antifreeze is also antiboil. It increases the temp at which water boils, or changes state. It also lubricates and inhibits corrosion; which is why one of many reasons coolant needs to be flushed occasionally.
When pure water boils it changes from liquid to gas at about 212°F or 100°C for the rest of the world. That is at room pressure at sea level, about 14.5psia. In a closed cooling system at 29 psia (15 psi cooling system pressure), the boiling temp increases to about 245°F. However-when a 50% antifreeze/distilled water mix is used in the system, the boiling point increases to 255°F or thereabouts. Two key points to be made here, or actually quite a few so bear with me.
One, by using 50% mix of glycol/water mix, the boiling point increases. Two, the freezing point decreases-dramatically. It is not the freezing point itself that is the problem, it is the expansion that is the problem. Inside the cooling system and in the engine itself ideally there is no space for expansion. As water cools, it also contracts up to a point (temp related), then it begins to expand-and it's not always AT the freezing point; it's also pressure related. As it's contracting, the pressure in the system drops off, thus the freezing point is raised a little. At roughly 34-35 deg F, it begins expanding-and quite a bit. Up to 10%, and the force exerted is astronomical. In school we paid a local machine shop to make up a steel vessel to house about a quart of water. The vessel was going to have a pressure gauge on it but had to be able to contain a huge amount of pressure. They made it out of 1/2" thick material, weighed roughly 25 lbs with the cap. In went the gauge fittings and we filled it with de ionized water, then into the freezer for a slow descent in temp. The pressure gauge went to 100,000 psi. The next day we came into the class and the vessel was BROKEN open and the pressure gauge's memory needle was buried at the 100,000 psi peg. So you can see that water's expansion rate and it's force exerted are HIGH. This is why you don't want water to get close to the freezing point. Pressure also increases as you get past the boiling point which is why the system's pressure cap is there-to relieve the excess pressure (but it don't work for relieve ice pressure, unfortunately).
Book says use water, but it doesn't say what kind of water. Tap water boils early and freezes late. It has all kinds of minerals and impurities in it due to the chemicals that the government wants us to have. Therein lies many problems. When tap water is used in a cooling system several things happen. One, it has zero lubricity which may not be important in some cooling systems. Second when it does boil (and it does sometimes), it pits the metal that it's in contact with. Thirdly, the energy given off as it boils is pretty intense. Fourthly, some cooling systems have radiators with "plastic" tanks, and boiling water will destroy the plastic tanks, which is evident by the whitish deposits left around the filler neck and down inside the top of the radiator; rendering the tank (and usually the radiator) useless scrap. Most antifreeze has additives to help with all this; which is why it's imperative to use it; ideally at 50/50 mix with de ionized water. Most of the 50/50 stuff you buy is already mixed up properly, so you just add it in. If not, you have to go find some de ionized water and mix it yourself at 50/50. Distilled water will still have some impurities in it, and the "distilled" stuff you get at the stores? You never know what's in that container...likely just tap water.
When a cooling system doesn't hold pressure for whatever reason, the boiling points and freezing points are going to be affected significantly. This is important consideration on older equipment.
