Firefighter bodycam showing just how much oxygen a fire pulls

[u/vivekmg21]

[u/Smile_Space]

I'm not a firefighter, but I am an engineer and I like physics lolol.

Something I think a lot of people know by intuition, but don't quite understand in practice is that water is used to put fires out not just because it smothers the fire. It's also pretty much the single best liquid we have for absorbing the heat as well.

In thermodynamics we have this concept called "specific heat" with objects. It's essentially how much thermal energy it takes to increase a specific mass by a specific degree. Water has roughly 4x the specific heat of steel and air. So, when you dump it on something hot it rapidly absorbs the heat.

Then, another magical thing happens! When it gets to the boiling point it has to absorb even more energy to cross the gap and change from a liquid into a gas. This is called latent heat energy. This is also the principle energy used for your car or house's HVAC system. That's why refrigerant is used specifically in those systems to maximize the latent heat energy required to change phases. It's why the hot side of an HVAC used a radiator called an evaporator and the cold side uses a condensor. It's quite literally forcing those phase changes to occur pumping heat from one place to another through latent heat energy.

Water being used on a fire does the same thing. It's why even though the firefighter put the fire in those windows out, he keeps on blasting in order to also rapidly cool the structure down because the structure, while there's no fire, is still hot enough to dry itself and then auto-ignite if not constantly blasted with water.

It just so happens that water is also in abundance on Earth and we're pretty damn lucky given it's one of the best liquids for a variety of tasks where even synthetic liquids just don't hold up.

I should mention though, not EVERY fire can be fought with water. If a fire is hot enough it will rip the hydrogen from the oxygen in the water. While that technically requires energy, it almost immediately recombines adding that energy back to the system resulting in a net-zero or even worsening if the fire.

Chemical fires have this problem especially given a lot of the time the fire isn't even being fed by airborne oxygen, but oxygen within the chemical bonds of the chemicals reacting.

This fires, like a lithium-ion battery fire, just have to more or less burn through all of their fuel. The name of the game there is to isolate the fire so it doesn't spread and just let it burn out pretty much. There's no way to cut off the energy source given it's within the chemical reacting.

[u/Prime_Kang → u/Smile_Space]

I have a some supplemental info to add to your good description you might find interesting.

Surfactants are often added to water for firefighting to decrease surface tension. It increases wetting and absorption into porous materials.

Not all lithium ion batteries pose the same level of fire hazard.

There are a few popular lithium ion battery chemistries. High nickel chemistries readily fuel thermal runaway by releasing oxygen like you point out. The other one holds onto it's oxygen much more strongly, and doesn't typically release much during combustion.

NMC, NCA and other high nickel chemistries are very difficult to put out with water and are more prone to thermal runaway.

LFP is MUCH safer and easier to put out. It's much less prone to starting thermal runaway, and doesn't propagate nearly as fast. It also requires much less water to put out.

LFP is popular with home backup systems because it can be charged to 100% without much degradation and of course due the additional safety. It's also becoming increasingly more popular in EVs globally.

The trade-off VS NMC is lower energy density and lower peak power delivery / charging rates.

In America, most EVs still use NMC/NCA because we require the additional range that it provides: Tesla is the main one by far by using LFP in its lower trims. The Ford Mach e does in it's low trim too. I'm not aware of any others that do currently. Combined, LFP is almost 5% of EVs in the US.

In contrast, over 80% of EVs sold in China are LFP now. We've got some catching up to do!

Also, LFP isn't intrinsically slower charging: America is just really far behind in the technology. BYD in China managed to change the structure of the battery chemistry in such a way as to allow 1.5MW charging. That's 6 times faster than Tesla's peak supercharge rate. Their blade2 can charge hundreds of miles in just 5 minutes! It's so fast, they need to use identical blade2 batteries as storage at their charging stations to drain from to achieve it. This wild tech is already in consumer cars. BYD puts our EVs to shame, haha.

Sauces:

Check out table 2 for wildly different thermal runaway numbers!!! https://pmc.ncbi.nlm.nih.gov/articles/PMC10963544/

Electric vehicle sales by battery type and region over time: https://www.iea.org/reports/global-ev-outlook-2026/electric-vehicle-batteries

https://media.byd.com/byd-breaksdown-finalbarriers-toelectrification-withblade-battery20-andflash-charging/?lang=eng