🌟 Module 3: Acidic and Basic Nature – The pH Concept

🎯 Objective:

To understand the pH scale, its scientific basis, and how acidic or basic nature of substances impacts daily life—ranging from the human body to agriculture and environment.

🔬 1. What is pH?

🔹 The term pH stands for "potential of Hydrogen" or "power of Hydrogen".

🔹 It is a measure of hydrogen ion concentration $[H⁺]$ in a solution.

🔹 pH is a logarithmic scale:

$$\text{pH} = -\log_{10}[H^+]$$

• If $[H^+]$ is high → pH is low → Acidic.

• If $[OH^-]$ is high → pH is high → Basic (Alkaline).

📈 2. The pH Scale (0–14)

pH Value	Nature of Substance	Example	Universal Indicator Color
0 – 3	Strongly Acidic	Hydrochloric acid (HCl), gastric juice	Red / Dark Orange
4 – 6	Weakly Acidic	Tomato juice, vinegar	Orange to Yellow
7	Neutral	Pure water	Green
8 – 10	Weakly Basic	Baking soda solution	Blue
11 – 14	Strongly Basic	NaOH, KOH solutions	Purple / Violet

🧠 Concept Insight:

• pH < 7 → Acidic

• pH = 7 → Neutral

• pH > 7 → Basic

The farther from 7, the stronger the acid/base.

🌿 3. Importance of pH in Everyday Life

👤 a. pH in the Human Body

• The human body maintains a blood pH around 7.35–7.45.

• Enzymes, hormones, and cell functions require stable pH.

• Drastic changes in pH can be fatal (acidosis or alkalosis).

🔍 Example:

• Stomach acid has a pH of 1.5–3.5 for digestion (HCl).

• Antacids like milk of magnesia (Mg(OH)₂) are basic to neutralize excess acid.

🌱 b. pH of Soil for Plants

• Soil pH affects nutrient absorption.

• Most crops grow well in pH 6 to 7.5.

• Too acidic soil → Add quicklime (CaO).

• Too basic soil → Add organic matter or sulfur.

🔍 Example:

• Tea grows well in acidic soils (pH < 6).

• Farmers often test soil pH before fertilizing.

🦷 c. Tooth Decay and pH

• Bacteria in the mouth produce acids after eating sugary foods.

• If pH of mouth falls below 5.5, tooth enamel starts dissolving.

• Toothpastes are basic to neutralize this acid and prevent cavities.

☔ d. Acid Rain

• Pollutants like SO₂ and NO₂ dissolve in rainwater to form H₂SO₄ and HNO₃.

• Rainwater becomes acidic (pH < 5.6) → Damages buildings, crops, aquatic life.

🔍 Equation:

$$\text{SO}_2 + \text{H}_2O \rightarrow \text{H}_2\text{SO}_3$$ $$\text{H}_2\text{SO}_3 + [O] \rightarrow \text{H}_2\text{SO}_4$$

🌈 4. Universal Indicator and pH Colors

• A universal indicator is a mixture of several indicators that shows a range of colors for different pH levels.

📊 Color Guide:

pH	Color	Nature
1	Red	Strong acid
4	Orange	Weak acid
7	Green	Neutral
9	Blue	Weak base
13	Purple	Strong base

🧪 5. Demonstration Activity – pH Testing

🎲 Materials:

• Universal indicator paper

• Solutions: Vinegar, soap water, lemon juice, milk, baking soda, cola, shampoo, detergent

• Small containers

🔬 Procedure:

1. Dip the indicator strip into each liquid.

2. Compare the color with the standard pH color chart.

3. Record observations.

📋 Sample Observations:

Substance	pH Range	Indicator Color	Nature
Vinegar	3	Orange-Red	Acidic
Lemon juice	2–3	Red	Acidic
Soap water	9–10	Blue	Basic
Milk	6.5–6.8	Pale Yellow-Green	Slightly acidic
Baking soda	8–9	Blue-Green	Weak base
Cola	2–3	Red	Strong acid
Shampoo	5–6	Yellow	Mild acid

💡 Misconception Alert

• A lower pH does NOT mean a “better acid”—it means a stronger acid.

• Acidic ≠ sour taste only. Some acids are dangerous to taste or touch.

• Not all alkaline solutions feel slippery. Some can be corrosive, like NaOH.

🧠 Quick Revision Capsule

• pH measures acidity/basicity using hydrogen ion concentration.

• Universal indicator helps determine pH through color change.

• Neutralization reactions help in controlling pH in soil, mouth, and body.

• pH control is essential in agriculture, healthcare, and environment.