pH and indicators

pH is a numerical scale from 0-14 that measures the acidity or alkalinity of a solution. The term stands for 'power of hydrogen' and indicates the concentration of hydrogen ions (H⁺) in a solution.

Key Definitions:

• Acids have pH < 7 and contain excess H⁺ ions
• Neutral solutions have pH = 7 (equal H⁺ and OH⁻ ions)
• Alkalis have pH > 7 and contain excess hydroxide ions (OH⁻)

The pH Scale:

• pH 0-3: Strong acids (concentrated HCl, H₂SO₄)
• pH 4-6: Weak acids (ethanoic acid, citric acid)
• pH 7: Neutral (pure water)
• pH 8-10: Weak alkalis (sodium hydrogencarbonate solution)
• pH 11-14: Strong alkalis (concentrated NaOH, KOH)

Indicators are substances that change colour depending on pH. They help identify whether solutions are acidic, neutral, or alkaline.

Common Indicators:

• Litmus: Red in acid, purple in neutral, blue in alkali
• Methyl orange: Red in acid, orange in neutral, yellow in alkali
• Phenolphthalein: Colourless in acid/neutral, pink in alkali
• Universal indicator: Shows full pH range with colour spectrum (red→orange→yellow→green→blue→purple)

Mnemonic for Universal Indicator colours: Richard Of York Gave Battle In Purple (Red, Orange, Yellow, Green, Blue, Indigo, Purple for pH 1-14)

pH measurement methods:

1. Universal indicator solution/paper (approximate pH)
2. pH probe/meter (precise digital reading to 0.01 pH units)

Understanding pH in Everyday Life:

Think of pH like a temperature scale for acidity—just as thermometers measure heat, pH measures 'sourness' or 'soapiness' of solutions.

Real-World Applications:

1. Human Body: Your stomach produces hydrochloric acid (pH 1-2) to digest food and kill bacteria. When excess acid causes discomfort, antacids (pH 9-10) like milk of magnesia neutralize it. Your blood must maintain pH 7.35-7.45—even small deviations can be life-threatening!

2. Agriculture: Farmers test soil pH because plants have preferences. Blueberries thrive in acidic soil (pH 4.5-5.5), while vegetables prefer neutral-slightly alkaline (pH 6.5-7.5). Adding lime (calcium oxide) raises pH; sulfur lowers it.

3. Swimming Pools: Pool water needs pH 7.2-7.8. Too acidic (pH < 7) corrodes equipment and irritates skin; too alkaline (pH > 8) makes chlorine ineffective and water cloudy.

4. Food Industry: Lemon juice (pH 2) preserves food because bacteria can't survive in strong acids. Cola drinks (pH 2.5) are acidic enough to dissolve teeth over time!

Analogy for pH Scale: Imagine a tug-of-war between H⁺ ions (acid team) and OH⁻ ions (alkali team). In acids, the H⁺ team dominates (low pH). In alkalis, OH⁻ team wins (high pH). At pH 7, it's perfectly balanced—neutral!

Why Indicators Work: Indicator molecules are like chemical chameleons—they change shape when H⁺ ions attach or detach, which alters the wavelengths of light they absorb, producing different colours we observe.

Example 1: Identifying Solutions Using Universal Indicator

Question: A student adds universal indicator to four solutions A-D. The colours are: A=red, B=green, C=yellow, D=purple. Identify each solution from: dilute sodium hydroxide, dilute hydrochloric acid, water, dilute ethanoic acid.

Solution:

• Red (pH 1-3) = strong acid = A: dilute hydrochloric acid
• Yellow (pH 5-6) = weak acid = C: dilute ethanoic acid
• Green (pH 7) = neutral = B: water
• Purple (pH 12-14) = strong alkali = D: dilute sodium hydroxide

Examiner note: Match colour to pH value first, then determine substance strength.

Example 2: Practical Investigation

Question: Describe how to determine the pH of lemon juice using universal indicator paper. State expected result and explain.

Step-by-step method:

1. Place drop of lemon juice on white tile
2. Dip universal indicator paper into the liquid
3. Compare colour change to pH colour chart immediately
4. Record pH value

Expected result: Colour changes to red/orange indicating pH 2-3

Explanation: Lemon juice contains citric acid, producing high concentration of H⁺ ions, making it strongly acidic.

Examiner note: Always state comparison with colour chart (3 marks available: method, result, explanation).

Example 3: pH Changes

Question: Sodium hydroxide solution (pH 13) is gradually added to hydrochloric acid (pH 1). Describe pH changes.

Solution: pH increases from 1→7 (slowly at first, rapidly near pH 7, then slowly toward pH 13). At pH 7, neutralization is complete: HCl + NaOH → NaCl + H₂O

Examiner note: Use equation to support explanation for full marks.