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I have read (thanks to Stack Exchange) that mixing water and sulfuric acid is exothermic, as is NaOH and HCl.

Are there two 'safe' liquids that can be mixed and produce anywhere near the same amount of heat? By 'safe' I mean they don't come with the same heavy warnings as sulfuric acid or lye. I would like them to be safe enough to be stored in a home and that a spill would probably cause some damage but clean up could be performed by the home owner safely.

I understand that the actual mixing might not be considered safe if they do release a lot of energy.

I'm only considering liquids because they can be kept separate and pumped together.

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    $\begingroup$ Being the two individual liquids safe means that they are both relatively inert. So the heat will be merely that of mixing. I don't expect any couple to release too much of heat by just that. Different is the case in which you mix an acid with a base, or when one liquid can be easily ignite. For instance interaction between flammable solvents and very active silica gels can cause fires. $\endgroup$
    – Alchimista
    Commented Feb 14, 2020 at 9:19
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    $\begingroup$ @Alchimista: Inert? Not necessarily. The reaction may require a catalyst that's present in only one of the two liquids. The question doesn't appear to require that both liquids are pure. $\endgroup$
    – MSalters
    Commented Feb 14, 2020 at 11:29
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    $\begingroup$ Acetone could well be found in the home, but I think that dichloromethane might be rarer. Either is likely to cause damage when spilt as they are both good solvents for a range of materials. Does "highly flammable" count as a "heavy warning" for the purpose of this question? $\endgroup$
    – Andrew Morton
    Commented Feb 14, 2020 at 11:34
  • $\begingroup$ @MSalters I said relatively inert. I think is pretty clear what I meant. $\endgroup$
    – Alchimista
    Commented Feb 15, 2020 at 7:10

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If you need a quick source of moderate heat, why not use a single liquid? Sodium acetate trihydrate ($\ce{CH3COONa·3H2O}$), for example, releases heat energy on crystallization... and it takes very little to trigger that crystallization! Commercial heat packs contain a "clicker", which makes a sharp vibration. You can make your own heat pack fairly safely and inexpensively: sodium acetate can be found for ~US$8/450 g, and a Safety Data Sheet states, "it's not classified for physical or health hazards under GHS."

Not only can you get a noticeable temperature rise quickly, it's reusable.

It would be more difficult to find liquids that quickly release energy that do not have physical hazards (e.g. ignition) or health hazards. Hypergolic fuels, for example, tend to be rather toxic, as well as explosive. You might get a bang out of reading John Drury Clark's Ignition!, though.

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