Chapter 7 of the CIE IGCSE Chemistry curriculum delves into the fascinating world of acids, bases, and salts, exploring their characteristic properties and interactions with various substances. This chapter serves as a crucial foundation for students, providing insights into the fundamental reactions that define acidic and alkaline substances.
From the reactions of acids with metals, bases, and carbonates to the distinct effects of these substances on indicators, the chapter offers a comprehensive exploration of the behaviour of acids and bases in different chemical scenarios. Additionally, it sheds light on the presence of hydrogen and hydroxide ions in aqueous solutions, introducing students to the pH scale and the neutralisation reactions that occur between acids and alkalis.
Understanding these principles is pivotal for students as they progress in their IGCSE chemistry studies.
1. Characteristic Properties of Acids:
(a) Reactions with Metals:
- Characteristic Reaction: Acids react with metals to produce hydrogen gas and a metal salt.
- Example:
- 2HCl+Zn→ZnCl2+H2
(b) Reactions with Bases:
- Characteristic Reaction: Acids react with bases to form salts and water in a neutralisation reaction.
- Example:
- 2HNO3+Ca(OH)2→Ca(NO3)2+2H2O
(c) Reactions with Carbonates:
- Characteristic Reaction: Acids react with carbonates to produce carbon dioxide gas, water, and salt.
- Example:
- 2HCl+CaCO3→CaCl2+CO2+H2O
2. Acids and Their Effect on Indicators:
(a) Effect on Litmus:
- Characteristics: Acids turn blue litmus paper red.
- Explanation: The acidic solution donates protons (H⁺ ions) to litmus dye, causing a colour change.
(b) Effect on Thymolphthalein:
- Characteristics: Thymolphthalein remains colourless in acidic solutions.
- Explanation: The acidic solution does not cause the indicator to change colour.
(c) Effect on Methyl Orange:
- Characteristics: Methyl orange turns red in acidic solutions.
- Explanation: The acidic solution donates protons, leading to the colour change of methyl orange.
3. Properties of Bases:
(a) Reactions with Acids:
- Characteristic Reaction: Bases react with acids to form salts and water in a neutralisation reaction.
- Example:
- 2KOH+H2SO4→K2SO4+2H2O
(b) Reactions with Ammonium Salts:
- Characteristic Reaction: Bases react with ammonium salts to produce ammonia gas, water, and salt.
- Example:
- 2NaOH+(NH4)2SO4→Na2SO4+2H2O+2NH3
4. Alkalis and Their Effect on Indicators:
(a) Effect on Litmus:
- Characteristics: Alkalis turn red litmus paper blue.
- Explanation: The alkaline solution accepts protons (H⁺ ions) from litmus dye, leading to a colour change.
(b) Effect on Thymolphthalein:
- Characteristics: Thymolphthalein turns blue in alkaline solutions.
- Explanation: The alkaline solution causes thymolphthalein to change colour.
(c) Effect on Methyl Orange:
- Characteristics: Methyl orange turns yellow in alkaline solutions.
- Explanation: The alkaline solution donates protons, causing the colour change in methyl orange.
5. Hydrogen and Hydroxide Ions:
- Statement: Aqueous solutions of acids contain H+ ions, and aqueous solutions of alkalis contain OH− ions.
- Explanation: In water, acids ionise to produce
- H+ ions, while alkalis ionise to produce OH− ions.
Supplement: pH and Neutralization:
6. Comparing Hydrogen Ion Concentration, Neutrality, Acidity, and Alkalinity:
- pH Scale: Universal indicator paper can be used to compare the acidity and alkalinity of solutions based on the pH scale and colour changes. Lower pH values indicate acidity (red to orange), neutrality is green, and higher pH values indicate alkalinity (green to violet).
7. Neutralisation Reaction:
- Reaction: The neutralisation reaction between an acid and an alkali produces water:
- H+(aq)+OH−(aq)→H2O(l)
- Explanation: The H+ ions from the acid combine with the OH− ions from the alkali form water in a neutralisation reaction.
Understanding these detailed properties of acids and bases, along with their interactions with indicators, is fundamental for IGCSE chemistry.
Classifying Oxides:
Acidic Oxides:
- Definition: Acidic oxides are oxides that, when dissolved in water, produce an acidic solution.
- Metallic Character: Typically associated with non-metallic elements.
- Examples:
- SO2 (Sulphur Dioxide):
SO2+H2O→H2SO
- CO2 (Carbon Dioxide):
CO2+H2O→H2CO3
Basic Oxides:
- Definition: Basic oxides are oxides that, when dissolved in water, produce a basic or alkaline solution.
- Metallic Character: Typically associated with metallic elements.
- Examples:
- CuO (Copper(II) Oxide):
CuO+H2O→Cu(OH)2
- CuO (Copper(II) Oxide):
- CaO (Calcium Oxide):
CaO+H2O→Ca(OH)2
Metallic and Non-Metallic Character:
- Acidic Oxides: Often associated with non-metallic elements, as nonmetals tend to form acidic oxides.
- Basic Oxides: Typically associated with metallic elements, as metals tend to form basic oxides.
Explanation:
Acidic Oxides:
- SO₂ (Sulphur Dioxide):
- Nature: Sulphur is a non-metal, and is a non-metallic oxide.
