A bonus question would be, if so, why? Is it because it decomposes and hence the expiry date is calculated out?
They do indeed. It could be for different reasons:
- A chemical might undergo chemical changes due to different storage conditions: it could react with other chemicals present in the air or in the environment. This could change its nature (reaction to create other compounds), properties and/or purity.
- A chemical might undergo physical changes as well: the temperature and pressure vary, as well as other conditions like humidity, light, electromagnetic fields. This could change it as stated above.
- Certain chemicals are simply unstable and decay or degradate over time to something else, without the need of externals input / changes. This could affect the chemical as stated above.
- Certain chemical suffers transpiration losses.
- Human factors, like contaminations, spillings, loose caps, etc.
However, it might be noticed that these reasons could be divided to produce two different dates: shelf life vs expiration date.