Glycolic acid is a chemical that falls into the larger class of compounds called glycols. Just as there are many different kinds of alcohols---another large compound class, including such molecules as ethanol (drinking alcohol), methanol (wood alcohol) and isopropanol (rubbing alcohol)---there are many different kinds of glycols, each of them with a unique set of chemical properties. The specific molecule glycolic acid is closely related to acetic acid, the molecule in vinegar, and has a number of interesting chemical properties.
Glycolic acid is a small organic molecule, consisting of two carbon atoms, four hydrogen atoms and three oxygen atoms. According to the "CRC Handbook of Chemistry and Physics," its molecular weight is 76.05 g/mol and its specific gravity is 1.27, making it approximately 25% denser than water. It is a colorless, odorless solid at room temperature, with a melting point of 167 degrees Fahrenheit and a boiling point of 212 degrees Fahrenheit, which is identical, incidentally, to the boiling point of water. Glycolic acid is exceedingly water soluble and is even hygroscopic, meaning that it absorbs water out of the air. This interesting property causes crystals of glycolic acid, if left open to the environment, to take up water until they become dissolved, liquefying the sample.
Although glycolic acid is stable under normal conditions, it can decompose if exposed to temperatures greater than 212 degrees Fahrenheit, according to the Material Safety Data Sheet (MSDS). The decomposition products of glycolic acid can be quite toxic and include carbon monoxide, a potent blood poison. Glycolic acid is also a strong acid and is therefore quite reactive with bases. The acidic nature of glycolic acid gives it utility in the cosmetic industry---applied to the skin, it will cause the outer layers of the skin to separate and peel. This can reduce the appearance of wrinkles, acne and uneven pigmentation and decrease the severity of flaking skin conditions. In essence, glycolic acid acts as a chemical exfoliating agent.
Like its chemical cousin ethylene glycol, which is commonly found in antifreeze, glycolic acid is exceedingly toxic if taken internally. The same liver enzymes that process ethylene glycol also process glycolic acid, producing oxalate in both cases. Oxalate then leads to kidney and liver failure, and in sufficient doses, to death. The acidic nature of glycolic acid makes it a skin, eye and respiratory irritant. Significant and repeated topical exposure can produce inflamed, reddened skin. The mucous membranes of the eye and respiratory tract are even more sensitive, and the MSDS recommends eye protection and adequate ventilation when handling glycolic acid crystals.