Why do dead bodies float on water but not when people are alive?

Question

It seems that when people are dead their bodies surface on water but when people are alive they go down unless they swim. Why is that?

Answer 1

What makes a corpse float or sink in water? Several factors:

* Lungs. Lungs are like a sponge. When someone drowns, the air sacs in the lungs fill with water. Since a body without air in the lungs is denser than water, it sinks. A person who is killed on the surface and then put in the water tends to float, since the lungs are still full of air (although see below). That's how pathologists can tell whether a person was drowned or was dead before hitting the water.
* Body position. A person who was dead before entering the water can still sink depending on the position of the body. If the body is upright when dumped into the water, water can enter the lungs while air escapes. Hence, the body sinks after a short time. If the body is prone (face down), the air in the lungs can't escape, so the body floats.
* Body fat. Body fat is less dense than water. The fatter a person is, the more buoyant the body. Muscle on the other hand is denser than water, so people with a lot of muscle--or people who are just plain lean--tend to sink.
* Clothing. Some fabrics trap air well; others don't. Natural fibers, like cotton and wool, absorb water and so tend to sink. An exception is silk, whose fine fibers can be woven tightly enough to trap air. Some synthetic fibers, like nylon and polyester, usually don't absorb water, so they can trap air. Other synthetic fibers, such as orlon and other fibers used in cold-weather clothing and sleeping bags, are manufactured like some pastas, with one or more tunnels in them. These tunnels are designed to trap air and insulate the wearer. These fibers float until they fill with water, at which point they sink like a stone.
* Putrefaction. A body that sinks doesn't necessarily stay sunk. After you die bacteria in your guts start eating you. They produce methane and other gases which collect in various body cavities. That's why corpses become bloated, whether in the water or not, giving us the charming term "floater."

The water can also affect buoyancy:

* Salinity. Probably the most important factor is salinity. Salt water is denser than fresh water, so corpses tend to float in the ocean but not in a river.
* Temperature. The water's temperature affects its density. Water is densest at 4 degrees Celsius (about 39 degrees Fahrenheit). Bodies of water often have a thermocline (a depth where there's a dramatic change in temperature). Warm waters "float" and circulate by convection when warmed by the sun; colder waters "sink" and tend to stay there. Hence, there is a slight (very slight) tendency for bodies in cold water to float, since they may be less dense than the water surrounding them.

Answer 2

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Answer 3

Dead bodies don't always float, it depends on how much water gets in the lungs. If the person does not die of drowning and the lungs stay clear, the body floats.

Answer 4

Living body only has gases in the lungs while a dead body, as it is decomposing, lets off gasses from the inside, thus making it buoyant.

Answer 5

After they die the bacteria in their guts continue to generate gasses like carbon dioxide and methane. They can also start digesting the corpse. All this extra gas makes them float.