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How much salt do we need in water to make an egg float?
Question Date: 2014-02-27
Answer 1:

This is a fun question! Objects float in materials that are more dense than the object itself. Density is a measure of how much substance there is per unit volume of material. So that means more dense objects have a higher amount of "stuff" in a given volume, and the opposite is true for materials that are less dense.

In order to make the egg float, we need to make it less dense than the water surrounding it. As your teacher has probably explained in class, adding salt makes the surrounding solution more dense. How much salt we need to add will vary from egg to egg, since the density of each egg will probably be slightly different. However, we can make a rough estimation! According to a couple of websites I found online, eggs have a density of about 1.03g/mL. That means in a volume of 1mL, there is about 1.03g of the stuff that comprises an egg. The density of pure water is about 1.00g/mL. So let's calculate how much salt we need to add to account for this difference in densities!

The first thing we need to keep in mind is the formula for density. Density is equal to mass divided by volume, or:

ρ = density = (mass in grams)/(volume in mL)

To make the calculation a little easier, let us assume we have 1L of water. We need to figure out how many grams of salt to add to the water to increase the density by at least 0.03g/mL. The equation we want to solve for this problem is:

0.03g/mL = (density of saltwater) - (density of water)

We can read this equation as: "0.03 is equal to the difference in the density of pure water and the density of water with salt added to it"

Now, use the equation for density:
0.03g/mL = (1000grams water + Xgrams salt)/(1000mL of water) - (1000grams water)/(1000mL of water)

Let's rearrange for the variable we are trying to solve for:

0.03 + 1000g/(1000mL water) = (1000grams water + Xgrams salt)/(1000mL of water)

Xgrams salt = (1000mL water)*(0.03g/mL + 1000gWater/1000mLWater) - 1000g water

X = 30g salt ~ 2 tablespoons

Now, it is important to keep in mind that the density of the egg varies and you may not use 1L of water, but some other amount. This means the amount of salt we would need to add will vary as well, but this is a ball-park estimate of about how much salt we should add (i.e. we would be adding tablespoons as opposed to cups of salt to 1L of water). I hope this helps!

Answer 2:

The reason the egg can float in saltwater is that saltwater has a higher density than freshwater. The density increases with salt concentration, so the amount of salt you need might depend slightly on the egg. A good starting point would be 6 tablespoons of salt in 1 glass of water (make sure it is fully dissolved by stirring). If the egg does not float, just add some more salt!

Answer 3:

You want to know how much salt it would take to have an egg float in water. An object, such as an egg or a boat, can float in water if it can displace enough water to match its own mass. There are 2 general ways to do displace water. One, you can design the object to displace a lot of water, such as boats. Two, you can make either the object less dense than the water. In the case of eggs, the second option is more appropriate since it is difficult to reshape an egg into a boat.

We need to make the eggs density less than waters. The density of an egg will vary slightly between each egg. To determine it exactly you will need to find its mass and its volume. Density is mass divided by volume. The average density of an egg is 1.031 g/cm3 (grams per centimeter cubed) or 1.031g/ml (grams per milliliter).

The density of water is 1 g/ml.

Salt adds mass while adding very little volume so it is a good additive to make the density higher. Adding salt increases the mass but volume stays constant. The density will increase with the amount of salt you are adding.

For every millileter of water we will need to add 0.031 grams of salt to match the density of the egg.

Lets say you want the egg to float in 1 liter of water, you would need 31 grams of salt.

I recommend you try this experiment for yourself with adult supervision.


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