Density is a physical property of a substance that can be determined by scientific experiment. You might have learned that density is mass divided by volume, which means that if you can measure both the mass and the volume of an object, you can calculate its density. A substance will always have the same density no matter the size of the sample, so that density can be used to help identify a substance. Since an egg is an object that has a mass and a volume, you can calculate its density.
Density is defined as the mass of an object divided by its volume. This statement can be written as an equation: D = m / V. An object that has a lot of mass in a small volume will have a large density, and an object with little mass in a large volume will have a small density. For example, lead has a very large density (11.35 g/cm3), and aluminum has a comparatively small density (2.70 g/cm3). This means that lead has a lot more mass packed into a 1-foot by 1-foot by 1-foot cube than does aluminum. In fact, an aluminum cube that size weighs about 170 lb., but a lead cube that same size weighs around 710 lb.!
First, let's specify what we mean by an "egg." Birds aren't the only creatures to lay eggs; so do fish, turtles, snakes, frogs and insects, just to name a few. In this article, we will limit our discussion to bird eggs (avian eggs)--specifically, hen eggs.
To determine the density of an egg, we first need to describe the components of an egg. It is these components, after all, that give an egg its mass and volume. According to the American Egg Board at IncredibleEgg.org, the main parts of an egg are:
- Shell, which is mostly calcium carbonate and makes up 9 to 12 percent of the total weight of the egg (and which is actually very porous so that air can pass through)
- Yolk (the yellow part consisting of fats, proteins, minerals and vitamins), which makes up about 34 percent of the liquid weight of the egg
- Albumen (the egg white consisting of proteins, among other things), which makes up about 66 percent of the liquid weight of the egg
- Air cell, which is a pocket of air found at the large end of the egg
There can be some variation of these parts.
Mass and Volume of an Egg
The mass of an object can be determined by use of a balance. Mass is typically measured in grams. The volume of an object can be measured in different ways. One way is to measure length with a ruler, and calculate the volume mathematically. This is easy to do if the shape of an object is something like a cube or a sphere. For objects that have irregular shapes, a common method is to use the water displacement method. Measure the volume of a certain amount of water (say, for example, 70 ml of water), then place the object into the water and see how much water it displaces (if the new volume is 100 ml, then 30 ml of water were displaced and that is the volume of the object). For smaller objects, volume is typically measured in milliliters or cubic centimeters.
Can the mass and/or volume of hen eggs vary from egg to egg? Yes, definitely.
According to IncredibleEgg.org, there are many factors that change the makeup of an egg. An egg can leave the uterus prematurely and not give the shell enough time to fully develop, so it is thinner than normal. There is the possibility of twin yolks (and even three or four are possible, or in the case of young hens, no yolk). Also, as the hen ages, her eggs are larger. The breed and size of the hen will also affect the size of the egg. Environmental conditions and nutrition will affect the size of an egg. Any of these can change the mass and/or the volume of the egg.
Most people know that an object will sink in water if it is more dense than water and will float in water if it is less dense than water. Many of us have put eggs into a pan of water when preparing to make boiled eggs. This event actually gave us our first indication of an egg's density: the eggs sank. Since water has a density of 1 g/ml, we now know the density of an egg is greater than 1g/ml.
Eggs don't always sink in water, however. According to the Nova Scotia Department of Agriculture, when an egg is first hatched, the air cell at the large end of the egg will expand as the egg cools, drawing air through the porous shell. As the egg ages, this air cell will increase in size. This will cause the density of the egg to decrease over time. In fact, Oakdell Egg Farms describes how you can use egg density to determine the freshness of an egg. If the egg sinks and sits horizontally in the water, it is very fresh. If the large end of the egg rises up from the bottom (because the air cell has become larger and contains more air), the egg is 1 or 2 weeks old. If the egg floats, it is very old.
Experimentally Determining Density
Realizing that the density of an egg will change over time, it still seems simple enough to calculate the egg's density: measure the mass and the volume of the egg, and then calculate mass divided by the volume. The fact that there is an air cell within the egg, however, complicates the mass measurement, and the unusual shape of an egg complicates the volume measurement.
In the general chemistry class at Boston College, the first experiment the students do is titled "How Dense Is an Egg?" Instead of measuring the mass and volume of the egg, the egg density is determined this way: Put an egg into water (it sinks), then slowly add salt until the egg just floats (which "means that the top of the egg just touches the top of the solution, without a significant amount of the egg protruding above the solution"). At this time, the egg and the salt water have the same density, and the mass and volume of the salt water can be easily measured.
There has been experimental research into the density of avian eggs. Here are the results from a few studies:
A.L. Romanoff and A.J.Romanoff in 1949 (in the book "The Avian Egg") gave the value of 1.033 as the density of the contents of a fresh hen egg.
In the 1974 issue of "The Condor," C.V. Paganelli, A. Olszowka and A. Ar developed an equation relating the density of an avian egg to the weight of the egg: egg density = 1.038 x egg weight^0.006.
In the 1982 issue of "The Condor", H. Rahn, Phyllis Parisi and C.V. Paganelli collected fresh egg samples from 23 different bird species to calculate the egg content density (average 1.031 g/cm3) and initial egg density (varying from 1.055 g/cm3 to 1.104 g/cm3). In fact, examination of the procedure they used to measure mass and volume of the eggs shows just how complicated the procedure is: "We collected fresh eggs ... and weighed them both in air and in water for determination of egg volume by Archimedes' principle. Gas in the air cell was then replaced by water injected with a hypodermic syringe, and the eggs were reweighed to obtain initial egg mass."
Although there has been some research to determine the density of an egg, the problem is that the density of even one egg can vary. If you need to know the density of a particular egg at a particular time, you will need to determine the density experimentally.