A beginner's guide to baking powder and bicarbonate of soda

Two small spoons, two very different jobs

Every British kitchen seems to have them: a slightly crusty tub of baking powder at the back of the cupboard and a bag of bicarbonate of soda that gets used twice a year. They look similar, they sit side by side on the shelf, and they both make things rise. Beyond that, they behave quite differently — and knowing which one to reach for is the difference between a scone that stands tall and one that spreads into a pale puddle.

This is a beginner's guide to what each one does, when to use it, and why half a teaspoon really can matter.

The chemistry, without the lecture

Both are chemical leaveners. Mix an alkali with an acid in the presence of moisture and you get carbon dioxide: thousands of tiny bubbles pushing their way through the batter or dough. Heat then sets the structure around those bubbles — gluten in bread dough, starch and egg in a sponge — so the rise is trapped rather than lost.

Bicarbonate of soda is the alkali. Baking powder is bicarbonate of soda already packaged with a solid acid, plus a little starch to keep the two apart until liquid is added. That single difference explains almost everything else.

Bicarbonate of soda: fast, fierce and fussy

Bicarbonate of soda is pure alkali, so it needs an acid partner in the recipe: buttermilk, natural yogurt, soured cream, lemon juice, vinegar, treacle and golden syrup, soft brown sugar or natural cocoa powder. It begins fizzing the moment it meets a wet ingredient, and those bubbles start escaping straight away. Mix it, shape it and get it into a hot oven without dawdling.

It is also powerful. Weight for weight, bicarbonate of soda is roughly three to four times as strong as baking powder. Too little acid and you can taste it — a soapy, metallic note at the back of the throat, and a crumb that browns alarmingly fast. That browning is not imagined, either: an alkaline mixture encourages the Maillard reaction, which is exactly why bicarbonate sits behind ginger nuts, soda bread and dark, sticky gingerbread. Use it where the recipe asks for it, in the quantity given.

Baking powder: the reliable all-rounder

Baking powder carries its own acid, usually cream of tartar or a phosphate, so it works in plain mixtures — a Victoria sponge, a plain scone, a batch of fairy cakes. Most baking powder sold in the UK is double-acting: a little gas is released when the liquid goes in, and the rest is triggered by oven heat, which gives you a more forgiving window between mixing and baking.

As a rough guide, allow one level teaspoon of baking powder per 100g of plain flour for a sponge, and check the recipe before altering it. Self-raising flour is simply plain flour with baking powder already blended through; you can make your own by adding about two level teaspoons to every 150g of plain flour. Don't swap one leavener for the other. Replacing two teaspoons of baking powder with two teaspoons of bicarbonate of soda would give you roughly four times the alkali and a thoroughly unpleasant cake.

Why precise measuring matters

Chemical leaveners are used in tiny quantities, so small errors are large in proportion. A heaped teaspoon instead of a level one can nearly double the amount, and too much raising agent makes a batter rise fast, set weak and then collapse — often with a coarse, tunnelled crumb and a lingering chemical taste.

• Weigh where you can. Digital scales in grams are more accurate than spoons and make repeatable results far easier.
• Level your spoons. Scoop, then sweep the back of a knife across the top. Never tap the spoon to pack it down.
• Test your tub. Stir a teaspoon of bicarbonate of soda into a little vinegar or lemon juice; it should fizz enthusiastically. For baking powder, drop a teaspoon into hot water and watch for bubbles. If either barely reacts, replace it.
• Store it properly. Airtight, cool and dry — humidity turns powder into clumps and starts the reaction early. Replace every six months or so, and write the date on the tub.

Putting it to work: scones, cakes and biscuits

Scones sit on the border between the two. Many recipes use self-raising flour with a little extra baking powder for lift, and any buttermilk brings acid into play. Handle the dough as little as possible, cut straight down with a sharp cutter, and bake hot — around 220°C — so the rise sets before the fat melts away.

Cakes are baking powder territory, baked at roughly 180°C. Fold the batter until just combined, get it into the tin promptly, and resist opening the oven door early, when a fragile rise is at its most vulnerable.

Biscuits — proper British ones — use bicarbonate when there is an acid to balance it, as with golden syrup in a ginger nut. The alkaline mixture helps the dough spread and colour, giving that crackled top and snappy finish. Leave space between them on the tray, and expect them to firm up as they cool.

Once you know which leavener you are using and why, the recipes stop feeling like guesswork. Keep the spoons level, keep the tubs fresh, and your scones, sponges and biscuits will reward you every time.