🕵️ The Detective's Problem
You need to make sodium chloride (NaCl) — table salt — from hydrochloric acid and sodium hydroxide solution.
Easy, right? Just mix them and... wait. Both reactants are dissolved in water. The product is dissolved in water. You can't filter it. You can't "add excess and filter off the excess."
⚠️ The "Excess Filter" Trap
If you add excess NaOH, it just dissolves invisibly into your solution. When you evaporate, you get NaCl contaminated with NaOH. Your "salt" would be impure and possibly corrosive!
💡 The Solution: Titration
Use an indicator to find the exact volume of acid that neutralises the alkali — drop by drop — until the colour just disappears. Record that volume. Then repeat with no indicator using those exact volumes. Evaporate. Pure crystals.
The Equation:
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Neutralisation — acid + alkali → salt + water