Alkalis are a special group of bases that dissolve in water to give a solution rich in hydroxide ions \$OH^-\$. Think of alkalis as the “water‑friendly” bases that are safe to handle in a laboratory, like sodium hydroxide \$NaOH\$ or potassium hydroxide \$KOH\$.
Litmus is a simple colour‑changing paper. In an acidic environment it turns red, while in a basic environment it turns blue. Alkalis, being basic, will always turn litmus paper blue.
Analogy: Imagine litmus paper as a mood ring – it changes colour based on the “mood” of the solution. Alkalis are the “happy” mood that makes the ring blue.
Exam Tip: If a question asks “What colour will litmus paper turn in a 0.1 M NaOH solution?” answer “Blue” and give the reasoning that NaOH is a strong alkali that fully dissociates to produce \$OH^-\$.
Thymolphthalein is a pH indicator that is colourless in acidic to neutral solutions but turns a vivid blue in solutions with a pH above about 9.5.
Analogy: Think of thymolphthalein as a “blue‑light” that only shines when the solution is “super‑basic”. Alkalis with a high enough pH will make it glow blue.
Exam Tip: When asked “What colour will thymolphthalein be in a 0.01 M KOH solution?” you should state “Blue” and explain that the pH is above 9.5, causing the indicator to change colour.
Methyl orange changes from red in acidic solutions to yellow in basic solutions, with a transition range around pH 3.1–4.4. Alkalis, being basic, will shift methyl orange to yellow.
Analogy: Picture methyl orange as a traffic light that turns from red to yellow when the road becomes “safe” (basic).
Exam Tip: For a question like “What colour will methyl orange show in a 0.05 M NaOH solution?” answer “Yellow” and note that NaOH is a strong base, so the solution is well above the transition pH.
| Indicator | Colour in Acidic | Colour in Basic (Alkali) | pH Transition Range |
|---|---|---|---|
| Litmus | Red | Blue | ~pH 7 |
| Thymolphthalein | Colourless | Blue | ~pH 9.5–10.5 |
| Methyl Orange | Red | Yellow | ~pH 3.1–4.4 |