MadSci Network: Chemistry |
Joel, The reason that putting a piece of bread in with the cookies makes them soft again is interesting. Good question. First you have to understand that cookies get hard and stale because they lose moisture to the surrounding air. Dry air passes over the cookies and essentially dries them out. When your aunt puts the bread in the tupperware with the cookies, they exchange moisture through the air. Bread contains a lot of water (fresh bread that is) compared to stale cookies. When you put them together in tupperware, the bread loses water to the surrounding air and then the cookies attract the water and become soft again. The key to this interaction however, and the reason the bread gets dry and hard, is because the cookies are more hygroscopic than the bread. This means that they have a higher affinity for water because they contain lots of sugar. Sugar, or sucrose in this case, has many hydroxyl groups in its molecules. These groups love to bind with water, more so than the molecules in the bread. Therefore, the cookies remove moisture from the air, which throws off the equilibrium, and the air continues to remove more moisture from the bread to compensate. Eventually you end up with soft cookies and dry bread. Hope this helped....
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