- Reaction:
- SO2+H2O→H2SO3
- Explanation: Sulphur dioxide dissolves in water to form sulphurous acid, indicating its acidic nature.
- CO₂ (Carbon Dioxide):
- Nature: Carbon is a non-metal, and
- CO2 is a non-metallic oxide.
- Reaction:
- CO2+H2O→H2CO3
- Explanation: Carbon dioxide dissolves in water to form carbonic acid, indicating its acidic nature.
Basic Oxides:
- CuO (Copper(II) Oxide):
- Nature: Copper is a metal, and
- CuO is a metallic oxide.
- Reaction:
- CuO+H2O→Cu(OH)2
- Explanation: Copper(II) oxide dissolves in water to form copper hydroxide, indicating its basic nature.
- CaO (Calcium Oxide):
- Nature: Calcium is a metal, and
- CaO
- CaO is a metallic oxide.
- Reaction:
- CaO+H2O→Ca(OH)2
- Explanation: Calcium oxide dissolves in water to form calcium hydroxide, indicating its basic nature.
Significance for IGCSE:
- Understanding the classification of oxides as acidic or basic based on their metallic and non-metallic character is essential for predicting their behaviour when dissolved in water. This knowledge helps students comprehend various chemical reactions and their outcomes.
1. Preparation, Separation, and Purification of Soluble Salts
CIE IGCSE Chemistry Topic 8: Acids, Bases and Salts Revision – PMT
(a) Alkali by Titration:
- Preparation: Acid reacts with alkali (base) in a controlled manner, using titration, until neutralisation occurs. The resulting solution is evaporated to obtain the salt.
- Separation and Purification: The salt solution is separated from excess reactants through filtration or decantation. Purification involves crystallisation from the solution.
(b) Excess Metal:
- Preparation: Acid reacts with an excess of metal. The excess metal is filtered, and the filtrate contains the salt. The solution is evaporated to obtain the salt.
- Separation and Purification: Filtration separates the excess metal. Crystallisation purifies the salt.
(c) Excess Insoluble Base:
- Preparation: Acid reacts with an excess of an insoluble base (e.g., metal oxide). The excess base is filtered, and the filtrate contains the salt. The solution is evaporated to obtain the salt.
- Separation and Purification: Filtration removes the excess base. Crystallisation purifies the salt.
(d) Excess Insoluble Carbonate:
- Preparation: Acid reacts with an excess of an insoluble carbonate. The excess carbonate is filtered, and the filtrate contains the salt. The solution is evaporated to obtain the salt.
- Separation and Purification: Filtration removes the excess carbonate. Crystallisation purifies the salt.
2. General Solubility Rules for Salts:
(a) Sodium, Potassium, and Ammonium Salts:
- Rule: Sodium (Na⁺), potassium (K⁺), and ammonium (NH₄⁺) salts are soluble in water.
(b) Nitrates:
- Rule: Nitrates (NO₃⁻) are soluble in water.
(c) Chlorides:
- Rule: Chlorides (Cl⁻) are soluble, except for lead (Pb²⁺) and silver (Ag⁺) chlorides.
(d) Sulfates:
- Rule: Sulfates (SO₄²⁻) are soluble, except for barium (Ba²⁺), calcium (Ca²⁺), and lead (Pb²⁺) sulphates.
(e) Carbonates:
- Rule: Carbonates (CO₃²⁻) are insoluble, except for sodium (Na⁺), potassium (K⁺), and ammonium (NH₄⁺) carbonates.
(f) Hydroxides:
- Rule: Hydroxides (OH⁻) are insoluble, except for sodium (Na⁺), potassium (K⁺), ammonium (NH₄⁺), and partially calcium (Ca²⁺) hydroxides.
- Hydrated and Anhydrous Substances:
Hydrated Substance:
- Definition: A hydrated substance is a compound that is chemically combined with water molecules. The water molecules are present as part of the compound’s crystal structure.
- Example: CuSO₄⋅5H₂O (copper(II) sulphate pentahydrate) has five water molecules per formula unit.
Anhydrous Substance:
- Definition: An anhydrous substance is a compound that contains no water molecules in its crystal structure.
- Example: CuSO₄ (copper(II) sulphate anhydrous) has no water molecules in its crystal structure.
Explanation:
- Hydrated substances have water molecules as an integral part of their crystal lattice. Anhydrous substances lack water molecules and are obtained by removing water from hydrated forms, often through heating or dehydration.
Understanding these methods of preparing, separating, and purifying soluble salts, along with the general solubility rules for salts, is fundamental for practical chemistry at the IGCSE level.
Summary:
In summary, Chapter 7 offers an in-depth examination of the characteristic properties of acids, bases, and salts. Students explore how acids react with metals, bases, and carbonates, witnessing the formation of hydrogen gas, salts, and carbon dioxide. The impact of these substances on indicators, such as litmus paper, thymolphthalein, and methyl orange, provides a tangible understanding of their acidic or alkaline nature. The chapter also covers the reactions of bases, including their neutralisation reactions with acids and the formation of ammonia gas when reacting with ammonium salts.
The distinction between acids and bases in terms of hydrogen and hydroxide ions is highlighted, contributing to a foundational understanding of the pH scale and neutralisation reactions.
Overall, the chapter equips students with essential knowledge for comprehending the behaviour of acids and bases, paving the way for more advanced studies in chemistry.
