pH Scale

pH SCALE

The pH scale is basically chemistry’s way of measuring how acidic or alkaline (basic) a solution is.
Think of it like a “mood scale” for substances:

• Acidic things = “angry” 😤

• Basic things = “chill” 😌

• Neutral = “peaceful middle ground” 😇

But scientifically, it measures the concentration of hydrogen ions (H⁺) in a solution.

1️⃣ What is pH?

pH is defined as:

pH = –log₁₀[H⁺]

Where [H⁺] means the concentration of hydrogen ions in mol/dm³.

You don’t need to calculate logs in WAEC/JAMB unless they give you a calculator-friendly number — but you MUST understand the concept.

2️⃣ The pH Scale (0 to 14)

The pH scale ranges from 0 to 14.

Here’s the breakdown:

• pH 0 – 6 → Acidic

• pH 7 → Neutral

• pH 8 – 14 → Alkaline (Basic)

The more H⁺ ions → the lower the pH → the stronger the acid.
The fewer H⁺ ions → the higher the pH → the stronger the base.

3️⃣ Characteristics of Acids, Bases, and Neutral Solutions

ACIDS (pH < 7)

• Sour taste

• Turns blue litmus → red

• React with bases

• React with metals to release hydrogen gas

• More H⁺ ions than OH⁻ ions

Examples:
HCl, H₂SO₄, ethanoic acid, lemon juice.

BASES / ALKALIS (pH > 7)

• Bitter taste

• Slippery/soapy feel

• Turns red litmus → blue

• Neutralize acids

• More OH⁻ ions than H⁺ ions

Examples:
NaOH, KOH, ammonia solution, magnesium hydroxide.

NEUTRAL SOLUTIONS (pH = 7)

• Neither acidic nor basic

• Equal number of H⁺ and OH⁻ ions

• Distilled water is the classic example

4️⃣ Strong vs Weak Acids/Bases (Very Exam-Relevant)

Strong acids / strong bases

• Ionize completely in water

• Produce lots of ions

• pH changes drastically

Examples:

• Strong acid: HCl, HNO₃, H₂SO₄

• Strong base: NaOH, KOH

Weak acids / weak bases

• Ionize partially

• Produce fewer ions

• pH values closer to 7

Examples:

• Weak acid: CH₃COOH (ethanoic acid)

• Weak base: NH₃(aq)

5️⃣ pH Indicators (VERY Important for Practical)

Indicators show pH through colour changes.

Universal indicator is the one used to show exact pH using a colour chart.

6️⃣ Common pH Values (Good for Memorization)

7️⃣ Real-Life Applications of pH

• Agriculture: soil pH affects crops

• Medicine: blood pH monitoring

• Water treatment: safe pH levels

• Food industry: acidity of drinks 🎉

• Cosmetics: skin-friendly pH

• Swimming pools: chlorine balance

8️⃣ Likely WAEC/JAMB Questions

1. State the pH range for acids and bases.

2. Describe how universal indicator works.

3. Explain the difference between strong and weak acids.

4. Name an indicator that turns pink in alkaline solution.

5. What is the pH of a neutral solution?

6. A solution turns blue litmus red. What can you conclude?

7. Why does pure water have a pH of 7?