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Can I have the purpose of the science project? "how salty does the sea have to be for an egg to float"?
Question Date: 2015-01-20
Answer 1:

A most interesting question! (I am going to assume that you mean a fresh egg. It turns out that bad eggs will also float in (relatively) fresh water due to the build up of gas inside the egg. This is because the shell is porous, and over time liquid evaporates from the shell and gas goes in.)

Whether or not an egg floats in a solution of salt water can be estimated using the concept of density. For something to float, it must have a smaller density than whatever it is floating in.

So let's estimate how much (table) salt it takes using density. The mass of an egg is about 50 g. To measure the volume, you can submerge (put just under the water surface) to see how much water is displaced; the volume of water it displaces is the volume of the egg! This is based on a concept behind the story of 'Eureka!' and Archimedes

The definition of density is mass per unit volume (density = mass/volume). For example, you could have a pound of steel and pound of feathers. While both would weigh the same, the amount of space the pound of feathers would take would be much larger! Thus, the density of feathers is smaller.

Luckily others were also curious to the density of an egg, so we will use their ) value of 1.031 g/cm3 (averaged across 23 bird species) for this estimate. The density of fresh water is taken to be 1.00 g/cm3. Likewise, the density of (table) salt is 2.16 g/cm3. For the egg to float, the density of the water needs to be just barely higher than the density of the egg. To make the math easier, let's say I have 1 g of water (i.e., 1 cm3). I'll assume that the masses and volumes of the water and salt just add together (this is not always the case; look to the mixing of water and ethanol where the mixing the two leads to a volume smaller than adding the two separate volumes together!). Although you probably have not learned the math behind this, I'll write down the equation used for this estimate for future reference:

total density

The total density we will take to be that of the egg, since this is the minimum density the salt water needs for the egg to float. The mass of the water is set to 1 g, and volume may be found from the original equation for density. What we solve for is the mass of the salt using algebra. In this estimate, we find that you need at least about 0.06 g of salt for every 1 g of water for an egg to float.

For reference, click here the ocean has on average 35 g of salt for every 1000 g of water. Our estimate is equal to 60 g of salt per 1000 g of water. So about twice as salty. If you were at the Dead Sea, you wouldn't need to add any more salt. It's salty enough that a human could float!

As with any estimation or experiment in science, there are errors, and it is good practice to understand where they come from. Nevertheless, our estimate gives us a ball park number of what to expect.

But don't take my word for it, try it yourself! There are several home style experiments you can try for yourself, like this click here please methodical one that tries different concentrations of salt.

Hope this helps! Best,

Answer 2:

Now, maybe you know this already or maybe you don’t, but I’ll first explain why something floats. If you already know about this, you can skip to the 3rd paragraph. The key property is density. You might know about density already even if you think that you don’t know. If you’ve ever tried to mix water and vegetable oil, you probably saw that the yellow oil vegetable oil sits on top of the water. This is because the density of oil is less than the density of water, and so it floats. A helium balloon “floats” because the density of helium is less than the density of the air we breathe. So even if you don’t know what density is, you’ve probably seen the property action before.

Density is normally defined as the amount of mass per volume. Mass is just how much stuff there is (like how something massive is really big; it has a lot of stuff!) Volume is how much space it occupies. If you had two dice, one normal and made of plastic and the other one made of lead, the lead die would feel heavier even though they’re the same size. That’s because there is more mass (more stuff) in the lead die than in the plastic die. The density of lead is larger!

So for something to float in the ocean, its density needs to be less than the density of the saltwater. As you put more salt into the water, you put more mass in the water, but the volume doesn’t change much (the density increases!) Now, I found out that some smart scientists did experiments like this in the past and found that the density of an egg is about 1.03 grams per milliliter. The density of water is about 1.00 grams per milliliter. That means if you had a liter (1000 milliliters) of water, you would have 1000 grams of water. You would need 1030 grams in that one liter to match the density of an egg. That means you could add at least 30 more grams of salt to the water. There is a little math involved to figure this out:

Density = mass/volume
Density of salt water = (mass of water + mass of salt)/volume
Density of our saltwater with 1000 g of water and 30 g of salt = (1000 g + 30 g)/(1000 mL) = 1.03 grams per milliliter

On average, there are about 32-37 grams of salt per liter of water. This would at first appear to be enough to make the egg float. However, the density of the ocean is about 1.027 grams per milliliter. It’s lower than we might expect because things like other minerals, biological material, and temperature can also impact the density.

This would actually be a great experiment to try on your own! A teaspoon of salt is about 5.7 grams, and you could keep adding salt and see how much salt is needed to make the egg float. You could compare your pure water and salt to ocean water. You could also try different temperatures of water and see how that changes things. And you could even try different types of eggs! For example, an old, rotten egg releases gases. Even though the shell looks like a solid, gases can escape (just like how a helium balloon will eventually deflate even though there’s no visible holes in it!). As the gases escape, the egg loses mass – but its volume is the same. So, the density (mass divided by volume) would decrease, and the egg might eventually float. But don't believe me; try it for yourself! Hope this helps!

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