Water to NaOH vs NaOH to water

What is the difference between the following two reactions, if any? I’m assuming equal amounts of both here.

NaOH(solid) + H2O -> lye(aq)

H2O + NaOH(solid) -> lye(aq)

Both reactions are exothermic, and both seem to be the same to me. 

However, everywhere, including in MSDS sheets, you will read that you should always add the solid sodium hydroxide to the water and never the other way round, because explosion risk (???) 

I’ve tried both ways and there are no explosions, and the end result seem to be the same. Am I missing something? Or is this one of those internet myths that are simply wrong?



  • PerryPerry Administrator, Professional Chemist
    You're missing something. It is not an Internet myth.

    The thing is this reaction is fast and exothermic (creates heat). Well, that heat has to go somewhere. When you add the solid to the liquid, heat gets transferred to the excess liquid. The temperature of the liquid water will increase and may even boil. However, water has a relatively high heat capacity which means it takes a lot of energy to raise the temperature. The more water, the more energy it takes to increase the temperature.  Also, the water will only interact with the surface of the solid so the reaction is slower.

    But imagine you do it the other way (add water to the solid). It's the same reaction but this time when the heat gets transferred to the liquid, there is a lot less liquid around. This smaller volume heats up more quickly and can rapidly boil & possibly explode. 

    Now, if you're working with small amounts and you're adding the water rapidly perhaps you won't experience a problem. But it's much safer to add the solid to the liquid.
  • How small an amount are you talking about? I just went outside and tried it again. I was going to post a video, but I can only post images.

    From what you are saying, a smaller amount of water would create a stronger reaction. I added about a tablespoon of water to half a cup of NaOH, expecting this stronger reaction, but nothing much happened. It barely got warm. It dissolved some of the solid, but that was it. 

    PS I'm not doubting you at all. I'm just flummoxed why the experiment doesn't back the theory. I used room temperature distilled water. And the NaOH is 99% pure according to the label. How can I create that boiling effect? 

  • Add 50g water to 50g NAOH and continue mixing it to reach more water to more NAOH quickly and see. 
  • MarkBroussardMarkBroussard Member, Professional Chemist

    This isn't "a theory" ... it's good lab practice.  You always add the solid base to water as Perry explained.  The main issue is safety ... if you add water to sodium hydroxide it may splash out of the container and into your eyes which can result in serious damage or blindness.  There is much less risk of this happening when you add the NaOH to water since you can better control the exothermic reaction.

    As to the end result ... both methods will yield the same NaOH solution.
    Chemist/Microbiologist formulating in the Organic & Naturals & Clean Beauty arena under ECOCert/Natural Products Assn/Whole Foods/National Organic Program/Clean At Sephora/Credo Clean guidelines focused skincare & haircare products. 

    See website for details www.desertinbloomcosmeticslab.com
  • PerryPerry Administrator, Professional Chemist
    @kiwigirl71 - There's a balance. If you have too little water there won't be much happening as the excess water may all just boil off.  If you have too much, then heat capacity effects take over. If you want to discover the range where you get a bad reaction, try an experiment where you adjust the ratios of solid and liquid. (I don't recommend this!!) But if you want to prove it to yourself start with 50 g of NaOH, add 5 g water. Then try the same experiment with 10 g water.  etc. up to 50g. Be sure to wear gloves, glasses and stand back.

    But as @MarkBroussard says, it's about safety and probabilities. Why take a chance when you don't have to?
  • @Perry and @MarkBroussard

    The reason for doing this is because the experiment doesn’t match the theory. And yes this is a ‘theory’, which should be able to be proven scientifically. I’m not a chemist, but a physicist, so the scientific method is familiar to me. 

    When I try something, in this case to test if adding water to solid lye ‘explodes’, I expect the theory to hold. And I expected a reaction. Because this didn’t happen, and I can’t for the life of me figure out why, that’s why I came here. 

    Out of curiousity, have either you actually tried this out? I have. And yes, wearing the whole Breaking Bad outfit, which really must have looked hilarious to anyone watching for lack of any reaction. 

    And as to why, to demonstrate that it really is a bad idea to add water to solid lye. 

    And lastly, I did the next batch of experiments, each time using 50 g solid NaOH. Each in a 250 ml pp beaker. Using 1 teaspoon, 1 tablespoon, 50 g and 100 g of distilled water at room temperature respectively. 

    Results: none of the samples reacted in a violent manner as expected. None of the samples boiled, although the temperature did rise, with the 50 g water sample showing the highest temperature increase. I did not see any gas (steam) coming off in the smaller samples, only in the two with 50 g and 100 g water. 

    But most importantly, none of them showed any exploding tendencies. 

    I don’t have any conclusions yet, only that maybe the samples are not large enough. 

    Soap makers usually make 1000 g of soap in a batch, using around 130-140 g of NaOH, depending on the type and quantity of the oils used. 

    So tomorrow, I’ll repeat the test using 200 g of NaOH with 5 g, 15 g, 100 g and 200 g of water respectively.
  • This seems dangerous, I hope you are taking all the necessary precautions. 

    Here are some YouTube videos that you might possibly want to watch that demonstrate this effect but with hot water used:

    Sodium hydroxide to hot water

    Hot Water to sodium hydroxide
  • PerryPerry Administrator, Professional Chemist
    @kiwigirl71 - I applaud your skeptical approach and inquiry. But I will add that just because you ran one experiment and didn't see any explosion doesn't mean that everyone who runs the experiment will not see an explosion. Or even that every time you run the experiment you will not see an explosion. You could have just created superheated water.

    But the two videos linked by @Sincityfire above show people who did mix water into Sodium Hydroxide and saw an explosive action. In fact, the first one showed violent reaction even when adding the solid to a liquid that was already too hot. This just means that it does happen, just maybe not every time. 

    So the advise is taking a "better safe than sorry" approach.

    I'm sure a person can cross a road numerous times without getting hit even without looking both ways.  But they should still look both ways before crossing. 
  • As you said, the water they used was already hot. Here’s a video, not mine using cold water. 

  • From what I learned here is not that you shouldn’t add water to the lye, but rather DO NOT USE HOT WATER. If you use hot water, regardless which method you’ll have a problem. 

    The heat released using cold water doesn’t seem to be sufficient to create an explosive exothermic reaction. And I guess my next step now will be to prove that mathematically. Luckily, we are now venturing into thermodynamics here, into my field of expertise. I’m back at work today, but will post the exact calculations later. 
  • PerryPerry Administrator, Professional Chemist
    Looking forward to it.
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