By-product

A by-product is something that is made or produced while a product is manufactured in a factory. Usually, the by-product is not used, and it becomes waste that is put in a landfill or garbage dump. Sometimes by-products are toxic materials which are dangerous to humans, animals, and plants.

Some companies are trying to find ways to reuse or recycle the by-products, instead of putting them into landfill or garbage dumps as waste.

Animal by-products are produced by slaughterhouses (places designed to kill animals for food). Animal by-products are the parts of the animals that cannot be eaten by humans. Animal by-products are sometimes included in non-human foods, such as pet food or animal feed (for farm animals).

In chemistry

In the chemical industry, a byproduct is usually any product other than the main reason for performing a chemical reaction. Chemists often use "byproduct" only to mean those chemicals that directly come from making the main product, and "side product" for products from alternative side reactions.

For example, a chemist might want to make chloromethane. The reaction for producing this is

CH₄ + Cl₂ → CH₃Cl + HCl

The hydrogen chloride is a byproduct, because it comes directly from the reaction that makes the main product. However, it's hard to make this reaction stop at just chloromethane, and some of that product will react with extra chlorine gas:

CH₃Cl + Cl₂ → CH₂Cl₂ + HCl

This dichloromethane would be called a side product, because in theory this second reaction doesn't need to happen to make chloromethane.