Science Buddies: "Ask an Expert"

Hi,
We just finished working on the floating egg experiment. Our original hypothesis was that an egg will float in ocean water. We set up the experiment as per the directions. In glass #5, the tap water glass, the egg sank. In glass #4, the egg sank. In glass #3, the egg seemed to suspend itself just shy of the bottom of the glass. In glass #2, it floated at the surface; in glass #1 it floated higher. We also had a sample of actual Pacific Ocean water, and the egg sank in it. We have no measuring equipment, so our hypothesis is that ocean water, at about 3 1/2 percent salt solution, is not enough to float an egg, that it takes much higher concentrations.
Can any one help with this? There are so many egg floating experiments that we actually were very surprised when the egg sank!

Egg shells are porous so the amount of gas and fluid inside can change with pressure, age, and temperature. Some of the uncontrolled variables might be the temperature of the water, how deep the water was in your glasses, how long you left the egg in the ones it sank in, and how old the egg was and what temperature the fluid inside the egg was, and what the barometric pressure was, and how well mixed your salt water was (see can water float on water in the advanced variations.

In glass 3, you found the magic neutral density buoyancy point. The pressure of water above the egg plus the air above the water caused the internal density to equal the displaced salt water density.

As for your actual pacific ocean water, where did you gather it? Was it near a fresh water outlet in a mixing zone? Did you gather it at high or low tide?

If you want to try to determine the salt concentration in your sea water sample, I can think of a couple of methods. If you have access to an accurate scale that can measure a few milligrams, you could slowly boil a known quantity of sea water until all you have left is solids and then weigh them.

Alternatively, you could measure the volume of a salt water sample and keep adding known quantities of NaCl (table salt) and mixing until you reached the same buoyancy point as you had in glass 3. At this point, you know how much salt you added, subtract this from how much an equal volume of the glass 3 concentration has, and get your answer.

Oops, forgot to mention solid suspension. Did you let the sea water sample settle or strain it through a paper filter (such as the ones used in many coffee makers)? If you want to boil off the water, you should do it with a filtered sample so that any suspended solids aren't left in what you weigh.