Acid meets base

Section 4 of 4

When an acid meets a base, the acid's H⁺ passes to the base, and the two use each other up. Chemists call this neutralisation.

Baking soda and vinegar show it with a fizz. The H⁺ from ethanoic acid passes to a hydrogencarbonate ion and makes carbonic acid, H₂CO₃. Carbonic acid falls apart at once into water and carbon dioxide gas, and the carbon dioxide bubbles out. The sodium ions and ethanoate ions stay dissolved in the water: together they are sodium ethanoate.

Baking soda and vinegar make sodium ethanoate, water and carbon dioxide Top: sodium hydrogencarbonate, a hydrogencarbonate ion, H-O-C with a double-bonded oxygen and an O minus, with a sodium ion, Na plus, below it. Plus ethanoic acid, CH3-C with a double-bonded oxygen and an O-H group whose O-H bond is amber. An arrow points down to the products: sodium ethanoate, the ethanoate ion with its O minus and a sodium ion below it, plus water, H2O, plus carbon dioxide, O=C=O. HOCOOCH3COOHCH3COOH2OOCO − Na + + − Na + + + sodium hydrogencarbonate ethanoic acid sodium ethanoate water carbon dioxide
Baking soda and vinegar: NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂. The H⁺ from the acid's O–⁠H bond (amber) passes to the hydrogencarbonate ion, which falls apart into water and carbon dioxide.

Count the atoms, as for any reaction, and both sides match.

Atoms on each side when baking soda meets vinegar
AtomNaHCO₃ + CH₃COOHCH₃COONa + H₂O + CO₂
sodium11
carbon1 + 2 = 32 + 1 = 3
hydrogen1 + 4 = 53 + 2 = 5
oxygen3 + 2 = 52 + 1 + 2 = 5

Sodium ethanoate is a salt: chemists use that word for any compound made of positive and negative ions. Table salt, sodium chloride, is one. The salt takes its name from the acid, so ethanoic acid gives an ethanoate, the same ending as in the ester ethyl ethanoate.

Fizz in the kitchen

Baking powder is baking soda mixed with a dry acid. In a wet cake mix the two react and give off carbon dioxide, and the bubbles make the cake rise. Sherbet and bath bombs mix baking soda with dry citric acid, and they fizz the moment they get wet. In the next lesson, a strong base, sodium hydroxide, breaks fats apart, and the sodium salts of their fatty acids are soap.

Quick check

Baking soda fizzes in vinegar. Which gas makes the bubbles?

That's the end of lesson 13

In this lesson you found out:

  • What makes a molecule an acid or a base
  • How the pH scale works
  • Why baking soda fizzes in vinegar
Start Lesson 14: Soap and washing up All lessons