Q & A: Eggs and People Float like Corks

Why Does an egg float when put in saltwater
- Kai (age 12)
Bossier , Louisiana , Us

Not only eggs but people float in saltwater like corks! But why does salt help? And why things float?

According to the old story, Archimedes (287 B.C.- 212 B.C.)
figured it out while bathing and ran out into town without
his clothes crying "Eureka!"

Let's start with plain water. It is pretty obvious that water can hold up water. This obvious fact is exactly how floating works: if something is heavier than water of the same volume, it sinks, if it is lighter than water of the same volume, it floats to the top. And what happens if it weighs the same as water of the same volume? You can test your guess with some water and two drops of different food colorings.

Now, back to salt water. If you weigh a cup of water, and then weigh a cup of salty water, it turns out that a cup of salty water is heavier. The more salt you dissolve, the heavier is the cup of salty water. (Books say that salty water has 'HIGHER DENSITY'. Density is how much a UNIT VOLUME weighs, such as cubic inch, or cubic mile, or a quart. )

Salty water can hold up salty water, that is it can hold up
heavier stuff than plain water can.

An egg weighs more than an imaginary egg made of plain water, but it weighs less than an imaginary egg made of salty water. So salty water can hold it up, and plain water cannot. That is how it works.

Because Dead Sea is much more salty than most world's oceans, and we are made mostly of water you can float in the
Dead Sea without even trying. are pictures of people floating in Dead Sea reading a newspaper!

Next question to ponder- why ships made of iron float? At the University of Illinois engineering students build concrete boats that float!

(published on 10/22/2007)

Hello! My son and and are doing a science fair project together. We decided to do a twist on the "make an egg float" experiment. Assuming that the egg would float in 12 oz. of ocean water (which we collected on a visit to the Jersey shore 3 days earlier), we were going to see how much salt we needed to add to our tap water to make the egg float at the same level as it did in the ocean water, thinking that we could then determine the salt concentration per 12 oz. of the ocean water. There was only one problem: the egg wouldn’t float in the ocean water!! We were wondering if you had any thoughts as to why the egg wouldn’t float in the ocean water? At this point, we are assuming that the salt concentration wasn’t sufficient for the egg to float. We’ve considered many possibilities, but are leaning toward that theory. Are we on the right track? We would greatly appreciate your input and would like to include it as part of our research for the project, if possible. Thank you for your time!
- Kristi
Chambersburg, PA, US

Your idea sounds very reasonable. You could see how much salt you have to add to the ocean water to get the egg to float. By comparison with the amount you need to add to tap water, you could still figure out the amount of salt in the ocean water.

Mike W.

(published on 10/22/2007)