1. UNIT: Chemistry and Measurement Objectives: Lesson 4 of 4 You will learn two methods for determining the volume of an object You will understand how the Density equation is derived You will gain a conceptual understanding of Density You will learn how to calculate Density given Mass and Volume TOPIC: Density

2. Quickwrite Iron Wood Styrofoam In 1-2 sentences answer one of the questions below: On a molecular or atomic level, what do you think density is? When you get an x-ray at the dentist's office, why do you think they place a lead vest over you? Consider the 3 blocks below, an iron block, a wood block and a styrofoam block; which one do you think is most dense? Imagine the dots represent the atoms or molecules that makeup each block; which block are the atoms more compact? In which block are the atoms more spread out?

3. Density Recall the relationship that exist between Mass and Volume Recall that Mass is the amount of atoms or molecules that makeup an object and Volume is how much space an object takes up By comparing the Mass and Volume of an object, you will be able to grasp the concept density The more mass an object contains in a given space, the more dense it is Density measures how tightly matter is compacted together Matter is made up of atoms, molecules and subatomic particles such as protons and neutrons

4. Density For example, consider the 3 cubes or blocks each made of a different substance One cube is made of Iron, one made of wood, and the other made of Styrofoam Now lets imagine the dots in each cube represent the arrangement of atoms or molecules in each substance Notice how some of the atoms or molecules are more spread out, and some are more compact Iron Wood Styrofoam Remember, Density measures how tightly matter (atoms/molecules) are compacted together Based on this definition, which of the 3 substances do you think is the most dense? The least dense? If you picked Iron as the most dense, you are correct, and if you chose Styrofoam as the least dense, you are also correct

5. What is Density? A measurement of how tightly matter are compacted together Matter is made up of atoms, molecules and subatomic particles such as protons and neutrons

6. Practice: Below are 3 fish made of lead, clay, and wood. Based on our definition of density, which fish is the most dense? Which fish is the least dense? Answer: The lead fish is the most dense because the atoms/molecules are the more compact; conversely, the wood fish is the least dense because the atoms/molecules are more “spread out” or less compact

7. Calculating Density Remember, density compares the mass of an object to its volume or size Density is calculated as a ratio of an object's mass to its volume, as shown in the equation below: In the above Density expression, Mass is measured in grams (g), and Volume is measured in either centimeters cubed (cm 3 ) or milliliters (mL) Iron Wood Styrofoam Volume Mass Density = (g) (cm 3 )(mL)

8. Styrofoam Calculating Density For example, let’s say we would like to find the density of the Iron Cube First we need to determine the mass of the Iron Cube To do this, we could place the Iron cube on a scale that measures in grams You can see that the Iron block has a Mass of 62.4 grams Iron Wood Volume Mass Density = 62.4 Grams

9. Calculating Density Now we that we have mass, we need to find the volume of the Iron cube The volume can be calculated two different ways We can measure length x width x height This gives us 2cm x 2cm x 2cm = 8 cm 3 or a volume of 8 cm 3 Iron Volume Mass Density = Length = 2 cm Width = 2 cm Height = 2 cm

10. Calculating Density Another way we can measure the volume of the Iron cube is by water displacement Notice the initial volume of water in the graduated cylinder is equal to 32mL What do you predict is going to happen to the water level if the iron cube is dropped in the graduated cylinder? That’s right its going to increase, but by how much? Let’s find out…. Notice, the water increased (32mL  40mL) or was displaced by 8 mL Therefore the volume of the Iron cube is equal to 8 milliliters (mL) Iron 32 mL  40 mL  10 20 30 40 50

11. Calculating Density Now that we have both mass and volume, we can calculate density If you recall, we obtained a Mass for the Iron cube of 62.4 grams and a volume of 8 milliliters If you further recall Density is equal to: Plugging in our values we get…. Iron Volume Mass Density = 8 milliliters 62.4 grams = 7.8 g/mL

12. How do we Calculate Density? Density is calculated as a ratio of an object's mass to its volume, as shown in the equation below: Mass is measured in grams and Volume is measured in either centimeters cubed or milliliters Note: 1cm 3 = 1 mL Volume Mass Density =

13. Practice: Calculate the density of an object that has a mass of 1.6 grams and a volume of 3 milliliters Answer: Volume Mass Density = 3 milliliters 1.6 grams = 0.6 g/mL

14. Density of Water Why is 1 cm 3 equal to 1 mL? One of the nice things about the metric system is that the Density of water is equal to 1 g/mL or 1 g/cm 3 To illustrate this point, lets say we want to find the volume of this cube…… To do this we need to measure length x width x height Notice each side of the cube is equal to 1 centimeter, which gives us a volume of 1cm x 1cm x 1cm = 1 cm 3 Now lets imagine the cube is completely empty How much water would it take to completely fill the cube? That’s right, 1 milliliter of water fills up 1 centimeter cubed, or 1 mL = 1 cm 3 Length = 1 cm Width = 1 cm Height = 1 cm Volume = 1 cm 3 = 1 mL

15. 1.0 Gram Density of Water Now lets find the mass of 1 milliliter of water Notice 1 milliliter of water has a mass of 1 gram Calculating the Density of water we get: Volume Mass Density = 1 milliliter 1 gram = 1.0 g/mL

16. Wood Block Density = 0.6 g/mL Iron Block Density = 7.8 g/mL Water Density Density = 1.0 g/mL Styrofoam Block Density = 0.01 g/mL Objects with a density greater than 1 will sink in water Objects with a density less than 1 will float in water Notice the densities of the each substance: Wood, Styrofoam, and Iron Based on the densities, try to predict which objects will float and which objects will sink Remember, dots represent the moleculararrangement Alright. Lets see which objects sink and which objects float float

17. What is the Density of water? The Density of water is equal to 1 g/mL Objects with a density greater than 1 will sink Objects with a density less than 1 will float

18. Summary (you can always write your own summary) Try to come up with and create your own definition for Density Describe the molecular arrangement for two different substances, one with a high density and one with a low density (are the molecules/atoms more spread out, more compact, explain….) Describe how you can calculate Density for an object Describe two methods you could find the volume of a cube Explain why the density of water is equal to 1 In terms of their density, explain why some objects float in water and some object sink in water