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In a Cambridge IGCSE question paper, the following question was asked:

Describe the reaction of sodium with cold water. Include the observations that can be made.

I was thinking of writing that "sodium explodes" when it comes into contact with water, but the marking scheme says that point is unacceptable:

Common incorrect answers include "sodium bursts into flames or explodes"

Doesn't sodium react explosively in cold water (at STP/RTP) or is that reaction limited to water at warmer temperatures?

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    $\begingroup$ Note that sodium explodes even in inert atmosphere, as it is primarily a coulombic explosion. $\endgroup$
    – Poutnik
    Aug 25, 2021 at 11:25
  • $\begingroup$ Other cases: chemistry.stackexchange.com/questions/108332/… ... chemistry.stackexchange.com/questions/72234/… $\endgroup$
    – Nilay Ghosh
    Aug 25, 2021 at 11:38
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    $\begingroup$ From having tested with ca 5 pounds of sodium into a fjord of the atlantic ocean in autumn, I can say that it makes a horrendous noise, sputters and hisses, and eventually catches flame, carried upwards by the column of hydrogen that emerges from it. It spreads outwards quickly, setting aflame your boat and your oars. At this point there is a general sense of urgency as to the "getting the f out of dodge with flaming oars" going between you and your mates in your pathetic little on-fire skiff. $\endgroup$
    – Stian
    Aug 25, 2021 at 12:40
  • $\begingroup$ Edits made to the question are not notified to the users who answered and so the edit request you made might just turn futile. $\endgroup$
    – Nisarg Bhavsar
    Aug 25, 2021 at 13:54
  • $\begingroup$ Depends on the form of sodium, a single chunk - no, sodium sand probably will $\endgroup$
    – Waylander
    Aug 25, 2021 at 14:59

2 Answers 2

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The question is vague. Does the sodium burn, or does the hydrogen, liberated by the sodium, burn? If you were to completely immerse sodium in water, with no air above (or with an inert gas, e.g., argon), would even the evolved hydrogen burn?

In your answer, try to distinguish amongst the various reactions, including the amount of energy from $\ce{2Na + H2O -> 2NaOH + H2}$ and from $\ce{2H2 + O2 -> 2H2O}$.

  • Would enough energy be liberated to vaporize the sodium in that first reaction?
  • Is the yellow glow from vaporized/ionized sodium
    • from the heat of that first reaction, or
    • from sodium reacting with air above the water, or
    • from the metal spattered by exploding hydrogen...

My complaint with the question is that it does not specify the conditions, e.g., temperature, amount of sodium, atmosphere... but then, perhaps, it's meant for the essayist to state.

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From personal experience doing a demonstration I can tell you that for similar amount of sodium (approx. 0.5 cm3) if you use cold water you would not see an explosion. Using small volumes of water at room temperature (around 20 °C) or hot water the explosion is almost guaranteed.

I think that is all due to kinetics of the reaction and thermal diffusion, thought it is a complex process and there is plenty of nuances.

I recommend you to access the article Exploding some myths publish by Declan Flemig in Education in Chemistry https://edu.rsc.org/feature/exploding-some-myths/2000082.article.

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