1. Leonard M. Fischer Horace Mann Middle School Center for Applied Technology 8950 NW 2nd Avenue El Portal, FL 33150 UNDERSTANDING DENSITY

2. WHAT IS MATTER? Before we learn what density is, we need to review what matter is. Basically, everything in the universe can be divided into two broad categories – Matter or Energy. Matter is anything that has mass and takes up space. Energy has no mass and does not take up any space.

3. WHAT IS MATTER? For example, water is matter because it has mass and takes up space. We can measure the mass by using a balance, and we can measure its volume by using a graduated cylinder. On the other hand, light is Energy. It has no mass and does not take up any space. Can you measure light on a balance? Does light take up space?

4. WHAT IS MATTER? What about air? Is air Matter or Energy? Well, even though we can’t see air, we can still apply our test. Does air take up space? Yes, a balloon proves that air takes up space. Does air have a mass? Yes, if we had a sensitive enough balance, we would find that a balloon filled with air has a greater mass than an empty balloon.

5. PROPERTIES OF MATTER Matter has both physical properties and chemical properties. Physical Properties – Are properties that can be observed or measured without changing the chemical makeup of matter. For example: size, color, smell, melting point and boiling point and are all physical properties. Observing or measuring them does not change the matter from one substance to another.

6. WHAT IS DENSITY? Density is another important physical property of matter. Density is a comparison of how much matter there is in a certain amount of space. It is a relationship between mass and volume. It can be expressed by the formula: density = mass_ volume

7. WHAT IS DENSITY? For example, which one of the squares below is more dense? Square ASquare B

8. WHAT IS DENSITY? Hopefully, you see that square B is more dense. It has the more matter, in the same size space. Square ASquare B

9. WHAT IS DENSITY? How about this? Now which square is more dense? Square A Square B

10. WHAT IS DENSITY? Hopefully, you see that Square B is more dense. It has the same amount of matter in a smaller space. Square A Square B

11. CALCULATING DENSITY? As stated above, density can be calculated by using the formula: density = mass/volume The units used in density are: Mass is measured in grams (g) Volume is measured in cubic centimeters (cm 3 ) Therefore, density will always be expressed in g/cm 3.

12. EXAMPLES Jack has a rock. The rock has a mass of 6g and a volume of 3cm 3. What is the density of the rock? d = 6g/3cm 3 = 2g/cm 3 Jill has a gel pen. The gel pen has a mass of 8g and a volume of 2cm 3. What is the density of the pen? d = 8g/2cm 3 = 4g/cm 3

13. WHY DO WE CARE? Since it never changes, we can use it to identify an unknown substance. Example – You buy what you think is a gold chain. How can you be sure? Compute the density. If it matches the known density for gold then it is real gold. If not, it is probably fake.

14. WHY DO WE CARE? We can also use it to predict how substances will interact. For instance, whether an object will sink or float. The density of water is 1.0 (It is the standard to which all other densities are compared) If the density of an object is greater than 1.0, the object will sink in water. If the density of an object is less than 1.0, then the object will float in water.

15. UNDERSTANDING DENSITY Any Questions?