Acids, Bases and Salts

Overview

Acids, bases and salts form a foundational chapter in upper-primary science, bridging everyday experiences (lemon sourness, soap slipperiness) with core chemical concepts. For WB TET Paper II, this topic tests your understanding of characteristic properties, behaviour with indicators, neutralisation reactions and practical applications. Questions typically ask you to identify substances as acidic or basic, predict indicator colour changes, write simple chemical equations for neutralisation, and recall common uses of acids, bases and salts.

Mastery here requires memorising indicator colour charts, understanding the pH scale conceptually, and knowing real-world examples. The topic also connects to environmental science (acid rain, soil pH) and health (antacids, oral hygiene), making it a favourite for application-based questions.

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/slippery and turn red litmus blue.

• **Alkalis** are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis (e.g., copper hydroxide is a base but not an alkali).

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

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

• **pH Scale** measures the strength of acidity or basicity on a scale of 0–14. pH 7 is neutral; below 7 is acidic; above 7 is basic.

• **Salts** are ionic compounds formed by the reaction of acids and bases. They may be acidic, basic or neutral depending on the parent acid and base.

• **Strong vs Weak Acids/Bases**: Strong acids (HCl, H₂SO₄) and strong bases (NaOH, KOH) ionise completely; weak acids (acetic acid) and weak bases (NH₄OH) ionise partially.

Formulas / Key Facts

**Chemical Formulas of Common Acids:**

• Hydrochloric acid: HCl
• Sulphuric acid: H₂SO₄
• Nitric acid: HNO₃
• Acetic acid (vinegar): CH₃COOH
• Carbonic acid: H₂CO₃

**Chemical Formulas of Common Bases:**

• Sodium hydroxide (caustic soda): NaOH
• Potassium hydroxide: KOH
• Calcium hydroxide (slaked lime): Ca(OH)₂
• Ammonium hydroxide: NH₄OH
• Magnesium hydroxide (milk of magnesia): Mg(OH)₂

**Indicator Colour Chart:**

| Indicator | In Acid | In Base | Neutral | |-----------|---------|---------|---------| | Litmus | Red | Blue | Purple | | Phenolphthalein | Colourless | Pink | Colourless | | Methyl orange | Red/Pink | Yellow | Orange | | Turmeric | Yellow | Reddish-brown | Yellow |

**Key Reactions:**

• Acid + Metal → Salt + Hydrogen gas (e.g., Zn + 2HCl → ZnCl₂ + H₂↑)
• Acid + Metal carbonate → Salt + Water + CO₂ (e.g., CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑)
• Acid + Base → Salt + Water (neutralisation)

**pH Values to Remember:**

• Gastric juice: ~1–2 (strongly acidic)
• Lemon juice: ~2
• Pure water: 7
• Baking soda solution: ~8–9
• Milk of magnesia: ~10
• Soap solution: ~9–10

Worked Examples

**Example 1: Indicator-Based Identification**

*Question*: A student adds a few drops of phenolphthalein to an unknown solution. The solution turns pink. Is the solution acidic, basic or neutral?

*Solution*:

• Phenolphthalein is colourless in acidic and neutral solutions.
• Phenolphthalein turns pink in basic solutions.
• Since the solution turned pink, it is **basic**.

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**Example 2: Neutralisation Reaction**

*Question*: Write the balanced equation when sodium hydroxide reacts with hydrochloric acid. Name the products.

*Solution*:

• NaOH + HCl → NaCl + H₂O
• Products: Sodium chloride (common salt) and water.
• This is a neutralisation reaction where the acid and base neutralise each other.

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**Example 3: Application Problem**

*Question*: A farmer finds his soil is too acidic for growing crops. What substance can he add to reduce the acidity?

*Solution*:

• To reduce soil acidity, the farmer should add a basic substance.
• Commonly used: Slaked lime (calcium hydroxide) or quicklime (calcium oxide).
• The base neutralises the excess acid in the soil, making it suitable for cultivation.

Common Mistakes

• **Confusing litmus colour changes** → Remember: "Acids turn blue litmus Red" (A-B-R sequence). Bases turn red litmus blue.

• **Assuming all bases are alkalis** → Only water-soluble bases are alkalis. Insoluble bases like Fe(OH)₃ or Cu(OH)₂ are not alkalis.

• **Forgetting that neutralisation always produces water** → Students sometimes write only salt as the product. Always include water: Acid + Base → Salt + Water.

• **Mixing up pH direction** → Lower pH means more acidic (not less). pH 2 is more acidic than pH 5. Higher pH means more basic.

• **Confusing strong/weak with concentrated/dilute** → Strong/weak refers to degree of ionisation; concentrated/dilute refers to amount of solute per unit volume. Dilute HCl is still a strong acid.

• **Assuming all salts are neutral** → Salts can be acidic (NH₄Cl), basic (Na₂CO₃) or neutral (NaCl) depending on parent acid and base.

Quick Reference

1. Acids release H⁺ ions; bases release OH⁻ ions in water.

2. Blue litmus → Red = Acid; Red litmus → Blue = Base.

3. pH scale: 0–7 acidic, 7 neutral, 7–14 basic.

4. Neutralisation: Acid + Base → Salt + Water (always produces water).

5. Common natural indicators: Litmus (lichen), turmeric, red cabbage juice.

6. Antacids contain bases (Mg(OH)₂, NaHCO₃) to neutralise excess stomach acid.