7 - Acid, bases and salts

Acids and Bases

Introducing Acids

• Definition: Acids are substances with a pH level ranging from 0 to 7, indicating their acidic nature.

• Key Component: The presence of hydrogen ions (H⁺) characterizes acids.

• Functionality: Acids are classified as proton donors, donating protons in chemical reactions, resulting in increased H⁺ concentration in solutions.

Properties of Acids

• Taste:

  • Generally sour.

  • Common examples:

    • Citric acid (lemons)

    • Acetic acid (vinegar)

• Reactivity:

  • Reactions with Carbonates: Produces salt, CO₂, and H₂O.

  • Neutralization: React with bases to form salt and water.

  • Reactions with Metals: Yield salt and hydrogen gas.

Strong and Weak Acids

Introducing Bases

• Definition: Bases have a pH level ranging from 7 to 14, indicating their basic nature.

• Key Component: The presence of hydroxide ions (OH⁻) characterizes bases.

• Functionality: Bases are classified as proton acceptors, accepting protons in chemical reactions.

Properties of Bases

• Taste:

  • Generally bitter.

  • Slippery feel (e.g., baking soda).

• Reactivity:

  • Reaction with Acids: Neutralizes to form salt and water.

  • Indicators:

    • Neutral litmus paper turns blue in basic solutions.

    • Methyl orange turns yellow in basic conditions.

    • Thymolphthalein turns blue in basic conditions.

Indicators and Oxides

• Oxides:

  • Acidic Oxides: Exhibit similar reactions to acids.

    • Examples:

      • Sulfur dioxide (SO₂)

      • Carbon dioxide (CO₂)

  • Basic Oxides: Behave like bases in reactions.

    • Examples:

      • Copper(II) oxide (CuO)

      • Calcium oxide (CaO)

  • Amphoteric Oxides: Exhibit both properties.

    • Examples:

      • Aluminum oxide (Al₂O₃)

      • Zinc oxide (ZnO)

The pH Scale and Universal Indicator

• Definition: The pH scale measures acidity or alkalinity, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral.

• Universal Indicator: A mixture of indicators showing different colors at different pH levels, indicating if a solution is acidic, neutral, or basic.

Preparation of Salts

Solubility Rules for Salts

• Salts:

• Soluble Salts:

  • Nitrates: All are soluble.

  • Chlorides: Soluble except lead(II) and silver chlorides.

  • Sulfates: Generally soluble, except barium, calcium, and lead sulfates.

• Insoluble Salts:

  • Most carbonates and hydroxides are insoluble, except for SPAN salts (sodium, potassium, ammonium).

Methods of Preparation

Key Terms

• Hydrated Substance: A substance with chemically combined water molecules.

• Anhydrous Substance: A chemical compound that lacks water in its structure.

• Water of Crystallisation: Water molecules that are part of a salt's crystalline structure.