Looking for an “egg-celent” hands-on science activity you can do without having to leave your house? Look no further than your kitchen! In this experiment, we are going to explore the principles of density using table salt, water, and eggs. You will also need a measuring cups and spoons, and some glasses and jars that are large enough for an egg to float.

Simply defined, the density of a substance is the measure of its mass per unit volume. As a mathematical equation, we can describe density as D = m/V, where D is density, m is mass (in grams), and V is volume (in mL). What do you think are the units for density?^{1}

Now, in general eggs are more dense than water. That means that one mL of egg has more mass than one mL of water. But, water is a very good solvent, and table salt (sodium chloride, or NaCl) dissolves very easily in the water when added. This means that the sodium and the chloride ions get mixed up with the water, creating a salt solution. As we add more salt, we are adding more ions to the water, making it heavier. If we have two solutions that are the same volume, but one is plain water (solution A) and one is salt and water (solution B), solution B is going to be heavier because of the added salt. Which solution is denser? Solution A or solution B?^{2}

Now, on to our experiment! The first thing to do is to test your egg by placing it in a glass of fresh water. A new egg will sink to the bottom of the cup, while an old egg will float. This is because eggshells are very slightly porous, and air can pass into the egg and get trapped. If your egg floats in pure water, discard the egg and try another one.

The second thing to do is to make up your solutions and to calculate the densities. Be sure to make a range of densities, starting with plain water. In order to calculate the densities, you need to know the following measurements:

- One tablespoon of salt weighs roughly 17 grams.
- One cup of water measures roughly 237 mL and weighs roughly 237 g. (Knowing these values, what is the density of water?
^{3})

Using these measurements, create some salt solutions with different densities. For example, if you dissolved one half tablespoon of salt in one cup of water, you would be dissolving 8.5 grams of salt in 237 mL of water. The mass of the solution would equal 8.5 grams of salt PLUS 237 grams of water. The volume of the solution is about one cup, which is 237 mL. So, the density of this solution equals (237+8.5)/237, or 1.04 g/mL.

Try making solutions of different densities – so, both more and less dense than this solution. You can do this by adding salt directly to water, or by making a highly concentrated solution and practicing your serial dilution skills. Calculate the density of each solution. Form a hypothesis about which cups that you think the egg will float in and write it down.

Now, try floating your egg in each of the cups, starting with the pure water. In which cups did your egg float, and in which did it sink? Did you prove or disprove your hypothesis?

Other questions to try – do different color eggs (brown versus white) have different densities? What about eggs from different species (quail versus chicken versus duck)? Or what if you use sugar instead of salt? The possibilities are endless!

If you try this experiment at home, be sure to let us know! Post your pictures to Twitter, Facebook, or Instagram and tag us @edvotek. Have fun experimenting!

Answers to footnotes:

- The units for density are g/mL.
- By adding salt to the water, we are adding mass. If solution A and solution B are the same volume, but solution B is NaCl dissolved in water, solution B will have a greater mass than solution A. This makes solution B denser than solution A.
- The density of water is roughly 1 g/mL.