The coolant that passes through the engine and associated plumbing accessories must have the ability to withstand temperatures well below zero without freezing. It must also be able to handle engine temperatures in excess of 250 degrees without boiling. Seems like a long margin for any fluid, but that isn’t all. The fluid must also posses rust inhibiters and a lubricant.
The coolant in modern vehicles is a mixture of ethylene glycol (antifreeze) and water. The recommended ratio is fifty-fifty. That’s to say that one part is antifreeze and one part water. This is the least recommended for use in automobile engines. Reduce antifreeze ratio and the boiling point would be too low. In certain climates where the temperatures can go well below zero, it is advisable to have as much as 75% antifreeze and 25% water, and no more than that. Pure antifreeze will not work very well and can cause a boil over.
Antifreeze is however poisonous and should be kept away from people and animals, especially dogs and cats, who are attracted by the sweet taste. Ethylene Glycol, if ingested, will form calcium oxalate crystals in the kidneys which can cause acute renal failure and death.