Vapor Pressure And Boiling Point
All liquids create vapors. The amount of vapor produced is defined by the chemical characteristics of the liquid. The pressure exerted by these vapors in the presence of the liquid is defined as vapor pressure. Vapor pressure increases as the temperature is elevated.
The boiling point of a liquid is defined as the temperature at which the vapor pressure is equal to the external pressure on the surface of the liquid. When a liquid is heated in an open vessel it will boil when its vapor pressure is equal to the atmospheric pressure. Keeping this in mind, as elevation is increased the atmospheric pressure decreases and the boiling point of a liquid is lowered. Back in the early days it was common for an engine to "boil over" when carrying a heavy load in high elevations like those found in the mountains out West. The early cooling systems were not pressurized, so increases in altitude were very problematic. The use of a pressure cap raises the boiling point of the coolant 3 degrees Fahrenheit for every psi of pressure over atmospheric.
Ethylene glycol and propylene glycol have lower vapor pressures than water, and their boiling points are higher than water. Glycols are considered high-boiling-point liquids because of their low vapor pressure. As an example, at 68 degrees the vapor pressure of water is more than 100 times greater that that of propylene glycol. The low volatility of glycols lessens their tendency to evaporate and has led to their use as antifreeze for engines.
Glycol/water mixes generally have physical properties between those of water and plain glycols. The addition of water to ethylene glycol lowers its boiling point over that of straight EG. The smaller the concentrate of EG, the lower the boiling point will be.
 The freeze point of the coolant...  The freeze point of the coolant is determined by the ratio of water to antifreeze. |  If the additives are depleted,...  If the additives are depleted, a bottle of SCA is all that's required to bring the coolant back to specification. |  Overtreated coolant brings...  Overtreated coolant brings along its own set of problems, so always adjust the ratio of SCA using test strips. |
Distilled And Deionized Water
If a traditional EG-based coolant is going to be used, it needs to be mixed with water to become an effective heat transfer medium. The proper procedure is to use either distilled or deionized water and not tap water. This leads to much confusion since many do not understand what makes water either distilled or deionized.
The water supplied by most city water departments contains some dissolved solids that cause scale deposits as well as corrosive ions, such as chlorides and sulfates, when used as a coolant. Although these elements are usually soluble in water, they can attack the cooling system. Water taken from wells can be especially high in mineral content.
The best water to use, according to the Dow Chemical Company (makers of EG and PG), has been distilled and deionized or passed through a reverse osmosis process to remove minerals and salts.