Why should you use water as a coolant?

You should not be using only water as a coolant it has no ethyl glycol which prevents it from freezing in the winter. It is only partially safe to use when there is no chance that it will be exposed to freezing temperatures. If your vehicle is a GM product you will need dex-cool. Mixing unlike coolants will cause them to coagulate.---

Water is used because of its ability to absorb huge amounts of energy.

A big problem is the properties of expanding when freezing solid. This can and most likely will destroy a car radiator.

Water is however not all that good for certain metals.

This in general due to different salts and minerals contained in tap water.

One can then add EthylGlycol and other substances both to reduce the corrosion and to lower the temperature of when water freezes to a solid.

Mixing different coolants Might cause the combined product to thicken.

One coolant can provide better corrosion protection over another product.