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When I mixed in a bowl baking soda, citric acid and Epsom salt, then put it in the microwave between 30-60 seconds, the mixture will start to fizz and puff up, kind of like dough rising. What is happening here?

I've done test with all of the ingredients individually in the microwave and nothing happens. I've tried various combinations of any two of the three ingredients and nothing happens. Only when all three are mixed together does the mixture fizz and puff up. It seems like the catalyst is the Epsom salt. What is in the Epsom salt that is setting off the other two ingredients?

I know that baking soda plus citric acid together with water will also fizz, as $\ce{CO2}$ is released. I was wondering when I have the three ingredients in the microwave and it is puffing up, is $\ce{CO2}$ also being released?

Furthermore, the resulting mixture that had puffed up, if I put it in water it will still fizz and create $\ce{CO2}$, just as if had I never put the mixture in the microwave and just put it straight in the water. What I'm also wondering about if the microwave had hurt the ability of the mixture to create fizz in the water later, i.e., would there have been more fizz in the water otherwise?

After asking my question I found this YouTube video that said heating up Epsom salt releases $\ce{H2O}$ gas. Which would make sense why my mixture is puffing up. Still I'd love to hear anyone else's opinion on this.

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The solid phase reaction of baking soda and citric acid to form monosodium citrate, water and $\ce{CO2}$ takes place very slowly, so you don't get the visible foaming and bubbling that you do when water is added, or if you were to use baking soda with an acidic juice or vinegar (~95% water).

The molecular formula for Epsom salt is $\ce{MgSO4\dot\ 7H2O}$, meaning it's actually more than 50% water by weight. At $\mathrm{150^oC}$ it starts giving up some of those waters of hydration. Although water is a gas at that temperature, enough of the water is able to wet the baking soda and citric acid to get that reaction going, which then releases more water:

$$\ce{NaHCO3 + C6H8O7 → NaC6H7O7 + CO2 ^ + H2O}$$

Once the above reaction is initiated by the release of enough water from the Epsom salt, the additional water the reaction releases makes it somewhat self-sustaining and the $\ce{CO2}$ provides the foam and bubble show. The fact that additional reaction was observed after adding more water just shows that there was unreacted baking soda and citric acid remaining in the solid "crust".

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