Whether an object floats or sinks is a matter of density. If the object is less dense than the surrounding liquid, it will float; if it is denser than the surrounding liquid, it will sink.
"Specific gravity" is the ratio of the density of a substance to the density of water at a standard temperature and pressure. (The density of water is right around 1.00 kg/L, depending on temperature and purity.) If the specific gravity of a substance is lower than 1.0, the substance or object will float in water. If it's greater than 1.0, it will sink.
For example, the specific gravity of styrofoam is 0.05. This means it floats on water, with about 95% of the styrofoam above the water and 5% below. The specific gravity of gold is about 19; it will quickly sink in water.
Human bodies are a combination of various substances. Muscle and bones have specific gravities above 1.0; fat has specific gravity below 1.0, and the air in our lungs has a specific gravity of nearly zero. The net result is that most human bodies have a specific gravity right around 1.0, but slightly below. So most people will float in water, though only a small portion of the body will actually be above the surface of the water. Fat people float better than thin people, because fat floats better than muscle. Extremely lean or muscular people will have difficulty floating.
Now let's move on to other liquids. Let's assume a typical human body has a specific gravity of 1.0 (though we already noted that it is usually slightly below 1.0). A human body should float in any liquid that has a specific gravity higher than its own, or higher than 1.0. Some examples are in the table below.
| Substance |
Spec. Gravity |
Result |
| Sulfuric acid (95%) |
1.84 |
Float |
| Chloroform |
1.49 |
Float |
| Corn syrup |
1.40-1.46 |
Float |
| Household vinegar |
1.05 |
Float |
| Milk |
1.02-1.05 |
Float |
| Salt water |
1.03 |
Float |
| Beer |
1.01 |
Float |
| Motor oil |
0.88-0.94 |
Sink |
| Olive oil |
0.80-0.92 |
Sink |
| Turpentine |
0.87 |
Sink |
| Gasoline |
0.73 |
Sink |
| Propane |
0.49 |
Sink |
The good news is that if you fall into a vat of sulfuric acid, you won't drown! The bad news is... everything else about it.
Sources: SImetric.co.uk, CSG Network, MadSci Network
As a kid I remember seeing a National Geographic photo of a worker sitting - or more correctly "floating" - on top of a large vat of mercury.
2009-08-12 11:51 submitted by John MMy vote for most entertaining Spotlight comment of the week goes to jmoeller for: "The good news is that if you fall into a vat of sulfuric acid, you won't drown! The bad news is... everything else about it."
Still, the idea of floating in beer is kind of... tasty?
2009-08-09 21:12 submitted by Sandi JonesI like the old format better. To see all the Q&A you just needed to scroll. Now you have to select for each question individually.
2009-08-08 17:03 submitted by BoneCome on, we learned this in high school!
Anyway, if anyone's interested, there's a really cool Mythbusters where they try to figure out how man can walk on water and end up creating some interesting liquid concoctions.
2009-08-05 15:20 submitted by PamplemousseHey Answerman,
2009-08-05 12:07 submitted by meeboNice new-look column but no number nine question? what gives?
That chart is hilarious.. as in "The good news is that if you fall into a vat of sulfuric acid, you won't drown! The bad news is... everything else about it." Yeah if I am floating in gasoline, turpentine, propane, motor oil, or chloroform(?!!), I'll probably be more worried about getting OUT that lying back and having a nice float...
2009-08-04 11:26 submitted by jmoeller