What a great question! This was in fact something I ran into just last week baking cookies. I used a pancake mix for the base, which had a lot more baking soda than I thought. The turtle-shaped cookies I was making became very fat turtles.

What is happening? All baking/cooking is to some degree like chemistry in the lab. Like any chemical reaction, you want to make sure that you have the right amount of each ingredient to react (i.e., stoichiometrically balanced). Too much or too little and you might end up with something you didn't anticipate (e.g., the turtle cookie example).

Every ingredient and process in baking/cooking has a purpose to achieve some kind of taste or texture, and baking soda is no different. Baking soda (aka sodium bicarbonate, NaHCO₃) acts as a leavening agent via an acid-base reaction. What does this mean? Basically, it makes things fluffy through reaction with another acidic ingredient, like applesauce, buttermilk, honey, brown sugar, cream of tartar, lemon juice, vinegar, and chocolate. You basic acid typically tastes sour and can donate a H+ hydrogen ion. In this instance, your base is the baking soda. When you react baking soda with something of acid, you make CO₂ carbon dioxide gas, which is what is responsible for making things fluffy. The chemical reaction for this is below.

NaHCO₃ + H+ → Na+ + CO₂ + H₂O

Baking powder is no different. It is simply baking soda mixed with a few other ingredients, like cornstarch to improve consistency and stability.

And in case you are curious about what the other ingredients do in baking, here are a few resources to give you an idea:

I think a great second example of the science of baking is the role of eggs and how they provide structure. I'll refer you to a set of related ScienceLine answers here that explains it more in detail.

Here is a great example of a scientific way of getting the perfect chocolate chip cookie for you. I think my favorite would be the one marked 'Both.'

Here are some pictures of my cookies
my cookies 1 !

The purpose of baking soda in baked goods is to react with an acid and produce bubbles that puff up the food. The acid that is usually used is vinegar or citric acid from a fruit. If too much baking soda is added, first of all it will taste terrible and have a “soapy” taste. Also, it may rise too much then collapse because the bubbles will get too big then pop. There are no health risks with adding too much baking soda though. If you accidentally add too much baking soda, the best thing to do is increase the amount of other ingredients so you don’t end up eating flat, bitter baked goods.

Too much baking soda will make the baked good taste bad, giving it a kind of soapy taste because the baking soda (sodium bicarbonate) is basic (basic substances in aqueous solution are slippery to the touch and taste bitter; they react with acids to form salts).

Baking soda is important to create CO₂, which helps baked goods rise. However, too much baking soda can cause things to rise uncontrollably, and then the baked goods might collapse after they finish cooking. It's not just about looks. Baking soda is also a base, so if you add too much, the baked goods can taste soapy.