The pressure will probably not stay for ages. But if the seal is good enough you can expect it to maintain the pressure atleast for few years. And yes this baking soda and water technique is really versatile for diy applications.
Anti Gravity Space PenView Instructable »
Cardboard Rubberband Car
yes you can use soap. but soap will dissolve faster so boat will run for less time, around 5 minutes.
Solid camphor should be available in almost all retail and medicinal stores. But if you dont find, then get it from ebay or amazon.
Micro camphor boatView Instructable »
Flapping Paper Plane
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