I've posted this on another forum, but wanted expertise from this one too.

I have a question that I've wondered about regarding radiators.

I've always been under the impression that you can pressurize both liquids and gasses,
but liquids do not expand whereas gasses will.

Soooo, what causes the expansion in our cooling systems?
Is is there always some air trapped in the system somewhere or does the heat generated
near the hot spots in the heads cause percolation?

So then the question follows; where is the recovered fluid in the overflow tank going to in the
cooling system??

I hope someone's explanation doesn't make me feel like a dolt....LOL

Joe

Your fine Joe. when the system cools, the resulting vacuum is supposed to draw the coolant back into the radiator, not sure how well that works

Fluids expand, you are confusing that they don't compress.

Specifically, as temp increases, solids, gas and fluid expand. if you will recall from your school days,
https://www.bbc.co.uk/bitesize/guides/zq7t4j6/revision/2#:~:text=Liquids%20expand%20for%20the%20same,they%20expand%20more%20than%20solids.&text=Heat%20causes%20the%20molecules%20to,of%20a%20solid%20or%20liquid.

It is literally the atoms moving faster causing them to push away from each other causing the expansion.

Compress ability is why we use fluids in like the braking system.
It causes the pressure to build up faster than a system with gas in it.
IF you think about the expansion due to heat the same way, molecules have more room to push together in a gas vs a liquid or a solid.

That is why when you get volume measurements it is at 70 degrees F not at any temp.
you can then adjust for the temp that the item is at for actual volume.

https://hypertextbook.com/facts/2007/AllenMa.shtml

Water is also the only substance that I remember that expands when it freezes. An exception to the rule about getting smaller as the temp goes down.
which is why the block cracks if you only have water in it and it freezes.

I'm sure most liquids expand when heated, the heads and blocks due when heated and they are trying to compress the coolant inside them adding to the expansion of the liquids in the cooling system

Aha Sniper, it's been 61 years since high school physics.

Probably not the only thing I've forgotten,

Thanks, Joe

Heat expands the coolant. There is a slight increase in volume. That will push out any air at the top of the radiator and coolant into the radiator cap and into the Bottom of the reservoir.
When you stop and cool. The coolant contracts, pulling in straight coolant from the Bottom of the reservoir.

After a couple cycles, there will be no air in a cooling system that has a reservoir.

Liquid's density is often temperature dependent. Part ( rather small) of the Ocean level rising we are currently experiencing is from the water simply getting warmer. Silicon brake fluid is a well known example of compressible fluids.
Any liquid that expands, can contract.

Based on the OP's question and context, temperature must be considered.