The Hydronium Ion Mystery
Watch H₃O⁺ meet OH⁻ and vanish into water
What this lesson covers
Why it matters
A drop of phenolphthalein turns pink in base, then vanishes as acid is added. The colour change is the visible signature of hydronium ions neutralising hydroxide.
The idea in plain words
Map the macroscopic observation (pink to colourless, warm) to the ionic equation: H⁺ + OH⁻ → H₂O.
Predict first
What happens to the pink colour when dilute HCl is added to the pink NaOH solution?
The hydronium ions from the acid react with hydroxide ions from the base, removing the species that causes the pink colour.
- It turns green
- It fades to colourless — correct
- It stays pink because acid is colourless
What you do
Add dilute HCl dropwise to the pink NaOH solution. Observe the colour change and feel the vessel for heat.
Check yourself
Which equation correctly represents the neutralisation observed?
The verified reaction data shows NaOH + HCl → NaCl + H₂O.
Why does the vessel feel warm during the reaction?
The observation 'heat felt: warm' indicates an exothermic neutralisation reaction.
What causes the phenolphthalein to turn from pink to colourless?
Phenolphthalein is pink in basic conditions (high OH⁻) and colourless in acidic/neutral conditions. The H⁺ ions neutralise the OH⁻ ions.
- NaOH + HCl → NaCl + H₂O — correct
- NaOH + HCl → NaH + Cl₂O
- NaOH + HCl → NaCl + H₂ + O₂
- The reaction is exothermic — correct
- The glass absorbs heat from the air
- Friction from stirring generates heat
- Dilution of the solution
- Removal of OH⁻ ions by H⁺ ions — correct
- The acid bleaches the indicator