Re: Does blood float or sink in salt water? Say at a water temperature of 12 C

Dear Student,

Whether a fluid floats or sinks in another fluid would depend on their specific gravities. If the specific gravity of blood were greater than that of salt water, the blood would sink; otherwise it would float.

Pure water, by definition, has a specific gravity of 1. The specific gravity of salt water depends on the concentration of NaCl in the water. A table of specific gravities (1) gave the following results:

• Sea water - 1.03
• NaCl 5% - 1.037
• NaCl 25% - 1.196

It's a little more difficult to determine the specific gravity of blood, because blood is a complex mixture of cells (of different sizes), proteins, electrolytes and water. One would expect the heavier components of blood (i.e., the cellular components) to sink, while the plasma (the non-cellular component with dissolved electrolytes) would dissolve itself into the salt water. The specific gravity of red blood cells themselves is 1.10. Thus, in sea water, for example, one would expect the red blood cells to sink.

Thanks for your question!

Stephen C. Lattanzi, M.D.

References

1. http://pump. net/liquiddata/specgravviscliquids.htm/