The density of an object is a measurement of its mass relative per unit of its volume. To put it another way. Each object on in the universe is made up of atoms. These atoms can either be close together or further apart. If the atoms are very close together then the material is very dense. If the atoms are very far apart, then then the material is not very dense.
To put it very simply, mass is the measurement of how much ‘stuff’ is in any particular object. Density is the measurement of just how tightly squeezed in together all of that stuff is. An easy density test is to try and float an item in water. Less dense items float but denser items will sink.
For an example, you can compare a Styrofoam cup with a rock. The cup is very light. It has atoms that are very far apart from one another. The rock is very heavy; even if it is much smaller than the cup. This is because its atoms are very close together. The Styrofoam is not very dense because its atoms are so far apart, whereas the rock is dense because the atoms are so close.
Be Accurate
The most accurate scientific term for density is volumetric mass density. This refers to the importance of the measurement of the volume of the item when compared to its mass, or weight. This should not be confused with the ‘specific weight‘ of the object. That is the measurement of the weight per unit volume of the material.
If you are dealing with a pure material, then its density will have the same value as its mass concentration. Mass concentration is a measurement that is used in chemistry when making blends and mixes of substances. It is defined as the mass of a constituent (an ingredient) divided by the volume of the mixture.
The Equation for Density.
Forumula for Density:
The equation for density is a very simple one. It is written as p = m/V. The ‘p’ is actually a Greek letter, it is the lower case ‘rho’ letter. It stands for density. The lower case ‘m’ is for mass. The upper case ‘V’ stands for volume. To get the density you simply divide the mass by the volume of your object and you get its density. Just replace the symbols with your data and divide.
Like all other formulas, you can also find the volume or the mass with it as well. As long as you know two properties of your object or material, you can use this simple formula to find the missing property. Simply plug in what you do know and complete your equation correctly.
Solving formulas for density, mass or volume:
- Calculate density when mass and volume are given: p = m/V
- Calculate mass when density and volume are given: m = p.V
- Calculate volume when density and mass are given: V = p/m
How To Compare Apples and Oranges
Many times people will measure different types of objects using different measurements. For example, people measure liquids in liters but they measure solids in cubic centimeters. These measurements are quite different. People need a way to compare these different measurements easily and accurately. They do this using two terms for the same concept.
The newer term is ‘relative density’. The traditional term is the ‘specific gravity‘ of the substance. The ratio of the density (the mass of a unit of volume) to the density of a control substance is what is known as relative density. This is the term that is generally preferred by modern scientists. The specific gravity is also the ratio of density of any given substance of a substance when compared to a reference substance. As you see, these two terms mean the same thing.
In modern books, you will see the relative density term used most often. But you may run into the older term from time to time, so it is good to know.
The Base Standard of Water Relative
Density is measured using a standard of comparison. Scientists use the density of water to measure other solids and even liquids. To measure the density of gases, most will use the standard density of typical atmospheric air. What this means is that water has a relative density of one. If an object will float in water, it has a density less than one. If an object sinks, than it will have a relative density that is greater than one. When scientists measure the density of gases, they will refer to air as having the standard density of one.
This means that that your floating Styrofoam cup has a density of less than one. The rock will have a density of greater than one. It is just that simple.
Sometimes Density Changes
Some materials can have changing densities. Think of water. As the temperature changes water goes from solid ice to liquid water and then to gaseous water vapor. Other materials can also change, but most do not change so dramatically. As temperature and pressure changes, the atoms in a material will contract or expand. Most solids and liquids barely change.
Gases, however, can dramatically change their densities. Warm gas can quickly expand. A hot air balloon is a good example of this. As the gases in the balloon are heated, they expand. As they are hot and light, they are then able to let the balloonist fly through the air.
Substances will become more dense with higher pressure on them. The atoms are literally squeezed together as pressure increases. Most substances will expand as they are heated. Heat is simply energy that is absorbed by a material. The extra energy that heat gives their atoms causes them vibrate faster. This makes them push each other away and create a less dense material. In science, the changeable nature of density is called the specific volume of a material.
Not That Dense After All!
Density may seem like a complex subject. In a way, this is true, but the principles are easy to grasp. As long as you remember that density is the property of how tightly packed the atoms in a material are, and that it is calculated by dividing the mass and the volume, you will have grasped the basics of density.
Check our table of densities