Acids, Bases, and Salts: ICSE Class X Study Guide

Characteristics of Acids

  • Etymology: The term 'acid' derives from the Latin acidus, meaning 'sour'.

  • Definition: Acids are compounds with one or more hydrogen (HH) atoms that produce hydronium ions (H3O+H3​O+) when dissolved in water.

  • Ionization Theory:

    • Acids furnish hydrogen ions (H+H+) upon dissolving in water.

    • H+H+ ions combine with water to produce hydronium ions.

    • Equation:

    • HCl→H++Cl−HCl→H++Cl−

    • H++H2O→H3O+H++H2​O→H3​O+

  • General Characteristics:

    1. Strong acids like HClHCl, H2SO4H2​SO4​, and HNO3HNO3​.

    2. Complete ionization in solution.

    3. Contain only ions.   

  • Carbonic Acid (H2CO3H2​CO3​): Weak mineral acid containing both ions and molecules in solution; turns blue litmus pink; non-corrosive.

  • Key Examples:

    • Hydronium ion (H3O+H3​O+) is formed when an acid dissolves in water.

    • Carbonic acid (H2CO3H2​CO3​) contains both ions and molecules.

    • Weak mineral acids include Carbonic acid and Formic acid (HCOOHHCOOH).

    • Methane (CH4CH4​) is hydrogen-containing but not an acid.

  • Commonality: Hydrogen is found in all acids.

Classification of Acids

Acids are classified based on their sources, basicity, and composition.

On the Basis of Sources

  1. Organic Acids: Obtained from plants; weak acids that do not ionize completely (e.g., Acetic acid (CH3COOHCH3​COOH), Formic acid (HCOOHHCOOH)).

  2. Inorganic Acids (Mineral Acids): Obtained from minerals (e.g., Hydrochloric acid (HClHCl), Sulphuric acid (H2SO4H2​SO4​)).

On the Basis of Composition

  1. Oxy Acids: Contain oxygen, hydrogen, and another element (e.g., Sulphuric acid (H2SO4H2​SO4​)).

  2. Hydracids: Contain hydrogen and a non-metallic element without oxygen (e.g., Hydrochloric acid (HClHCl)).

Basicity of Acids

  • Definition: Basicity is the number of hydronium ions (H3O+H3​O+) formed by the ionization of one molecule of acid in an aqueous solution.

Classification based on Basicity

  1. Monobasic Acids: Produce one hydronium ion (H3O+H3​O+) per molecule (e.g., HClHCl).

  2. Dibasic Acids: Produce two hydronium ions per molecule (e.g., H2SO4H2​SO4​).

  3. Tribasic Acids: Produce three hydronium ions per molecule (e.g., H3PO4H3​PO4​).

Preparation of Acids

  1. By Synthesis: Direct combination of elements (e.g., H2+Cl2→2HClH2​+Cl2​→2HCl).

  2. By Action of Water: On non-metallic oxides for oxy-acids (e.g., SO3+H2O→H2SO4SO3​+H2​O→H2​SO4​).

  3. By Oxidation of Non-metals: (e.g., S+6HNO3→H2SO4+2H2O+6NO2S+6HNO3​→H2​SO4​+2H2​O+6NO2​).

  4. By Displacement: (e.g., NaCl+H2SO4→NaHSO4+HClNaCl+H2​SO4​→NaHSO4​+HCl).

Bases and Alkalis

  • Definition: Bases are ionic compounds that form hydroxide ions (OH−OH−) when dissolved in water, reacting with H3O+H3​O+ to produce salt and water.

  • Alkali: A soluble base that produces hydroxyl (OH−OH−) ions in solution (e.g., KOHKOH, NaOHNaOH).

Classification of Bases

  1. On the Basis of Strength:

    • Strong Bases: Almost completely dissociate in water (e.g., NaOHNaOH).

    • Weak Bases: Partially dissociate (e.g., NH4OHNH4​OH).

  2. On the Basis of Acidity:

    • Monoacidic: Produce one OH−OH− per molecule (e.g., NaOHNaOH).

    • Diacidic: Produce two OH−OH− (e.g., Ca(OH)2Ca(OH)2​).

    • Triacidic: Produce three OH−OH− (e.g., Al(OH)3Al(OH)3​).

Salts and Methods of Preparing Salts

  • Definition: A substance obtained by the partial or complete replacement of the ionizable hydrogen atoms of an acid by a metallic or ammonium ion.

  • Classification of Salts:

    1. Normal Salts: Complete replacement of ionizable hydrogen atoms (e.g., NaClNaCl).

    2. Acid Salts: Partial replacement, giving H3O+H3​O+ in solution (e.g., NaHSO4NaHSO4​).

    3. Basic Salts: Partial replacement of hydroxyl groups (e.g., [Cu(OH)Cl][Cu(OH)Cl]).

    4. Double Salts: Formed by two different cations or anions.

    5. Mixed Salts: Consist of multiple ions.

    6. Complex Salts: Contain complex ions.

  • Methods of Preparing Salts:

    1. Direct Combination: Heating elements (e.g., 2Fe+3Cl2→2FeCl32Fe+3Cl2​→2FeCl3​).

    2. Simple Displacement: Active metals with dilute acids (e.g., Zn+H2SO4→ZnSO4+H2Zn+H2​SO4​→ZnSO4​+H2​).

    3. Precipitation (Double Decomposition): Exchange radicals to form a precipitate (e.g., BaCl2+H2SO4→BaSO4+2HClBaCl2​+H2​SO4​→BaSO4​+2HCl).

    4. Neutralization (Insoluble Base): Acid with insoluble bases (e.g., MgO+2HCl→MgCl2+H2OMgO+2HCl→MgCl2​+H2​O).

    5. Neutralization (Alkali): Acid neutralized by soluble base (e.g., NaOH+HCl→NaCl+H2ONaOH+HCl→NaCl+H2​O).

    6. Decomposition of Carbonates: Action of acids on carbonates (e.g., CuCO3+2HCl→CuCl2+H2O+CO2CuCO3​+2HCl→CuCl2​+H2​O+CO2​).

Solubility Rules for Salts

  • Soluble: All ammonium, sodium, and potassium salts; all nitrates and nitrites.

  • Exceptions: Most chlorides, bromides, and iodides are soluble except for Hg2Cl2Hg2​Cl2​, AgClAgCl, and PbCl2PbCl2​ (soluble in hot water).

  • Most sulphates: Soluble except for BaSO4BaSO4​ and PbSO4PbSO4​.

  • Insoluble: Most carbonates, sulphides, sulphites, and phosphates except for ammonium, sodium, and potassium.

  • Most metallic oxides and hydroxides: Insoluble except sodium, potassium, and ammonium; Ca(OH)2Ca(OH)2​ is slightly soluble.