Demonstrate chemical oxidation (loss of electrons) with the browning of freshly cut fruit.
At the beginning of class, cut a fresh apple (or a banana or avocado) into thin slices. Next, slice a lemon, but do not allow the lemon juice to come in contact with the other fruit. After 15-30 minutes, the flesh of the apple should turn noticeably brown. This is due to the interaction of the interior pigments of the apple (or other fruit) with atmospheric oxygen. Lemons do not turn brown because lemon plants can synthesize vitamin C from glucose. Vitamin C inhibits oxidation of the lemon because it inte racts very quickly with oxygen, preventing the oxygen from interacting with the rest of the fruit. Vitamin C is often referred to as an anti-oxidant.
Using a hot plate or bunsen burner, fry an egg in a beaker. Heating denatures the protein in egg whites, causing them to turn from clear to white in appearance. Heating the proteins of the egg causes them to unfold. When newly exposed amino acids are chemically attracted to other newly exposed amino acids, new peptide and new hydrogen bonds form.
Milk contains two kinds of proteins, whey and casein proteins. The casein proteins are involved in the curdling of milk. These proteins normally exist as negatively charged groups that repel each other, and are thus distributed evenly throughout the milk. When vinegar or lemon juice, both acids, are added to milk, this causes the groups of casein proteins to lose their negative charges and then lose their ability to repel each other. They then bond with each other, causing coagulation, or curdling (sourin g) of the milk.