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

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?

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

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

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

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

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 (cm3) or milliliters (mL)

Iron

Wood

Styrofoam

VolumeMassDensity =

(g)

(cm3)(mL)

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

62.4 Grams

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 cm3 or a volume of 8 cm3

IronLength = 2 cm

Width = 2 cm

Height = 2 cm

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

4040

50

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

MassDensity =

8 milliliters62.4 grams

= 7.8 g/mL

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: 1cm3 = 1 mL

Volume

MassDensity =

Practice:• Calculate the density of an object that has a mass of

1.6 grams and a volume of 3 millilitersAnswer:

Volume

MassDensity =

3 milliliters1.6 grams

= 0.6 g/mL

Density of Water• Why is 1 cm3 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/cm3

• 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 cm3

• 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 cm3

Length = 1 cmWidth = 1 cm

Height = 1 cm

Volume = 1 cm3 = 1 mL

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:

VolumeMassDensity =

1 milliliter1 gram

= 1.0 g/mL

WoodBlock Density= 0.6 g/mL Iron

Block Density= 7.8 g/mL

Water Density= 1.0 g/mL

StyrofoamBlock Density

= 0.01 g/mL

Objects with a density greater than 1 will sink in waterObjects 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 molecular arrangement

Alright. Lets see which objects sink and which objects float

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

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