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

Acids, Bases and Salts – Characteristic Properties of Alkalis (Cambridge IGCSE 0620)

Learning objectives

• AO1 – Knowledge & terminology: Define bases and alkalis, classify the main types, describe the colour changes of litmus, thymolphthalein and methyl orange, and state the pH ranges for each transition.
• AO2 – Application: Use the indicator data to predict the observable result of a neutralisation or dilution experiment.
• AO3 – Practical skills: Design, carry out and evaluate a short investigation of alkali‑indicator reactions, including safety, recording and evaluation.

1. What is a base? What is an alkali?

• Base (general definition): A substance that produces hydroxide ions (OH⁻) in water or that can accept protons. In the Cambridge syllabus bases include:
  • Metal oxides (e.g. CaO, MgO)
  • Metal hydroxides (e.g. NaOH, KOH, Ca(OH)₂)
  • Ammonia (NH₃) – a molecular base that reacts with water to give NH₄⁺ and OH⁻
• Alkali (specific definition): A **soluble** metal hydroxide. In aqueous solution it dissociates completely to give a high concentration of OH⁻ ions (pH > 7).
  Typical alkalis: NaOH, KOH, LiOH, Ca(OH)₂, Ba(OH)₂. (Group 1 hydroxides are always soluble; a few Group 2 hydroxides are sufficiently soluble to be classed as alkalis.)
• Strong vs weak alkalis: Alkalis listed above are strong – they dissociate completely. Weak bases such as NH₃ are not called alkalis because they are only partially ionised.

2. Classification summary

3. Effect of alkalis on common acid‑base indicators

Why do the colours change?

• Litmus: Exists in two tautomeric forms. In acidic media the protonated (red) form dominates; removal of protons by OH⁻ converts the molecule to its blue form.
• Thymolphthalein: The phenolic groups are protonated (colourless) at low pH. Deprotonation above pH ≈ 9.3 creates a conjugated anion that absorbs blue‑green light, giving a vivid blue colour. Above pH ≈ 12 the molecule is fully deprotonated and the colour fades.
• Methyl orange: Has three visible forms. The fully protonated form is red; loss of one proton produces an orange species; further deprotonation yields the yellow form. The transitions occur in a narrow acidic range (3.1–4.4).

4. Predicting the colour change of an alkali (AO2)

Example 1 – Litmus

1. 0.20 M NaOH solution is added dropwise to 10 mL of red litmus solution (initial pH ≈ 3).
2. Each 0.5 mL drop adds n(OH⁻)=0.20 mol L⁻¹×5.0×10⁻⁴ L=1.0×10⁻⁴ mol.
3. After the first drop the solution pH rises above 5, entering the litmus transition range, so the colour changes from red to purple and then to blue as more alkali is added.

Example 2 – Thymolphthalein

1. A 0.05 M NaOH solution is added to 20 mL of thymolphthalein solution (initially colourless, pH ≈ 7).
2. When the total added OH⁻ raises the pH above 9.3, the solution becomes blue. This typically occurs after adding about 1 mL of the 0.05 M NaOH.

Example 3 – Methyl orange

1. Adding a strong alkali to a methyl‑orange solution that is red (pH ≈ 2) will first give an orange colour once the pH passes 3.1, and then yellow once the pH exceeds 4.4.
2. Because the transition range is narrow, only a small amount of alkali is required to see the full colour change.

5. Practical investigation of alkali‑indicator reactions (AO3)

Apparatus and reagents

Procedure

1. Label three sets of four test tubes as L (litmus), T (thymolphthalein) and M (methyl orange). Number the tubes 0–3 within each set.
2. Pour 5 mL of the appropriate indicator solution into every tube.
3. Record the initial colour in tube 0 (no NaOH added).
4. Add 0.5 mL of 0.10 M NaOH to tube 1, swirl gently, and record the colour.
5. Repeat the addition for tube 2 (total 1.0 mL added) and tube 3 (total 1.5 mL added), recording the colour after each addition.
6. Optional: Dip a strip of pH paper into each mixture and note the pH. Compare the observed colour with the pH ranges in the table above.
7. Dispose of the alkaline solutions in the designated waste container, rinse the test tubes with water and return equipment to its place.

Safety notes

• NaOH is caustic – wear gloves and goggles, avoid skin contact.
• Work in a well‑ventilated area; if splashes occur, rinse immediately with plenty of water.
• Label all solutions clearly to prevent mix‑ups.

Evaluation points

• Did the colour changes occur at the expected volumes? If not, consider errors in NaOH concentration, indicator dilution, or incomplete mixing.
• Visual observation can be subjective – using a standard colour chart or a digital colour sensor improves reliability.
• Limitations of each indicator:
  • Litmus gives only a binary response (red/blue) and is not precise for pH measurement.
  • Thymolphthalein does not change colour below pH ≈ 9.3, so it is unsuitable for weakly basic solutions.
  • Methyl orange works only in a narrow acidic range (3.1–4.4); it quickly becomes yellow in any moderately basic solution.

6. Relationship between pH, [H⁺] and [OH⁻]

pH is defined as pH = -\log[H⁺]. In pure water at 25 °C, [H⁺] = [OH⁻] = 1.0×10⁻⁷ mol L⁻¹ (pH = 7). Adding an alkali supplies OH⁻ ions, which combine with H⁺ to form water, thereby reducing [H⁺] and raising the pH. Each indicator changes colour when the ratio [A⁻]/[HA] (deprotonated/protonated form) reaches a value that falls within its characteristic pH interval.

7. Key points to remember

• All alkalis are soluble metal hydroxides; they dissociate completely to give OH⁻ ions (pH > 7).
• Litmus: red → blue; transition 5 < pH < 8.
• Thymolphthalein: colourless → blue; transition at pH ≈ 9.3 (useful up to ≈ pH 12).
• Methyl orange: red → orange → yellow; transition 3.1 < pH < 4.4.
• Indicator colour changes are a visual expression of the change in [H⁺] (or [OH⁻]) caused by an alkali.
• When planning experiments, list all apparatus, follow safety rules (gloves, goggles, avoid skin contact with NaOH), and record observations systematically.