Re: Building a submarine

Interesting problem. I like your idea of the Alka-Seltzer. However, if you just seal the Alka-Selzer in a bottle, the weight of the stuff inside the bottle can't change -- just it's 'state' will change -- so the bouyancy won't change either and the desired effect won't occur. What you need is some way for the gas generated to push water out of the container, that is the bottle (or whatever) can't be sealed away from the water. The problem then is to prevent the gas from getting out and letting the water back in.

One idea that occurs to me is to put some vinegar into a balloon. then put a small container, a test tube would be good, with baking soda in it into the balloon, holding it so that the two chemicals don't mix. Then tie of the ballon. Drop it into the water. When it reaches the bottom, it will flop over, letting the vinegar and baking soda mix. That will make carbon dioxide gas (harmless) and inflate the balloon. If there is enough gas, then the ballon should then float up to the surface. I haven't tried this! Could be messy, so try it in a sink. Too much gas and the balloon pops. Too little, and the balloon won't inflate enough to rise.

Another thought. Take a cork. Stick something heavy on it with a water-soluable glue (Paste maybe? sticky sugar syrup? Flour/water mix?). Drop it in the water. The weight makes it sink, but when the glue dissolves, the cork will bob back to the surface. You could even attach a thread connecting the cork and the weight, so the cork woulld only go part way, sort of like an underwater sonar buoy.

There are probably a lot of other ways to do this. And I'm sure that all of them will take some patient fiddling around to make them work, but that's science for you.