Density: A Fundamental Physical Property
Density is one of the fundamental physical properties that is essential to understanding many scientific disciplines including materials science and engineering. It is also used in everyday life to determine whether objects will float or sink.
How to Measure Density
The most common way to measure density is by weighing an object and dividing it by the volume of that object. This can be done with a graduated cylinder or a beaker and will usually be more accurate than using the standard method of putting an object on a weight scale.
A fun way to teach students about density is by doing a demonstration. Use a container with some water and some cooking oil (about the same volume) and drop in a couple of small objects to see what happens.
Do you notice that some objects float and others sink? If so, tell them that the density of these different substances will determine which objects float.
Explain to the class that density is a measure of how heavy something is compared to its size or volume. For example, a block of lead has a higher density than a block of Styrofoam because the lead is heavier and has more atoms packed closely together than the Styrofoam.
Have the students connect this to the ideas they learned about in Chapter 1 when they discovered that heat increased molecular motion and caused molecules to move a little closer together. They also saw that when a substance was cooled, the molecular attraction between the molecules caused them to move a little further apart.
How Does Density Affect Layering?
If you put some different liquids in a jar and then measure their weight, you can see that the more dense liquids float on top of the less dense ones. This can be useful in demonstrating how density affects the layers of liquids and how they will sink when a particular layer is empty.
What Liquids are Denser than Water?
When you measure the density of a liquid, it is important to be sure that you are measuring at normal temperature and pressure. A change in pressure can change the density of a liquid, but the changes are small and can be difficult to detect with an instrument.
The official units for density are grams per cubic centimeter, g/cm3, but there is a wide variety of other common units used. They range from grams per milliliter g/mL to kilograms per liter, kg/L.
Some of these common units can be confusing, so it is important to understand the difference between the units before you begin. A good resource for this is the Wikipedia page on density.
What Are the Best Tools for Measuring Density?
The best tool to use when measuring density is a graduated cylinder. It is a very accurate and precise way to measure the density of a solid and can be used on all kinds of surfaces.
A good rule of thumb is to use the same weight for the cylinder as you would for a beaker. This ensures that you are getting an accurate measurement of the density and will help avoid any mistakes when comparing measurements.
