If it does evaporate, then how much in comparison to the water?

If it does not evaporate, can I be safe to assume I only need to add water when replenishing lost coolant?

Edit #1

I just want to know in order to keep the right coolant to water ratio. If I ran a higher ratio of either water or coolant, theoretically it will either enhance or lower the benefits of either. Is this too extreme of an approach to coolant or should I just not bother with the ratio accuracy?

  • coolant also evaporates, its rate might be lower than water, to be on safe side I usually use a higher than advised ratio of coolant in mixture, so that if I run low on mixture I will only have to add water until next flush
    – Nilabja
    Oct 24, 2017 at 5:33
  • @Nilabja - Do you have a reference for the statement that it's lower than water, and if it is how much?
    – Narcotixs
    Oct 24, 2017 at 7:05
  • No, thats why I said "might"
    – Nilabja
    Oct 24, 2017 at 7:07

3 Answers 3


Usually it is the chemicals that prevent freezing / corrosion that evaporate which is why the annual service / test includes checking the Specific Gravity to make sure that the anti-freeze will still do its job.

Make sure that the ratio is correct then, in future, add or top-up with the correct mix for the season.

Edit to add information due to a comment : The water will evaporate, but the portions that have a lower boiling / vapourization temperature get evaporated first - think about adding wine to a stew during cooking - the alcohol gets boiled off first - that, in fact, is how some sprits are made....

  • So you're saying water doesn't usually evaporate, only the anti-freeze portion.
    – Narcotixs
    Oct 24, 2017 at 16:28
  • So based on the adding alcohol to soup example, wouldn't that mean anti-freeze evaporates at a faster rate than water, and if so, at what rate? The boiling point of ethylene glycol for example is 197.3°C, which is higher than water (i.e., 100°C), so how is it possible that it evaporates at a faster rate?
    – Narcotixs
    Oct 29, 2017 at 23:48
  • The notion that ethylene glycol evaporates faster than water is simply false. I have added an answer which explains why.
    – mongo
    Jun 11, 2019 at 16:10
  • @mongo so all those cars that I tested the SG on and found the concentration insufficient to protect the engine components from freezing damage were wrong... OK, so a mate had a Sunbeam Tiger with the cast iron V8 which had freezing damage (we CI welded the plate back into place and it was then fine), turned out the anti-freeze solution had degraded - sufficient water ... but not protection. Note ethylene glycol is not the only chemical...
    – Solar Mike
    Jun 11, 2019 at 16:16
  • The vapor pressure of vodka (40% ethyl alcohol) is higher than water, and it will boil off first. Ditto for menthol alcohol. However 99% or more of vehicle antifreeze products in use today are ethylene glycol, diethylene glycol or propylene glycol. Look at the products at your parts store. Read the label. Propylene glygol has a vapor pressure similar to ethylene glycol, but diethylene glycol is 10 times lower than ethylene glycol. I can't say what antifreeze chemical your mate was running, but I can talk to the chemical and physical properties of chemicals.
    – mongo
    Jun 11, 2019 at 16:31

I never heard of any freeze plugs that popped due to a degraded coolant/water ratio protection that degraded to, say, only -15 degrees C. OTOH, I don't live on the artic circle. But bet is to change the stuff completely on a regular basis and enjoy the benefits of all worlds...

It's not that expensive and good insurance on a number of fronts.

  • It is true that adding even a sub-recommended amount of ethylene glycol to a a radiator will help create more of a mush, than a highly expanded ice cube, and it does provide some block freeze protection. As an example, on one VW when I got caught up north with a presumed -34F mix, it was -45F, and things had not frozen. It was a mush. The engine turned over OK, and the decision was to start, idle for 3 or 4 minutes, then shut down. That was repeated until the block measured -20F, and then the engine was left to run. No problems. Your mileage may vary.
    – mongo
    Jun 11, 2019 at 15:36

Common ethylene glycol is the antifreeze agent in most, but not all automotive antifreeze solutions. Common mix ration is 50/50 with water, and additive packages are included which commonly include dyes and more recently a taste agent to make the product taste bitter overcoming the normally sweet taste which has been a factor in the poisoning of children and pets.

At common temperatures the vapor pressure of water is far higher (300x) than the vapor pressure of ethylene glycol. For example, at 20C, water is 17.53 mmHg, and ethylene glycol (also called MEG) is 0.06 mmHg.

At higher temperatures that spread gets wider. Bottom line, an open container of 50/50 ethylene glycol and water, will result in most of the water evaporating first. If heated, at normal atmospheric pressure, then the water will evaporate even faster.

In older cars, tractors and other internal combustion engines, it is a common but not universal practice to run the cooling system unpressurized so as to reduce the stress on aging radiators and hoses. Overtime, with water vapor being released from a hot system, the mixture in the cooling system will become more concentrated. This will lower the thermal transfer properties a bit, so maintaining a mix mix near 40/60 to 50/50 of ethylene glycol and water will maintain a good thermal transfer rate.

Two common methods for measurement of the concentration are hydrometer (liquid density), or optically by use of a refractometer. In the US, presently, hydrometers are sold at car parts stores for under $10. The refractometer is a more accurate instrument, and can be used for battery electrolyte specific gravity estimations also is is commonly found for about $22.

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