Acids, Bases and Salts

Overview

Acids, bases and salts form one of the most fundamental chapters in chemistry for Assam TET Paper II. This topic builds the foundation for understanding chemical reactions, everyday substances and industrial processes. Questions typically test your ability to identify acids and bases by their properties, understand the pH scale, write neutralisation reactions and recognise common salts with their uses.

For the Assam TET, expect 2–4 questions from this topic. The examiner often links this chapter to real-life applications — why turmeric changes colour with soap, why antacids relieve acidity, or why farmers add lime to acidic soil. Mastering the properties, indicators and the pH concept is essential for scoring well.

Key Concepts

• **Acids release hydrogen ions (H⁺)** when dissolved in water. They taste sour, turn blue litmus red and react with metals to release hydrogen gas.

• **Bases release hydroxide ions (OH⁻)** in water. They taste bitter, feel soapy, turn red litmus blue and react with acids to form salt and water.

• **Neutralisation** is the reaction between an acid and a base producing salt and water: Acid + Base → Salt + Water. The H⁺ and OH⁻ ions combine to form water.

• **pH scale** measures the concentration of H⁺ ions on a scale of 0 to 14. pH 7 is neutral, below 7 is acidic, above 7 is basic/alkaline.

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

• **Salts** are ionic compounds formed by neutralisation. They consist of a cation from the base and an anion from the acid.

• **Strong acids and bases** ionise completely in water (HCl, NaOH), while **weak acids and bases** ionise partially (acetic acid, ammonia).

• **pH of body fluids matters** — stomach acid has pH around 1.5–2, blood maintains pH 7.35–7.45, and any imbalance causes health problems.

Formulas / Key Facts

| Formula/Fact | Context | |--------------|---------| | pH = −log₁₀[H⁺] | Mathematical definition of pH (conceptual understanding enough for TET) | | pH + pOH = 14 | Relationship between pH and pOH in aqueous solutions at 25°C | | HCl + NaOH → NaCl + H₂O | Neutralisation of hydrochloric acid with sodium hydroxide | | 2HCl + Zn → ZnCl₂ + H₂↑ | Acid reacting with metal releases hydrogen gas | | Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ | Acid reacting with carbonate releases carbon dioxide | | CaO + H₂O → Ca(OH)₂ | Quick lime reacting with water to form slaked lime (base) | | pH of pure water = 7 | Neutral solution at 25°C | | pH of gastric juice ≈ 1.5–2 | Highly acidic, contains HCl | | pH of blood ≈ 7.35–7.45 | Slightly alkaline, tightly regulated |

**Common Acids and Their Sources:**

• Hydrochloric acid (HCl) — stomach, laboratory
• Sulphuric acid (H₂SO₄) — car batteries, industrial chemical
• Acetic acid (CH₃COOH) — vinegar
• Citric acid — citrus fruits
• Lactic acid — curd, sour milk

**Common Bases and Their Uses:**

• Sodium hydroxide (NaOH) — soap making, drain cleaner
• Calcium hydroxide Ca(OH)₂ — whitewashing, treating acidic soil
• Magnesium hydroxide Mg(OH)₂ — antacid (milk of magnesia)
• Ammonium hydroxide (NH₄OH) — cleaning agents

**Important Salts:**

• Sodium chloride (NaCl) — common salt, food preservation
• Sodium bicarbonate (NaHCO₃) — baking soda, antacid
• Sodium carbonate (Na₂CO₃) — washing soda, glass making
• Calcium sulphate (CaSO₄·2H₂O) — Plaster of Paris
• Bleaching powder CaOCl₂ — disinfection, textile bleaching

Worked Examples

**Example 1: Identifying Acid or Base Using Litmus**

*Problem:* A solution turns red litmus blue but shows no change with blue litmus. Is it acidic, basic or neutral?

*Solution:*

• Red litmus turning blue indicates a base
• Blue litmus showing no change confirms it is not acidic
• Therefore, the solution is **basic (alkaline)**

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**Example 2: Writing Neutralisation Reaction**

*Problem:* Write the balanced equation when sulphuric acid reacts with potassium hydroxide.

*Solution:*

• Acid: H₂SO₄ (sulphuric acid)
• Base: KOH (potassium hydroxide)
• Products: Salt (K₂SO₄) + Water (H₂O)

Balanced equation: **H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O**

Note: Two KOH molecules needed because H₂SO₄ has two replaceable hydrogen atoms.

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**Example 3: pH and Nature of Solution**

*Problem:* Arrange these solutions from most acidic to most basic: Solution A (pH 3), Solution B (pH 9), Solution C (pH 7), Solution D (pH 1).

*Solution:*

• Lower pH = more acidic
• Higher pH = more basic
• pH 7 = neutral

Order from most acidic to most basic: **D (pH 1) → A (pH 3) → C (pH 7) → B (pH 9)**

Common Mistakes

• **Confusing pH direction:** Students often think higher pH means more acidic. Correct: Lower pH = more acidic, higher pH = more basic. Remember "pH goes up, acidity goes down."

• **Mixing up litmus colour changes:** Red litmus turns blue in base; blue litmus turns red in acid. Memory aid: "Blue to Red = Acid" (BRA).

• **Assuming all salts are neutral:** Students believe every salt has pH 7. Correct: Salts of strong acid + weak base are acidic (NH₄Cl); salts of weak acid + strong base are basic (Na₂CO₃).

• **Forgetting water as a product:** In neutralisation reactions, students write only salt as product. Correct: Always include water — Acid + Base → Salt + Water.

• **Confusing sodium carbonate with sodium bicarbonate:** Na₂CO₃ is washing soda (strongly basic), NaHCO₃ is baking soda (mildly basic, used as antacid). Their uses differ significantly.

Quick Reference

• **Acid = H⁺ donor** | **Base = OH⁻ donor** | **Salt = product of neutralisation**

• **pH scale:** 0–6 acidic | 7 neutral | 8–14 basic

• **Litmus test:** Blue→Red = Acid | Red→Blue = Base

• **Universal indicator:** Shows range of colours — red/orange for acids, green for neutral, blue/purple for bases

• **Turmeric indicator:** Yellow in acid/neutral, red/brown in base (why soap turns turmeric stains red)

• **Antacids contain bases** (Mg(OH)₂, NaHCO₃) to neutralise excess stomach acid