Describe acids in terms of their effect on: (a) litmus (b) thymolphthalein (c) methyl orange

Objective

Describe acids in terms of their effect on: (a) litmus, (b) thymolphthalein, (c) methyl orange.

Litmus is like a mood ring for solutions. When you dip a strip of litmus paper into a solution, it changes colour based on the solution’s acidity or basicity.

• 🔴 Acidic solutions turn blue litmus paper red.
• 🔵 Basic solutions turn red litmus paper blue.
• Neutral solutions do not change the colour.

Think of it as a traffic light: red means “stop” (acidic), blue means “go” (basic).

Key point for exams: If a solution turns blue litmus red, it is acidic. If it turns red litmus blue, it is basic.

Thymolphthalein is a pH indicator that behaves like a colour‑changing chameleon.

• 🔵 Colourless in solutions with \$pH < 9.3\$ (mostly acidic).
• 🟣 Blue in solutions with \$pH > 9.3\$ (mostly basic).

Imagine a sea that turns from clear to blue as the water becomes more alkaline.

Exam tip: Remember the threshold \$pH = 9.3\$ – below it, no colour; above it, a bright blue.

Methyl orange is another pH indicator that changes colour across a different range.

• 🟠 Red in solutions with \$pH < 3.1\$ (strongly acidic).
• 🟢 Yellow in solutions with \$pH > 4.4\$ (weakly acidic to neutral).
• Between \$pH 3.1\$ and \$4.4\$ it shows a mixture of red and yellow (orange).

Think of it as a traffic light that turns from red to green as the solution becomes less acidic.

Exam hint: If a solution turns methyl orange from red to yellow, it is moving towards neutrality.

Exam Tips

1. Always write the colour change first, then the pH range.
2. Use the analogy of a traffic light or mood ring to remember the colour changes.
3. When given a pH value, quickly decide which indicator would show a colour change.
4. Practice sketching the indicator colour changes in a table for quick reference.