OK so I've got a clear liquid here... and a different clear liquid here... and I'm going to add a white powder to this one -- ooh, bubbles -- and then a different white powder to this one... Also bubbles, but different this time... Now while these are sitting around I'm going to hop over here and get a water bath up to 75 degrees Celsius And now I'm going to take both of these - - which have stopped bubbling -- and put them... in this water bath, and wait a few minutes for these to come up to temperature, and... what....?

Where are these bubbles coming from?

[splash] Alright, so you've probably guessed that the liquid in this bowl was vinegar and that the white powder was baking soda, so you also know the bubbles are carbon dioxide.

In this bowl, the clear liquid was just plain old water, and the powder was a mixture of baking soda, monocalcium phosphate, and cornstarch.

You know this mixture as double acting baking powder, and it's quite different than that other white powder -- baking soda.

Here's how.

Baking powder contains at least one solid acid.

In this case, monocalcium phosphate.

It also contains a solid base, almost always baking soda.

The acid and base react with each other when water is added to produce carbon dioxide gas.

But... we waited for this reaction to finish, in other words we waited for all the acid and the base to react with each other... so... Where are these extra bubbles coming from?

Well it turns out that the acid doesn't dissolve very much in water.

So, while the baking powder is just sitting here at room temperature, most of the acid doesn't actually react with the base.

Heat helps the acid dissolve more in water, which means more of it will react with the baking soda, which means more CO2 is produced, which means... more bubbles.

There may also be other reasons heating makes more CO2, depending on which baking powder you use, but that's a long and convoluted story involving Henry's law and Le Chatelier's principle... so we'll cover that in another video.

With cookies next time.

Mmm.

Cookies.

Anyway, this is where the "double acting" thing comes from -- it "acts" once out here in the batter, and then it "acts" again in the oven.

By the way, almost all baking powder sold today is double acting.