Which of the following is the best way to explain how baking a cake or frying an egg is a chemical change? Both are changing size and shape. Heat is changing the texture of both substances. Heat is being added to both, causing a phase change. The end products of both cannot be reversed to their original forms.

Cooking an egg is a set of chemical changes because chemical bonds are broken and new ones are made, resulting in the formation of new substances. In this case many of the bonds broken are those which cause the egg's proteins to be folded into a globular shape. The protein chains unfold and become entangled causing the setting effect. This is often called denaturing. Whether or not a change can be reversed has no bearing on whether it is chemical or physical. This particular change is almost impossible to reverse because of entropy effects but that simply isn't true of all chemical reactions, and there are many physical changes which are hard to reverse.