Baking Soda vs Baking Powder: What's the Difference?

Baking soda is a single chemical, sodium bicarbonate, that only makes gas when it meets an acid; baking powder is baking soda pre-mixed with its own dry acids and a little starch, so it only needs moisture and heat. That one difference decides which recipes call for which, how much you use, and why swapping them carelessly gives you flat pancakes or a soapy-tasting cake. Both are chemical leaveners: they puff batter with carbon dioxide instead of waiting for yeast.

What actually happens when baking soda fizzes?

Sodium bicarbonate (NaHCO₃) is a mild base. Drop it into something acidic and the two react: the acid hands over a hydrogen ion, the bicarbonate falls apart, and you get carbon dioxide gas, water and a salt. In a bowl of batter, that CO₂ inflates the tiny air bubbles you beat in while mixing. In the oven, the heat makes those bubbles expand further until the eggs and starch set around them and lock the rise in place.

The catch is that soda needs a partner. Common acidic ingredients that will set it off include:

• Buttermilk, yogurt, sour cream and kefir
• Lemon juice, vinegar and orange juice
• Brown sugar, molasses and honey (mildly acidic)
• Natural (non-Dutched) cocoa powder
• Cream of tartar
• Chocolate, applesauce and some fruit purées

Plain milk is only very slightly acidic, so it can't do much on its own.

Soda is also strong. Teaspoon for teaspoon it delivers roughly three to four times the lift of baking powder, which is why recipes use it in small amounts, often 1/4 to 1/2 teaspoon. A handy rule: about 1/4 tsp of soda neutralises the acid in 1 cup (240 ml) of buttermilk or yogurt.

What if there isn't enough acid?

Heat alone will eventually break baking soda down, at around 80°C (176°F) and up. But that reaction leaves behind sodium carbonate, a stronger base with a bitter, soapy, metallic taste. Leftover soda also pushes the batter's pH up, which speeds browning and can turn baked goods a darker, yellowish colour with a coarse crumb. If you've ever bitten into a scone that tasted faintly of soap, that's unreacted bicarbonate.

So what is baking powder made of?

Open a tin of baking powder and you're looking at three things blended together:

Because the acid is already there, baking powder works in recipes with no acidic ingredients at all, like a classic vanilla cake made with plain milk. Its manufacturers also balance the soda and acid so nearly all of it reacts, leaving the batter close to neutral, so the flavour stays clean.

What does double-acting mean?

Most baking powder sold in North America is double-acting. It contains a fast acid, usually monocalcium phosphate, that reacts as soon as it gets wet, plus a slow acid, such as sodium aluminum sulfate or sodium acid pyrophosphate, that barely reacts until the batter heats up in the oven.

That gives you two bursts of gas. The first happens in the bowl and starts aerating the batter. The second arrives in the oven, just as the batter is warming and starting to set, which is exactly when you want lift. Double-acting powder is forgiving: you can rest pancake batter or leave muffin batter on the counter for a few minutes without losing everything.

Single-acting powder, which only has a fast acid, is less common now. With it, most of the gas is released during mixing, so the batter has to go into the oven fast.

Chef Fizz says: Baking soda is a solo act that needs a duet partner. Baking powder showed up with its own band.

Why do so many recipes use both?

Banana bread, buttermilk biscuits and chocolate cake often list soda and powder together. That isn't redundancy. Each one does a different job.

1. Soda neutralises the acid. Buttermilk, brown sugar or natural cocoa bring acid. A measured amount of soda reacts with it, so the final bake isn't sharply sour.
2. Soda boosts browning and flavour. A slightly higher pH speeds up the Maillard reaction, giving deeper colour and toastier notes. It also loosens gluten a little, which is part of why soda makes cookies spread wider.
3. Powder handles the rest of the lift. Once the acid is used up, there may not be enough soda left (or you'd need so much it would taste soapy). Powder tops up the rise in a balanced, self-contained way.

So a recipe with 1 cup of buttermilk might use 1/4 tsp soda to match the acid, then 1 to 2 tsp baking powder for height.

Want to test your instincts? The Cooking Scientist app has a quick lesson on baking soda vs baking powder where you pick the right leavener for each batter.

How do I substitute one for the other?

You can swap them, but not one for one.

Replacing baking powder with baking soda. Make your own powder: for each 1 tsp of baking powder, use 1/4 tsp baking soda plus 1/2 tsp cream of tartar. Mix it into the dry ingredients and bake immediately, because this homemade version is single-acting.

Replacing baking soda with baking powder. Use roughly three times as much powder: 1/2 tsp soda becomes about 1 1/2 tsp powder. Expect a paler colour, a slightly tangier flavour (the recipe's acid is now left over) and a cakier texture. It works in a pinch but isn't identical.

Using soda with no acid. Don't. Without an acid, most of the soda breaks down in the oven into sodium carbonate, and the bake tastes bitter.

How much leavening should a recipe have?

These are common starting points, not laws. Recipes vary with how much egg, fat and whipped air they rely on.

More is not better. Too much leavening makes bubbles grow so big they merge and burst, and the cake rises dramatically and then collapses. That's one of the classic reasons a cake sinks in the middle. Too much also leaves a chemical aftertaste and a crumbly, coarse texture.

Do baking soda and baking powder expire?

Baking soda on its own is very stable as long as it stays dry. It slowly picks up moisture and odours, so seal the box, but it keeps working for a long time.

Baking powder is more fragile. Humidity lets the acid and base react slowly inside the tin, using up their power before they ever reach your batter. An opened tin generally stays reliable for about 6 to 12 months. Keep the lid tight, store it away from the stove and kettle, and always use a dry spoon.

Try it at home

This three-glass test shows the difference in five minutes.

1. Set out three clear glasses. Put 1/2 tsp baking soda in the first two and 1/2 tsp baking powder in the third.
2. Pour 60 ml (1/4 cup) of cold tap water into glass one. You'll see almost nothing happen. Soda without acid just dissolves.
3. Pour 60 ml of white vinegar into glass two. It foams up fast and fiercely, then dies down within seconds. That's the full acid-base reaction happening all at once.
4. Pour 60 ml of cold water into glass three. You'll get a steady, modest fizz: the fast-acting acid in the powder at work.
5. Once that fizz slows, top glass three with 60 ml of very hot tap water. The bubbling picks up again. That's the slow acid kicking in with heat, the second act of double-acting powder.

What you've just watched is exactly what happens in your batter: soda needs acid, while powder reacts once when wet and again when warmed.

Common mistakes

• Swapping teaspoon for teaspoon. Soda is three to four times stronger than powder. Using equal amounts gives you either a soapy, over-risen bake or a flat one. Use the ratios above.
• Using soda in a recipe with no acid. The soda never fully reacts and turns into bitter sodium carbonate. If there's no buttermilk, yogurt, citrus, brown sugar or natural cocoa, you want powder.
• Letting batter sit too long with homemade or single-acting powder. The gas escapes before the oven can trap it. Mix, pan and bake straight away.
• Trusting an old tin of powder. Expired powder gives a dense, short bake. Do the hot-water fizz test if you can't remember when you opened it.
• Not sifting or whisking the leavener into the flour. Clumps of soda leave bitter brown speckles and uneven holes. Whisk it thoroughly through the dry ingredients first.
• Swapping Dutch-processed cocoa for natural cocoa. Dutched cocoa has been alkalised, so it won't react with soda. If a recipe relies on natural cocoa plus soda, switching cocoas can leave the cake flat and soapy.
• Adding extra leavening for a taller cake. Past the right amount, bubbles merge and collapse. For more height, check your oven temperature and pan size instead.