Acids, Bases and Salts

Overview

Acids, Bases and Salts form a foundational chemistry topic in JKTET Paper II, bridging theoretical concepts with everyday applications. This topic tests your understanding of chemical properties, reactions, and the pH scale—concepts essential for teaching upper primary and secondary students.

For the exam, expect questions on identification of acids and bases using indicators, properties and reactions of these substances, neutralisation, and pH values of common substances. The topic connects directly to practical chemistry and environmental science, making it a favourite for application-based questions. Mastering this unit requires clear conceptual understanding rather than rote memorisation.

Students must be comfortable with chemical equations, indicator colour changes, and real-world examples from daily life in J&K—like the acidity of apples from Kashmir orchards or the alkaline nature of lime used in traditional construction.

Key Concepts

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

• **Bases** are substances that release hydroxide ions (OH⁻) in water. They taste bitter, feel soapy, and turn red litmus blue. Water-soluble bases are called **alkalis**.

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

• **pH scale** measures the hydrogen ion concentration in a solution, ranging from 0 to 14. Values below 7 indicate acidic nature, 7 is neutral, and above 7 is basic.

• **Indicators** are substances that show different colours in acidic and basic solutions. Common indicators include litmus, phenolphthalein, and methyl orange.

• **Neutralisation** is the reaction between an acid and a base to form salt and water: Acid + Base → Salt + Water.

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

• The pH of a solution determines its corrosive nature, biological activity, and suitability for various applications.

Formulas / Key Facts

**Chemical Equations:**

• Acid + Metal → Salt + Hydrogen gas

Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑

• Acid + Metal carbonate → Salt + Water + Carbon dioxide

Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑

• Acid + Base → Salt + Water (Neutralisation)

Example: NaOH + HCl → NaCl + H₂O

• Acid + Metal oxide → Salt + Water

Example: CuO + H₂SO₄ → CuSO₄ + H₂O

**pH Values to Remember:**

| Substance | pH | |-----------|-----| | Gastric juice | 1.0–2.0 | | Lemon juice | 2.2 | | Vinegar | 2.5–3.0 | | Pure water | 7.0 | | Blood | 7.35–7.45 | | Milk of magnesia | 10.5 | | Sodium hydroxide | 14.0 |

**Indicator Colour Changes:**

| Indicator | In Acid | In Base | |-----------|---------|---------| | Litmus | Red | Blue | | Phenolphthalein | Colourless | Pink | | Methyl orange | Red | Yellow |

**Important Salts:**

• Common salt (NaCl) — table salt, food preservation
• Baking soda (NaHCO₃) — antacid, baking
• Washing soda (Na₂CO₃.10H₂O) — cleaning agent
• Plaster of Paris (CaSO₄.½H₂O) — casts, moulds
• Bleaching powder (CaOCl₂) — disinfection, bleaching

Worked Examples

**Example 1: Identifying the Salt Formed**

*Question:* What salt is formed when hydrochloric acid reacts with sodium hydroxide?

*Solution:*

• Acid: HCl (provides Cl⁻)
• Base: NaOH (provides Na⁺)
• Reaction: NaOH + HCl → NaCl + H₂O
• Answer: Sodium chloride (common salt)

**Example 2: pH Calculation Concept**

*Question:* A solution has pH = 3. Another solution has pH = 6. Which is more acidic and by how much?

*Solution:*

• Lower pH means higher acidity
• pH 3 is more acidic than pH 6
• Each pH unit represents 10 times difference in H⁺ concentration
• Difference = 6 - 3 = 3 units
• pH 3 solution is 10 × 10 × 10 = 1000 times more acidic
• Answer: pH 3 solution is 1000 times more acidic

**Example 3: Reaction with Metal Carbonate**

*Question:* What happens when dilute sulphuric acid is poured over limestone (calcium carbonate)?

*Solution:*

• Limestone is CaCO₃
• Reaction: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂↑
• Observation: Brisk effervescence occurs due to CO₂ gas
• Products: Calcium sulphate, water, and carbon dioxide
• This reaction explains why acid rain damages marble monuments

Common Mistakes

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

• **Mixing up indicator colours** → Students forget which colour indicates acid or base. Memory aid: "Blue to Red = Acid" (BRA) for litmus paper.

• **Assuming all bases are alkalis** → Only water-soluble bases are alkalis. Copper hydroxide is a base but not an alkali because it does not dissolve in water.

• **Writing incomplete neutralisation products** → Students forget to include water in neutralisation reactions. Always write: Acid + Base → Salt + Water.

• **Confusing baking soda and washing soda** → Baking soda is NaHCO₃ (sodium bicarbonate), washing soda is Na₂CO₃ (sodium carbonate). Baking soda is used in cooking; washing soda is used for cleaning.

Quick Reference

• pH 7 = neutral; below 7 = acidic; above 7 = basic

• Litmus: Blue → Red (acid), Red → Blue (base)

• Neutralisation always produces salt + water

• Strong acids: HCl, H₂SO₄, HNO₃ | Weak acid: CH₃COOH

• Strong bases: NaOH, KOH | Weak base: NH₄OH

• Antacids work by neutralising excess stomach acid (HCl)