Acids, Bases and Salts

Overview

Acids, Bases and Salts forms a foundational chapter in chemistry for CG TET Paper II, connecting everyday substances with chemical principles. This topic carries direct questions in the science section and also links to related areas like chemical reactions, metals and non-metals, and environmental chemistry.

For the exam, you must understand the defining properties of acids and bases, the pH scale for measuring acidity and alkalinity, and how salts are formed through neutralisation reactions. Questions typically test identification of acidic/basic substances, pH values of common solutions, and practical applications like antacids, baking soda uses, and industrial processes. Mastering this topic also helps in understanding soil chemistry, water treatment, and biological processes—areas that appear in EVS-linked questions.

The key is to move beyond rote memorisation toward understanding why acids taste sour, why bases feel soapy, and how the hydrogen ion concentration determines the nature of a solution.

Key Concepts

• **Acids release H⁺ ions** in water—this hydrogen ion is responsible for all acidic properties including sour taste, reaction with metals, and colour change with indicators.

• **Bases release OH⁻ ions** in water—the hydroxide ion causes bitter taste, soapy feel, and turns red litmus blue.

• **Arrhenius definition**: Acids produce H⁺ in aqueous solution; bases produce OH⁻. This is the definition most relevant for school-level chemistry.

• **Brønsted-Lowry concept**: Acids are proton donors; bases are proton acceptors. Useful for understanding reactions beyond water.

• **pH scale** runs from 0 to 14—values below 7 are acidic, exactly 7 is neutral, above 7 is basic/alkaline.

• **Neutralisation reaction**: Acid + Base → Salt + Water. This is an exothermic reaction where H⁺ and OH⁻ combine to form water.

• **Salts are ionic compounds** formed from the cation of a base and the anion of an acid. Their properties depend on the parent acid and base.

• **Indicators** are substances that show different colours in acidic and basic solutions—litmus, phenolphthalein, and methyl orange are common examples.

Formulas / Key Facts

**pH Definition**: pH = -log₁₀[H⁺], where [H⁺] is hydrogen ion concentration in moles per litre. Lower pH means higher acidity.

**Neutralisation**: HCl + NaOH → NaCl + H₂O (general pattern: Acid + Base → Salt + Water)

**Reaction of acid with metal**: Zn + 2HCl → ZnCl₂ + H₂ (hydrogen gas evolves)

**Reaction of acid with carbonate**: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ (carbon dioxide evolves)

**Reaction of acid with hydrogen carbonate**: NaHCO₃ + HCl → NaCl + H₂O + CO₂

**Common pH values to remember**:

• Gastric juice: 1–2 (strongly acidic)
• Lemon juice: 2.2
• Vinegar: 2.5–3
• Pure water: 7 (neutral)
• Blood: 7.35–7.45 (slightly basic)
• Milk of magnesia: 10
• Household ammonia: 11–12
• Sodium hydroxide solution: 13–14

**Strong acids**: HCl (hydrochloric), H₂SO₄ (sulphuric), HNO₃ (nitric)—dissociate completely

**Weak acids**: CH₃COOH (acetic), H₂CO₃ (carbonic)—dissociate partially

**Strong bases**: NaOH, KOH—dissociate completely

**Weak bases**: NH₄OH (ammonium hydroxide), Mg(OH)₂—dissociate partially

Worked Examples

**Example 1: Identifying acid or base using litmus**

*Question*: A solution turns blue litmus red but does not affect red litmus. What is the nature of the solution?

*Solution*:

• Blue litmus turning red indicates acidic nature
• Red litmus remaining red confirms it is not basic
• Therefore, the solution is acidic

**Example 2: pH calculation concept**

*Question*: If the hydrogen ion concentration of a solution is 0.001 M, what is its pH?

*Solution*:

• [H⁺] = 0.001 M = 10⁻³ M
• pH = -log₁₀[H⁺] = -log₁₀(10⁻³) = -(-3) = 3
• pH = 3, which means the solution is acidic

**Example 3: Neutralisation reaction**

*Question*: What happens when dilute sulphuric acid reacts with sodium hydroxide? Write the balanced equation.

*Solution*:

• This is a neutralisation reaction
• H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
• Products: Sodium sulphate (salt) and water
• The reaction is exothermic (releases heat)

**Example 4: Practical application**

*Question*: Why is baking soda used to treat ant bites?

*Solution*:

• Ant sting contains formic acid (methanoic acid)
• Baking soda (NaHCO₃) is a mild base
• Applying baking soda neutralises the acidic effect of formic acid
• This reduces pain and irritation

Common Mistakes

**Confusing pH values** → Students think higher pH means more acidic. Correction: Higher pH means more basic. pH 2 is more acidic than pH 5.

**Mixing up litmus colour changes** → Blue litmus turns red in acid (not the reverse). Memory aid: "Acid = A for Angry = Red colour".

**Assuming all salts are neutral** → Students forget that salts of strong acid + weak base (like NH₄Cl) are acidic, and salts of weak acid + strong base (like Na₂CO₃) are basic. Only salts of strong acid + strong base (like NaCl) are truly neutral.

**Forgetting water in neutralisation** → Students write Acid + Base → Salt but forget that water is always a product of neutralisation.

**Confusing concentration with strength** → Concentrated HCl and dilute HCl are both strong acids. Strength refers to degree of dissociation, not amount dissolved.

**Ignoring practical applications** → Many questions ask about real-life uses—antacids for acidity, lime for acidic soil, chloride of lime for water treatment. These are frequently tested.

Quick Reference

• Acids: sour taste, pH < 7, turn blue litmus red, release H₂ with metals

• Bases: bitter taste, pH > 7, turn red litmus blue, soapy feel

• pH 7 = neutral; pH < 7 = acidic; pH > 7 = basic

• Neutralisation always produces salt + water and releases heat

• Universal indicator shows full pH range through colour spectrum (red → green → violet)

• Antacids contain Mg(OH)₂ or NaHCO₃ to neutralise excess stomach acid