Why Does Water Put Out Fire (not Oil Fire)?

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Discussion Overview

The discussion centers around the reasons why water is effective in extinguishing certain types of fires, specifically comparing its effectiveness on general fires versus oil fires. Participants explore the chemical interactions involved, the role of oxygen, and the implications of using water on different fire types, including sodium and potassium fires.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions the role of free oxygen impurities in water and how they relate to fire suppression and fish respiration.
  • Another participant confirms the presence of dissolved gaseous oxygen in water but notes that it is insufficient for fire needs, emphasizing that fish require gills to extract oxygen from water.
  • A participant explains that water reduces available oxygen and removes heat, which is crucial for extinguishing fires.
  • It is noted that while water can remove heat from an oil fire, it can also create droplets of oil that increase oxygen availability, complicating the situation.
  • Concerns are raised about using water on sodium or potassium fires due to their reactivity with water, which can produce hydrogen and oxygen, potentially leading to explosive reactions.
  • Participants clarify that sodium reacts with water to produce hydrogen and heat, while potassium reactions are described as explosive.

Areas of Agreement / Disagreement

Participants generally agree on the chemical principles involved in fire suppression with water, but there are differing views on the implications of using water on oil and reactive metal fires, indicating unresolved concerns regarding safety and effectiveness.

Contextual Notes

The discussion includes assumptions about the sufficiency of dissolved oxygen in water for combustion and the specific reactions of sodium and potassium with water, which may depend on varying conditions not fully explored in the thread.

americanforest
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Why does water put fire (not oil fire) out? I understand that the bound Oxygen atoms in the H2O molecules can't be used by the fire but doesn't water contain free oxygen impurities; If not, how do fish breathe? Is it just because the amount of oxygen contained in the impurities is insignificant compared to the amount in the atmosphere that the fire was previously in?
 
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Yes - there is a small amount of dissolved gaseous oxygen in water, but you have to filter a lot of water to extract it that's why fish need gills. Fire can't extract the oxygen from the water in sufficent quantitites.

ps, The reason not to put water on a oil fire isn't any different chemistry it's that the water and oil mix slightly, then the water boils carrying droplets of oil into the air.
 
A fire requires three things: fuel, oxygen, heat. Water dramatically reduces the amount of oxygen available as described above, and it also evaporates and removes a large amount of heat.

On an oil fire water still removes some heat, but by throwing droplets of oil into the air it dramatically increases the oxygen available.
 
You wouldn't want to throw water onto a sodium or potassium based fire though.
 
Jeff Reid said:
You wouldn't want to throw water onto a sodium or potassium based fire though.

Why?
 
Sodium and potassiaum are reactive enough that they can break apart water into hydrogen and oxygen.
If you drop sodium into water it will give off hydrogen and oxygen which will burn, potassium pretty much explodes.
 
Last edited:
mgb_phys said:
Sodium and potassiaum are reactive enough that they can break apart water into hydrogen and oxygen.
If you drop sodium into water it will give off hydrogen and oxygen which will burn, potassium prety much explodes.

Well just hydrogen. (and a great deal of heat)

2Na + 2H2O => 2 NaOH + H2
 

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