Neutralisation, watched with an indicator
An acid and a base cancel each other. That much is a sentence in a textbook; the difficulty is that the product — salt and water — looks like nothing at all, so there is no way to see it happen. An indicator solves that. Phenolphthalein is colourless in acid and pink in alkali, so adding it to sodium hydroxide stains the whole beaker pink and adding acid drains the colour away at the moment the alkali runs out. The pH paper afterwards confirms what the colour claimed.
5 steps · Acids, Bases & Salts
What you will use
Apparatus
- Beaker
Chemicals
- Sodium Hydroxide
- Phenolphthalein
- Dilute Hydrochloric Acid
The steps
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Pour sodium hydroxide solution into the beaker.
Colourless, and it will stay that way until something is added to reveal it.
-
Add a few drops of phenolphthalein.
Bright pink. The indicator has not changed the alkali; it has made it visible.
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Swirl the beaker to spread the indicator through the alkali.
The streak of pink evens out. Mixing is a thing you do, not something that happens.
-
Pour in dilute hydrochloric acid.
The pink drains away, and the beaker warms: neutralisation gives out heat.
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Dip the pH paper.
Green — around neutral pH. The alkali is gone and no acid is left over.
What it shows
An acid and a base neutralise each other to give a salt and water — here sodium chloride, dissolved and invisible. The indicator only reported the moment the alkali ran out, which is the whole reason a titration can be read at all.