ + H₂
• - Na₂CO₃ + HCl → NaCl + H₂O + CO₂
• - CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
• - HCl + NaOH → NaCl + H₂O
• - H₂SO₄ + KOH → K₂SO₄ + H₂O
• - Normal rain is slightly acidic due to dissolved CO₂.

Remember

• - Acids = sour taste.
• - Blue litmus → red in acids.
• - Metal + Acid → Salt + H₂.
• - Carbonate + Acid → Salt + H₂O + CO₂.
• - Acid + Base → Salt + H₂O (neutralization).
• - Examples:
• - Zn + HCl → ZnCl₂ + H₂
• - Na₂CO₃ + HCl → NaCl + H₂O + CO₂
• - HCl + NaOH → NaCl + H₂O

Common Mistakes

• 1. Thinking acids turn red litmus blue. Acids turn blue litmus red; bases turn red litmus blue.
• 2. Forgetting that acids react with metals to produce H₂ gas. Not all metals react — but many do.
• 3. Confusing the products of acid-carbonate reactions. Products are salt + water + CO₂.
• 4. Thinking neutralization produces only salt. It produces salt and water.
• 5. Forgetting to balance equations. Always check that atoms are balanced.
• 6. Thinking all acids react the same way. Strong and weak acids react differently in terms of rate and completeness.

Quick Revision

• General properties of acids:
• Metal + Acid → Salt + Hydrogen
• Zn + HCl → ZnCl₂ + H₂
• Mg + H₂SO₄ → MgSO₄ + H₂
• Metal carbonate + Acid → Salt + Water + Carbon dioxide
• Na₂CO₃ + HCl → NaCl + H₂O + CO₂
• CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
• Acid + Base → Salt + Water
• HCl + NaOH → NaCl + H₂O
• H₂SO₄ + KOH → K₂SO₄ + H₂O
• Normal rain is slightly acidic due to dissolved carbon dioxide.

60-Second Revision

• Acids = sour taste.
• Blue litmus → red in acids.
• Metal + Acid → Salt + H₂.
• Carbonate + Acid → Salt + H₂O + CO₂.
• Acid + Base → Salt + H₂O (neutralization).
• Warning: acids turn blue litmus red, not red litmus blue.