LXguy said

Because antifreeze has a higher boiling point than water.

Guess what, the freezing point is lower with a mixture of antifreeze and water too! LOL!

http://www.hotrod.com/techarticles/texaco_dex_cool_antifreeze/index.html

"While it is true that straight water is the best heat-transfer liquid you can use in an automotive application, straight water, even deionized or distilled water, is still highly corrosive. And the addition of corrosion inhibitors doesn’t deliver a balanced anticorrosion package as does the new Dex-Cool."

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