Home candymaking is one of those old-school skills that's fun to exercise, any time you can get the kids out from underfoot. For example, peanut brittle is an old-fashioned favorite that's welcome at any time, either as a holiday treat or an everyday indulgence. Most recipes use baking soda to lighten the brittle, making it less tooth-breakingly hard to eat. Although baking powder can be used in place of soda for most recipes, it's not suitable for brittle.
Soda vs. Baking Powder
Both baking soda and baking powder play the same role, lightening your baked goods more quickly and reliably than yeast. Baking soda is mildly alkaline substance, called sodium bicarbonate. When it comes into contact with acidic ingredients, it foams vigorously and releases bubbles of carbon dioxide gas. Those bubbles, in turn, make your baked goods light and delicate. Baking powder is made by combining baking soda with a dry acid, such as cream of tartar, and buffering ingredients, such as cornstarch, to prevent them from reacting with each other prematurely.
Brittle is one of many candies made by boiling sugar until it reaches a specific temperature, called the "hard crack" stage, or just over 300 degrees Fahrenheit. At that temperature the sugar is liquid, but when it cools it forms into a smooth-textured, hard, dense sheet. Sugar can be chancy to work with, and at the slightest opportunity it will re-crystallize into its normal, grainy form. To prevent that, most recipes include either an acidic ingredient, such as lemon juice or vinegar, or an invert sugar, such as corn syrup or glucose. Both help keep the sugar from crystallizing before you're ready.
If you pour the sugar onto a sheet at that stage, it makes a solid, smooth-textured slab of very hard candy, like commercial hard candies or lollipops. What makes brittle different is that at the very end, before you pour it onto an oiled or parchment-lined sheet, you stir in baking soda. The soda reacts to the acidity in the sugar, making the candy foam up to several times its previous volume. That makes it lighter and easier to break and chew -- more brittle, in fact, hence the name -- and also helps give the candy its pleasant brown color.
Although baking powder creates a somewhat similar effect, it's not suited for use in candy. It's formulated to generate carbon dioxide more slowly, so you have time to get your baked goods into the oven before all the leavening power is used up. That's good in baking but unsuitable in brittle, where the sugar cools quickly.
If you don't make candy regularly it's best to use a candy thermometer, because if the temperature isn't right, your brittle won't have the correct texture. Be sure to use a large pot with plenty of room for the candy to expand after you add the soda, because if it boils over you're not going to be happy. At best you'll be scrubbing sugar from your stove and counter for days. At worst, you might need treatment for burned hands, or even worse a child with burns.