Why baking soda and vinegar fizz
Acid–carbonate neutralization · Acid–base · Gas-forming · Endothermic
The foam from baking soda and vinegar is the same gas that makes pancakes fluffy, and the cup actually gets colder.
Reactants approach
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ΔH = +31 kJ · endothermic, absorbs heat
ΔH per mol NaHCO3; slightly endothermic.
Meet the molecules
- Reactantsolid
NaHCO3
sodium hydrogen carbonate (baking soda)
- Reactantdissolved in water
CH3COOH
acetic acid
- Productdissolved in water
CH3COONa
sodium acetate
- Productliquid
H2O
water
- Productgas
CO2
carbon dioxide
What you notice
Pour vinegar onto baking soda and the mixture foams up within a second, often spilling over the cup. Vinegar is about 5% acetic acid in water; baking soda is sodium hydrogen carbonate. The bubbles are carbon dioxide, the same gas that makes cake batter rise when recipes pair baking soda with buttermilk, yogurt or lemon juice. Hold the cup while it fizzes and it feels slightly cooler, a sign that the reaction absorbs a little heat from its surroundings. When the fizzing stops, what is left is a dilute solution of sodium acetate.
Try it
In a cup standing in the sink, add a few spoons of vinegar to a teaspoon of baking soda and feel the cup cool slightly; stretch a balloon over a bottle containing both to collect the CO2.
Safety: Never seal the mixture in a closed jar or bottle, because the gas pressure can burst it.
Step by step
- 1
The acid hands over a proton
An acetic acid molecule transfers the H+ from its –COOH group to a hydrogen carbonate ion, HCO3–. The acid becomes an acetate ion, CH3COO–.
- 2
Carbonic acid forms
Accepting the proton turns HCO3– into carbonic acid, H2CO3, which is unstable at room temperature.
- 3
It falls apart into gas
H2CO3 splits into water and CO2. Very little CO2 can stay dissolved, so it escapes as the bubbles that make the foam.
- 4
Spectator ions stay put
The sodium ions do not change. Left in solution with the acetate ions, they make sodium acetate.