[reremedies]

I'm a magician, and I'm trying to get an effect where a crumpled up "used" sugar packet slowly uncrumples and apparently re-fills itself.  I tried using baking soda and vinegar.  I lined the packet with plastic and the vinegar in a separate plastic bubble that could be broken so it would then mix with the baking soda.  It's not expanding enough though, and not mixing well.

Are there other materials that can be mixed together easily to create a slowly expanding volume inside a sealed space?  By "slowly" I mean around 5 seconds.  Maybe a foam of some kind?  Also, obviously, I'd like to deal with materials that are safe and easy to handle, store, and transport.

Thanks for any help you can give!

[Enthalpy]

I'm not the most qualified here for your query, but it's so sad nobody answers! Hope more people will jump in.

Baking soda is a candidate, yes. You will get a faster reaction using a more convincing acid than vinegar, which doesn't mean corrosive. Tartaric acid (among others) is civilized and reacts promptly
http://en.wikipedia.org/wiki/Tartaric_acid
beware baking soda produces much gas, so use the right amount of soda or acid.
Either the right amount exactly, or have a vent that opens when the desired volume or overpressure is obtained, or inflate a very strong container (less secure).
Baking soda uses to produce a foam, as the bubbles appear within the mixed liquids.

Among foams, I know polyurethane, easily available. I feel it less suitable, as it is viscous and needs to be mixed, or expands from a bottle. Harder to remove thereafter.

Could you inflate your object with gas from a small bottle? CO₂ exists as sparkler for drinking water. May be loud.

[fledarmus]

You might try pre-dissolving your baking soda in a minimum amount of water and putting it in a second plastic packet. This would speed up your reaction. Switching to muriatic acid (dilute hydrochloric acid, usually available in pool supply stores) might also help, but it is a stronger acid and you would need to be more careful handling it. (If you use sufficient baking soda to neutralize it, disposal after the trick isn't a problem, but the preloading and storage could be).

[nj_bartel]

You might try pre-dissolving your baking soda in a minimum amount of water and putting it in a second plastic packet. This would speed up your reaction. Switching to muriatic acid (dilute hydrochloric acid, usually available in pool supply stores) might also help, but it is a stronger acid and you would need to be more careful handling it. (If you use sufficient baking soda to neutralize it, disposal after the trick isn't a problem, but the preloading and storage could be).

Exactly what I was going to recommend.  Just run the stoichiometry beforehand (if you find and give us the volume of gas desired, we can help).  But with dilute HCl and a small excess of dissolved baking soda, I wouldn't be concerned unless it got in my eyes.

[Enthalpy]

I did the reaction as a child with tartaric acid and it began quickly, lasting few tens of seconds, with both the acid and the baking soda already dissolved in water. Safe acid, as opposed to hydrochloric.

In any case, a limited vent should be foreseen to prevent overpressure, and this means the reaction is allowed to last longer than the visual effect.