What type of change is it when cake batter becomes a cake?

* New substances are formed: The ingredients in cake batter (flour, sugar, eggs, etc.) undergo chemical reactions during baking. The heat causes the proteins in the flour to denature and form a gluten network. The sugars caramelize and contribute to the flavor. The baking powder or soda releases carbon dioxide gas, creating the airy texture. These reactions create entirely new substances that weren't present in the batter.

* Irreversible: You can't easily reverse the process and turn a cake back into batter.

* Changes in properties: The cake has a different texture, taste, and appearance compared to the batter.

While some physical changes occur during baking (like the batter expanding due to heat), the chemical changes are the most significant and responsible for transforming the batter into a cake.