Canned Heat

Next time

you’re wondering whether there will be enough propane for Saturday’s barbecue or if the flow will peter out somewhere between the tank and dinner, try this science demo. Pour some hot tap water down the side of the tank, then run your hand along it to feel the boundary where cold metal becomes warm: that’s the level of propane.

The hot water test shows that most of the propane in your cylinder is liquid, sloshing around the bottom and absorbing more heat from the water than the pocket of propane gas on top. If it were all gas in there, like a balloon or a scuba tank, the entire metal sheath would absorb heat evenly.

And that there’s a liquid in there is more than just a fun science fact. When liquid propane becomes gas, as it does when it leaves the cylinder, it expands to about 270 times its volume. (Technically, your tank is called a “cylinder” if it can be legally transported with propane inside.) Propane gas has about 2.5 times as