There are a few chemicals that turn reddish when exposed to water and oxygen, but the most common and well-known is iron.

Here's why:

* Iron Oxidation: When iron reacts with water and oxygen, it undergoes a process called oxidation. This forms iron oxide, commonly known as rust. Rust is a reddish-brown compound that can be seen on many objects made of iron.

Other possibilities:

* Some copper compounds: Certain copper compounds can also exhibit a reddish coloration when exposed to air and water. However, the color change is often more greenish or bluish, rather than a distinct red.

Important Note: It's crucial to remember that handling chemicals requires caution. Always follow safety procedures and guidelines provided by manufacturers and your local authorities.