Density can be defined as the ratio of an object's mass per unit volume. In other words. D=m/v! More speficially, density depends upon two things:
- The mass of each molecule that makes up the material.
- The amount of space or volume the material takes up.
For example, a real boulder has a greater density than a styrofoam boulder. Why? The individual styrofoam molecules have a lower mass and are not "packed" tightly together. Whereas, the individual molecules that compose a rock are higher in density and "packed" more closely together. As a result, the real boulder has a greater density than a styrofoam boulder.
Another fact to consider is the density of a material remains the same under the same conditions. For example, the density of aluminum is 2.7 g/cm^3. Whether you are talking about aluminum foil or aluminum wire, the density remains the same provided the sample is not hollow or contains a mixture of materials.
What if we have an irregular-shaped object? It is not as straightforward to determine the density because it is more difficult to calculate the volume. There is a way, though! The volume of odd-shaped objects can be found by using a graduated cylinder and the fact that 1ml = 1cm^3. Here's an example.
Let's say we want to find the volume of a rock you found while hiking. Fill a graduated cylinder with water to a set amount, say 50ml. Then, add your rock. Be careful not to splash any of the water out! Now, note the new water level. Let's say it rose to 61ml. Then, the volume of the rock could be found by the following method:
Volume of Rock: 61ml - 50ml = 11ml or 11 cm^3
Pretty easy, right? Let's face it, most objects in nature are irregular inshape. This provides an easy way for determining volume and, ultimately, calculating density.
Activity 1: Floating Carrots
Begin by asking your students if they thing a carrot will sink or float if placed into a container of tap water. tally your responses. Have someone drop the carrot into the container and observe what happens. It should sink because the density of the carrot is greater than the density of the tap water. Now, ask your students what they think would happen if you added salt to the water. Again, tally your responses. Begin adding salt in measured amounts. Eventually, the carrot should float. Make note of how much salt you added. At this point, the carrot floats because the density of the salt water is greater than the density of the carrot. It is very easy at this point to displace enough water to create a buoyant force that flaots the carrrot.
Activity 2: The Dead Sea- Sink or Float
Visit the Dead Sea (at least virtually) using Google Earth. Go online and explore this amazing place. Find out answers to the following:
- How was the Dead Sea formed?
- Why it is easier for people to float here?
- Is it really deadly?
Extreme science is a great place to begin!