The clear protein of an egg white becomes opaque and firm when cooked because the heat:

The clear protein of an egg white, known as albumen, becomes opaque and firm when cooked due to a process called denaturation. Here's the step-by-step explanation:

1. Raw egg whites are made up of water and proteins. These proteins are in a complex, folded structure that is held together by various types of chemical bonds.

2. When you cook an egg, you're essentially adding heat. This heat energy is absorbed by the egg white.

3. The absorbed heat energy causes the proteins in the egg white to vibrate more and more.

4. As the temperature continues to rise, these vibrations become so intense that the bonds holding the protein structure together start to break.

5. Once these bonds break, the proteins unfold from their original, complex structure. This process is known as denaturation.

6. The denatured proteins then interact with each other in new ways, forming a network of proteins.

7. This network of proteins traps water molecules, which causes the egg white to become firm.

8. The process of denaturation also changes the way light interacts with the egg white, making it appear opaque rather than clear.

So, in summary, the heat from cooking causes the proteins in an egg white to denature and form a network, which traps water and changes the way light interacts with the egg white, making it appear opaque and firm.