If you leave a hole in the middle of a cookie and cook it, yes, the cookie gets bigger and the hole gets smaller. Why?
In nature everything depends on the temperature. That is, the numerical value of any physical magnitude is affected, to a greater or lesser extent, by temperature. This dependency is characterized by the coefficient of thermal expansion, which is a specific property of each material.
Answer and Explanation:
All known matter is made up of particles (atoms, neutrons, etc.). The distance between these particles depends on the temperature. Microscopically, as the temperature increases, the separation between particles increases, which, macroscopically, translates into an increase in the dimensions of the material.
In the case of the cookie, when we bake it, we increase its temperature, this produces an increase in volume. Since the air in the central hole does not offer resistance to the increase in volume, its diameter is reduced. On the other hand, as the tray where they are baked offers resistance to the increase in volume, the cookie does not grow in that direction. This is why the center hole decreases when baking cookies and its bottom remain flat.
In addition, by increasing the temperature we also remove water from the mass so we also induce a phase change of the atomic structure. By cooling the cookie, its volume is also reduced but in a smaller portion as a result of the new atomic structure.
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Learn more about this topic:
from UExcel Physics: Study Guide & Test PrepChapter 11 / Lesson 8