Mineral oil is a colorless, odorless and tasteless liquid that can be used for many different purposes. It's often used as a lubricant or base ingredient in products such as cosmetics, pharmaceuticals and even some types of food. However, one thing it isn't very good at is maintaining its original appearance over time. One of the most common issues associated with mineral oil use is that it can quickly turn black due to an oxidation process caused by exposure to air.
This oxidation reaction occurs because oxygen from the air reacts with the hydrocarbons in the oil, creating new molecules called carboxylic acids which are dark in color. This chemical reaction also produces other by-products depending on how much heat or light was present when exposed to air. If temperatures were high enough during exposure then this could cause other molecules known as quinones to form which are also dark in color but instead of being soluble they're not water-soluble like carboxylic acids so they often remain suspended within the oil giving it an even darker look overall.
The best way to prevent mineral oils from turning black is storing them correctly away from excessive light and heat sources; however if you do find your product has already turned it's best practice not to consume anything containing oxidized mineral oils as this may have potentially harmful consequences for your health due to all of their different by-products created through oxidation reactions taking place inside them over time!