The question I have every year is the following: Chametz is everything that is from the five grains and comes into contact with water. It then ferments. From what I remember from my chemistry classes, fermentation means yeast. So now the question – is yeast (that you can buy in the supermarket for baking) also chametz?
The answer to the question is NO, yeast itself is not chametz. Confirmed by my rabbi and Chabad (in reply to a question if wine, fermented grapes, is chametz): "Of the hundreds of species of yeast, the Passover prohibition only applies to yeast which is a product of one of the following five grains: wheat, barley, oat, spelt, or rye. Yeast which is the product of grapes, or its sugars, is not considered chametz (leavened food)."
Chametz also doesn’t include baking powder and baking soda. These are pure chemistry (sodium bicarbonate + acid in case of baking powder) and have nothing in common with yeast except the effect. The Coffee shop rabbi probably has the shortest possible explanation why they are not chametz: "chometz is basically wet grain" 😉
So, I will keep my baking powder and see what it does to matza flour! I’ll keep you posted.