Acid cancels base
An acid and a base cancel each other: neutralisation.
The pink liquid
A flask holds a base (sodium hydroxide) with a few drops of phenolphthalein. The liquid is pink: pink means base. A teacher adds acid drop by drop.
What will happen to the pink colour?
What this lesson covers
The idea
An acid and a base nullify each other's effect; the reaction between them giving a salt and water (Base + Acid → Salt + Water) is a neutralisation reaction.
The pink liquid
A flask holds a base (sodium hydroxide) with a few drops of phenolphthalein. The liquid is pink: pink means base. A teacher adds acid drop by drop.
What will happen to the pink colour?
- It fades and finally disappears
- It becomes darker pink
- It turns blue
Drop by drop
Add HCl one drop at a time. Watch the pink and the see-saw: how much base is left?
Acid and base cancel out
An acid and a base nullify each other’s effect. The reaction between them that gives a salt and water is a neutralisation reaction: Base + Acid → Salt + Water.
At the neutral point the base is used up and the liquid is neither basic nor acidic.
- Base + Acid | → Salt + Water
- In words | sodium hydroxide + hydrochloric acid | sodium chloride + water
- With symbols | NaOH + HCl | NaCl + H₂O
Notes
An acid and a base nullify each other's effect; the reaction between them giving a salt and water (Base + Acid → Salt + Water) is a neutralisation reaction.
Check yourself
Which pair is a neutralisation reaction?
Base + Acid → ?
The pink colour has just disappeared. What does that tell you?
NaOH + HCl → ? + H₂O. What is the salt?
- Zn + HCl. A metal and an acid give a salt and hydrogen gas. That is not neutralisation.
- Mg + H₂SO₄. A metal and an acid give a salt and hydrogen gas. That is not neutralisation.
- NaOH + HCl — correct. Yes! A base meets an acid and they make a salt and water.
- Salt + water — correct. Yes!
- Salt + hydrogen. Hydrogen comes when a metal meets an acid.
- Salt + carbon dioxide. Carbon dioxide comes when a carbonate meets an acid.
- No reaction took place. The pink went because a reaction took place.
- The acid has cancelled the base — correct. Yes! The base is used up.
- The acid is used up. It is the base that is cancelled. The acid has done its job.
- NaH. The salt joins the metal part of the base with the acid part: Na with Cl.
- ClOH. The salt joins Na of the base with Cl of the acid.
- NaCl — correct. Yes! Sodium chloride, common salt.