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When you mix 2 chemical compounds together, you alter its overall physical chemical behaviour (compared with each individual item).
For antifreeze & water - what happens is the freezing point is lowered (from 0 Celsius - to a -ve value - which is dependent upon how much antifreeze is added). Also, the boiling point of the solution increases beyond the 100 Celsius of pure water (again, depends on how much antifreeze is added) - this behaviour is part of a string of changes called Colligative Properties - there are other differences such as electrical conductivity, specific heat capacity, surface tension, etc - but those that I've gone into detail, are the main reasons behind its use.
Davesss is perfectly correct in stating that it should be used constantly, especially as modern engines carry less coolant than before, but use ever narrower channels around the engine to duct the heat away - any 'furring up' through hard water or corrosion will prove disasterous - its a common fault on Rover 'K' series engines (one of the earliest adopters of this modern philosophy).
Hope this helps.
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2005 FIAT Ducato 2.8JTD LWB Gran Volume, 2000 Kawasaki ZZR 1100 D7 'fullpower' in black + full GIVI & KAPPA luggage, 2007 MB R-Class 320 CDi Sport LWB uprated by Brabus to 300Bhp don't know if there's a 155mph limiter - will be fun finding out! 2004 Mercedes Vaneo 1.7 CDi Ambiente 7-seater