Baking Soda, Explained in Two Minutes

Baking soda is the cheapest genuinely useful thing in most kitchens, and almost nobody knows what it does. Which is a shame, because it only does two things — and once you know what they are, every use makes immediate sense and you stop wasting it on jobs it was never going to manage.

Trick one: it cancels out acid

Baking soda is a mild alkali. Put it next to something acidic and the two neutralise each other. That is why it takes the sharp edge off a smell (most bad kitchen smells are acidic), why it cuts through a tomato stain in a pan, and why a pinch fixes a sauce that has gone a bit too tangy.

Trick two: it makes bubbles

Mix it with an acid, or heat it up, and it fizzes off carbon dioxide. In a cake batter those bubbles get trapped and the sponge rises around them. In a sink it makes a satisfying foam and achieves very little — more on that below.

Three white powders, easily confused

• Baking soda (bicarbonate of soda) is the pure stuff. It needs something acidic in the recipe — buttermilk, yoghurt, lemon, cocoa, honey, brown sugar — or it just sits there tasting soapy.
• Baking powder is baking soda with a powdered acid already mixed in, plus a bit of starch to keep them apart in the tub. It works in recipes with no acid of their own.
• Washing soda is a different chemical altogether. Much harsher, great for laundry, and not something to cook with.

Ran out of baking powder? One teaspoon is roughly a quarter teaspoon of baking soda plus half a teaspoon of cream of tartar. Going the other way rarely works.

Is your box still any good?

Baking soda goes flat over time, especially in an open box. Tip a quarter teaspoon into a splash of vinegar. A big immediate fizz means it is fine. A sad little bubble means it has been quietly doing nothing for a year, and a new box costs about a pound.

Two boxes, not one

Keep a sealed tub for baking and a separate cheap bag for cleaning. The open box in the fridge is busy absorbing the smell of last week's onions, and you do not want that turning up in a Victoria sponge.

Where to go next

The practical guides: twelve jobs it does brilliantly, the smell guide, scrubbing without scratching, and why your bakes come out flat.