Baking soda fizzes when vinegar is added because the reaction releases carbon dioxide gas.

The longer version

Vinegar is an acid, and the powder known as bicarbonate of soda is a mild base. When the two meet they swap parts, and one product of that swap is carbon dioxide, a gas that cannot stay dissolved. It escapes as bubbles, and the froth is simply that gas pushing its way out through the liquid.

The other products are water and a dissolved salt, so the mixture ends up far less acidic than the vinegar alone. Anything that produces a gas on mixing tends to fizz in the same way, which is why the reaction has long been used in classroom demonstrations of chemical change.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.