|MadSci Network: Chemistry|
Liquid PlumR, which is a commercial drain opener works in a couple of ways.
Liquid PlumR is a mix of sodium hydroxide, also known as lye, and sodium chlorate. Check out the Material Safety Data Sheet for Liquid Plumr.
When sodium hydroxide is mixed with water, the reaction is exothermic. It releases energy, which in turn heats up the solution. The heat from the solution helps to melt any grease or oils that may be clogging the drain. If the clog is in a pipe that is easily accessible (like under your kitchen sink) a hair dryer heating the pipe will produce the same result.
Also, when sodium hydroxide comes in contact with fats or oils a reaction between the two can take place. This reaction is called saponification, and means "soap-making". When a fat molecule is broken down by hydroxide, the product is a salt of a long-chain fatty acid. This is soap. Soap helps to emulsify (make soluble in water) oils and grease and allows them to be washed down the drain without resolidifying elsewhere.
Most drain cleaners are very basic (high pH) and can be dangerous to work with. Read the warning label carefully and follow the directions.
Try the links in the MadSci Library for more information on Chemistry.