A great question! I'm going to interpret your question as why you need to put batter into the oven before you eat it. I'm always thinking about this every time I start a new recipe. Baking transforms your goopey batter into something solid and edible. How does this happen? It all comes down to the role of temperature in chemistry.

Chemistry (and actually many things in materials science in general) are governed through a combination of thermodynamic and kinetics. Thermodynamics tells you how energetically favorable a reaction is and kinetics (the motion of things, i.e., molecules) tells you how fast it is going to happen.

In a basic chemical reaction, there is an activation energy associated with the reaction. This is because during a reaction, there is a point in the reaction where your reactants come together as an activated complex that is a fusion of the reactant and product and is energetically unstable. This is at the peak of the activation energy (E_activation, in the graph below), and is like an energetic hill that your reactants have to overcome to react and get to the other side (i.e., to the products). This activation energy is (in part) determined by the thermodynamics of your system.

graph

How does temperature contribute to this? Temperature is an average kinetic energy of the particles in your system. You have probably learned that kinetic energy is:

click here, please

It is the energy associated with the motion of the particles colliding with each other. Because temperature corresponds to an average kinetic energy, that means there is a range of kinetic energies present in the system. The figure below shows a possible distribution of such kinetic energies for two temperatures, T1 and T2 (for T2 > T1). You can imagine T1 to be room temperature and T2 to be the temperature of the oven. As you can see in the graph, the higher temperature T2 is broader and shifted to the right. This means it has a higher average kinetic energy, which means more molecules are above E_a (i.e., have the minimum needed energy E_a to react). This means a reaction is more likely to occur, so overall for many particles, your reaction happens faster. You could in principle wait for a reaction of this kind to proceed without putting it under a higher temperature. It would just take a lot longer (and your batter would probably go bad and be inedible for other reasons). (Likewise, it's the same reason you put things in the refrigerator- lower temperatures hinder the progress of reactions that may cause your food go bad.)

There are additional influences that temperature has since the reactions that occur in baking are not just particles colliding and reacting. The best example is the use of egg in almost every baking recipe. Why is that? It is because egg is used to make your batter structural. That is, it is used to turn what is before liquid batter into a solid baked good! In order to do that, you need to crank up the temperature.

An an uncooked egg consists of many proteins, which are large and complex macromolecules. When you increase the temperature, you activate a process known as denaturing. The basic idea is shown in this link . For raw egg white, the proteins are folded; this makes them easy to pull apart (like with a whisk!). The protein is folded because lined along each protein chain are many different types of chemical groups that are attracted to each other. Increasing the temperature disrupts this attraction and promotes the formation of bonding between chains (i.e., aggregation) . This creates a stronger network of proteins, which is what makes a cake or brownie solid. (This process of denaturing also happens to be the same process when you cook meat. Similarly, the formation of bonds between molecule chains is also akin to making the rubber in tires.)

If you are curious about how other ingredients or conditions (like pressure) influence the chemistry of baking, you can visit this website baking with altitude . And because this question is about baking, I'll also point you to an NPR article for making your ideal chocolate chip cookie .

Hope this helps!
Best,

Baking is a great example of physics in the kitchen.

When we put our cake in the oven he heat it up. The heat slowly diffuses from the surface of the cake to the inside, that's why baking takes so long!

Sometimes we want to make a fluffy cake, like in sponge cakes. In this case, we want to introduce a lot of air bubbles. We also want the cake to stand up and not collapse like a soufflé when it has a lot of bubbles. To do that, we add flour and eggs.

So here is how we achieve the perfect result:
We whisk the butter and the sugar to introduce many air bubbles. The molecules of oil go to the surface of the bubbles and they prevent them from collapsing under pressure. If we let it sit too long, the bubbles will meet each other and collapse, that's why we cannot make a batter today and use it tomorrow. Our cake will be flat.

When we heat up the batter which has bubbles, the gas heats up too and expands, so the gas bubbles grow larger. If we want more air bubbles, we can add baking soda. When heated, baking soda releases carbon dioxide which is a gas. Carbon dioxide also expands when heated. These are the two ways to introduce bubbles.

Another important part is the addition of eggs. When we heat butter it melts. But when we heat eggs they become solid, just like when we fry them! It is because of the proteins inside them, which lose their shape as we heat them up. So, by becoming solid, eggs help us to trap the bubbles because they create a solid structure around them. For the same reason we add flour. When heated it creates a network which prevents our cake from collapsing. Soufflés don't have much flour and that's why sometimes they collapse at the end as they cool !

Different processes take place at different temperatures. For example, chocolate will melt at 34C (degrees Celsius), but the white of the egg becomes solid at 63C and the yellow at 80C.

Finally, don't overcook your cake! Because the outside which has a higher temperature because of heat conduction will get harder and become solid. In this case, the inside bubbles as they heat up and expand will increase the inside pressure and then the cake surface will burst at the top!

By baking you probably mean how foods that start as dough and are heated in the oven form breads or other “baked goods.” The key to getting baked goods to puff up and solidify is carbon dioxide. Carbon dioxide is a gas which fills in the holes in the dough’s “web of gluten” causing it to rise. There are two ways to get the carbon dioxide into the baked good. The first way is using a living organism, yeast, which eats the flour and produces carbon dioxide as a waste product. The second way is using a simple chemical reaction with baking powder where you combine an acid and a base and get CO₂. The yeast method leads to traditional breads, while the baking powder method leads to “quick breads” like biscuits and cakes. When the baked good is put in the oven, the steam will cause it to rise more leading to the fluffy texture of quick breads or the fine crumb of yeasted breads. Additionally, the baked good will solidify at the high temperatures due to water boiling off and various chemical reactions.

In summary, baking occurs when CO₂ and steam are trapped in the gluten web causing the baked good to puff up.

All chemical reactions are dependent on temperature. The warmer, the faster they happen. Cooking is a result of very rapid chemical reactions due to high temperature.