# Why isn't calcium carbide more corrosive?

Acetylene has a pKa of 26, I think. While that's pretty acidic ... for a hydrocarbon ... I still expect calcium acetylide (carbide) to be a very strong base indeed. Yet the NFPA fire diamond health code is just a 1, compared to a 3 for both sodium and calcium hydroxides, even though each mole of the carbide ought to create a mole of calcium hydroxide the moment it touches water. Little kids play with toy cannons powered by somewhat impure calcium carbide, taking no special lab precautions. Why is that possible?

Update: I have since noticed that those NFPA diamonds don't all agree. The Santa Rosa Fire Department and Wikipedia say Health rating is 1. Fisher Scientific says it is 2. NOAA says it is 3. I may never look at one of those little diamonds the same way again...

In these Safety Data Sheets for $$\ce{Ca(OH)2)}$$ and for $$\ce{CaC2}$$, both compounds are rated at "Causes serious eye damage – Category 1" and "Causes skin irritation – Category 2".
That said, $$\ce{CaC2}$$ is often sold as pellets or "stones". Because of large particle size, only a small amount of $$\ce{Ca(OH)2)}$$ is produced at a time from the action of water on $$\ce{CaC2}$$ since the release of acetylene bubbles slows penetration of water into the pellets.
$$\ce{Ca(OH)2)}$$, on the other hand, is more often sold as a finely divided powder. That dust is, in my experience more of a respiratory hazard. The "lime", as it's slaked in water, also evolves considerable heat, which exacerbates the caustic effect on skin.
Of course, $$\ce{CaC2}$$ has it's own hazards... it can cause considerable damage from an acetylene explosion.