This guide covers the properties of acids, bases, and salts, the concept of pH, and the crucial reaction of neutralization. Understanding the definitions and reactions is key to success in the Chemistry section of your exam.

1. Acids

Short Notes & Key Concepts

• Definition: An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water.

• Properties of Acids:

  • Have a sour taste (e.g., lime juice).

  • Are corrosive; can damage skin, cloth, and metal.

  • Turn blue litmus paper red.

  • Have a pH value less than 7.

  • Reaction with Metals: React with metals above hydrogen in the reactivity series to produce a salt and hydrogen gas.

    • Acid + Metal → Salt + Hydrogen Gas

    • e.g., 2HCl + Mg → MgCl₂ + H₂

  • Reaction with Carbonates: React with carbonates and hydrogencarbonates to produce a salt, water, and carbon dioxide gas.

    • Acid + Carbonate → Salt + Water + Carbon Dioxide

    • e.g., 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂

• Strong vs. Weak Acids:

  • Strong Acids: Completely ionize (break apart) in water to release all their H⁺ ions. Examples: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄).

  • Weak Acids: Only partially ionize in water, releasing a small portion of their H⁺ ions. Examples: Acetic acid (CH₃COOH - vinegar), Carbonic acid (H₂CO₃).

2. Bases and Alkalis

Short Notes & Key Concepts

• Definition: A base is a substance that reacts with an acid to form a salt and water only. Many bases are metal oxides or metal hydroxides.

• Alkali: An alkali is a base that is soluble in water. It produces hydroxide ions (OH⁻) when dissolved in water. All alkalis are bases, but not all bases are alkalis.

• Properties of Alkalis/Bases:

  • Have a bitter taste.

  • Feel soapy or slimy to the touch.

  • Turn red litmus paper blue.

  • Have a pH value greater than 7.

• Strong vs. Weak Alkalis:

  • Strong Alkalis: Completely ionize in water to release all their OH⁻ ions. Examples: Sodium hydroxide (NaOH), Potassium hydroxide (KOH).

  • Weak Alkalis: Only partially ionize in water. Example: Ammonia solution (NH₄OH).

3. The pH Scale & Indicators

• pH Scale: A numerical scale from 0 to 14 that measures the acidity or alkalinity of a solution.

  • pH < 7 = Acidic (The lower the number, the stronger the acid).

  • pH = 7 = Neutral (e.g., pure water).

  • pH > 7 = Alkaline (The higher the number, the stronger the alkali).

• Indicators: Substances that show different colors in acidic and alkaline solutions.

4. Neutralization & Salts

• Neutralization: The reaction between an acid and a base (or alkali) to form a salt and water. The acidity and alkalinity cancel each other out.

  • General Word Equation: Acid + Base → Salt + Water

  • Core Ionic Equation: H⁺(aq) + OH⁻(aq) → H₂O(l)

• What is a Salt? A salt is an ionic compound formed when the hydrogen ion (H⁺) from an acid is replaced by a metal ion or an ammonium ion (NH₄⁺).

• Practical Applications of Neutralization: This is a very common exam topic!

  • Indigestion: Using an antacid (a weak base like magnesium hydroxide) to neutralize excess stomach acid (HCl).

  • Agriculture: Adding lime (calcium oxide/hydroxide, which are basic) to neutralize acidic soil.

  • Bee Stings: Bee stings are acidic. They can be neutralized by a weak alkali like baking soda (sodium bicarbonate).

  • Wasp Stings: Wasp stings are alkaline. They can be neutralized by a weak acid like vinegar.

Exam Tips & Tricks

1. Memorize the Properties: The properties of acids and bases (taste, feel, effect on litmus, pH range) are fundamental and frequently asked.

2. Know the Three Key Reactions of Acids: Acid + Metal, Acid + Carbonate, and Acid + Base. Be ready to state the products for each. The gas produced is a common question (H₂ from metals, CO₂ from carbonates).

3. Neutralization is Key: Understand that H⁺ + OH⁻ → H₂O is the essential reaction. The practical applications (indigestion, soil treatment, stings) are very popular exam questions.

4. Indicator Colours: You must know the colour changes for litmus, methyl orange, and phenolphthalein. They are often used in questions about titrations or identifying unknown solutions.

5. Strong vs. Weak: The key difference is the degree of ionization (complete vs. partial). This is a good way to earn higher marks in an explanation question.