7th Grade: November 4thTrim and Glue in Journal:

“Cornell: Practice Problems”

Hint: You can cut BOTH at the same time!

REMINDER:

Quiz is Monday

Homework due Monday

Density is defined as mass per unit volume. It is a measure of how tightly packed and how heavy the molecules are in an object. Density is the amount of matter within a certain volume.

What is density?

Density is a comparison of how much matter there is in a certain amount of space.

Each element on the periodic table has a specific density.

The density of an element is the same no matter what the size or mass of the sample.

Density can be used to identify elements

Density is a physical property of a substance that is used to identify that substance.

The size of the object does not change the density. The density of lead is always going to be the same.

DensitDensity of y of LeadLead

DensitDensity of y of LeadLead

Density Density of Leadof Lead

MassMeasurement of the amount of matter

(or stuff) in an objectMeasured in grams (g)

There are 3 states of matter:

Solid

Liquid

Gas

There are 3 states of matter:

Solid

Liquid

Gas

Volume

Measurement of the amount of space an object takes upMeasured in milliliters (ml) or cm3

Finding Volume

To find the volume of a liquid, use a graduated cylinder

Make sure to read the bottom of the meniscus

• Identifying your units!

The base unit for mass (m) of an object is grams (g).

The base unit for volume of a solid is cubic centimeters (cm3).

The base unit for volume of a liquid is milliliters (ml)

Which one is more dense?

Demonstration: People in a square

How about this: Which square is more dense?

Which one is more dense?

Now which one is more dense?

What is density?

Density = mass OR mass ÷ volume.

volume

Units for density: g/cm3

g/mL

Why are these the units for density?

ALWAYS REMEMBER

UNITS!

The Triangle forVolume, Mass, and Density

M ÷ ÷D X V

Density = MV

Volume = MD

Mass = D x V

1) Find the mass of the object

2) Find the volume of the object

3) Divide : Density = Mass - Volume

To find density:

Ex. If the mass of an object is 35 grams and it takes up 7 cm3 of space, calculate the density.

1) Find the mass of the object

2) Find the volume of the object

3) Divide : Density = Mass - Volume

To find density:

Set up your density problems like this:

Given: Mass = 35 grams Unknown: Density (g/ cm3) Volume = 7 cm3

Formula: D = M / V Solution: D = 35g/7 cm3

D = 5 g/cm3

Ex. If the mass of an object is 35 grams and it takes up 7 cm3 of space, calculate the density.

Solving for Density

Jack has a rock. The rock has a mass of 6g and a volume of 3cm3. What is the density of the rock?

Jill has a gel pen. The gel pen has a mass of 8g and a volume of 2cm3. What is the density of the rock?

Solving for Mass and Volume

Calculate the mass of a liquid with a density of 3.2g/mL and a volume of 25mL.

*Find the volume that 35.2g of carton tetrachloride (CCl4) will occupy if it has a density of 1.60g/mL.

A 2.75 g sample of a substance occupies a volume of 250.0cm3. Find its density.

The density of ethanol is 0.780g/mL. Find the mass of a sample of ethanol that has a volume of 150.0mL.

A rectangular block of lead measures

2cm x 3cm x 4.5cm. If the density is 11.34g/cm3, calculate the mass of the block.

Jake has a book, a ruler, and a balance. How can Jake find the density of the

book with the tools he has?

Measure Book : Length, Width, HeightCalculate Volume: LxWxHMeasure mass with TBBPlug into Density Formula

Finding Volume

Water Displacement Method- Read the initial volume of water in a graduated cylinder- Place object into cylinder- Read the final volume of water- Final – Initial = Volume of the object

Reading a Graduated Cylinder

Meniscus

Initial Height of Water

350 mL

1st Water Level

With an Irregular Shaped Object

Initial Height of Water

350 mL350 mL

Place object into water and read level again.

1st Water Level 2nd Water Level

With an Irregular Shaped Object

Initial Height of Water

350 mL

425 mL

Place object into water and read level again.

1st Water Level 2nd Water Level

With an Irregular Shaped Object

Initial Height of Water

350 mL

425 mL

Place object into water and read level again.

1st Water Level 2nd Water Level

With an Irregular Shaped Object

Subtraction Problem

2nd water level

- 1st water level

425 mL

- 350 mL

75 mL

DENSITY OF AN IRREGULAR SOLID

Find the Mass of the solid Find the Mass of the solid on a balance.on a balance.Fill the Measuring Cylinder Fill the Measuring Cylinder with Water to a known with Water to a known Volume.Volume. Add the Object.Add the Object. Work out the Volume of Work out the Volume of Water that is displaced.Water that is displaced. Calculate the Density.Calculate the Density.

50 cm3

80 cm3

m = 360 g = m = 360 =12.0 g/cm3

V 30

Water Displacement Practice

An irregular object with a mass of 18kg displaces 2.5 L of water. Calculate the density of the object.

An irregularly-shaped sample of Aluminum has a mass of 43.6g. The student uses water displacement to find the volume. The initial volume reading is 25.5mL, and after the Al is added, the water has risen to 41.7mL. Find the density of the Al.

One more Example….

A cube of gold (Au) has a side length of 1.55cm. If the sample is found to have a mass of 71.9g, find the density of Au.

Liquid Layers

If you pour together liquids that don’t mix and have different densities, they will form liquid layers.

The liquid with the highest density will be on the bottom.

The liquid with the lowest density will be on the top.

Liquid Layers

Which layer has the highest density?

Which layer has the lowest density?

Imagine that the liquids have the following densities: 10g/cm3. 3g/cm3.6g/cm3. 5g/cm3.

Which number would go with which layer?

Liquid Layers – Try with your neighbor

Which liquid has the highest density?

Which liquid has the lowest density?

Which liquid has the middle density?

Liquid Layers – Try on your own!

Imagine that the liquids on the right have the following densities:15g/cm3 10g/cm3

3g/cm3 9g/cm3

7g/cm3 12g/cm3

Match the colors to the correct densities.

3g/cm3

7g/cm3

9g/cm3

10g/cm3

12g/cm3

15g/cm3

Review

What is the formula for density?What happens if you pour together liquids

that have different densities?Will the liquid on the top have the highest

or lowest density?Will the liquid on the bottom have the

highest or lowest density?

Supplies: Each person needs 1 cranberry, 1 Powerade, 1 diet 7 up, and 1 clear plastic cup of ice.

Prediction: Which substance is most dense? Which substance is least dense?

Task: Poured Cranberry juice in first over a piece of ice; Gatorade in slowly over a piece of ice; and then diet 7 up slowly over a piece of ice. Observe what happens.

Conclusion: Why was cranberry juice the most dense?

Reason Why: They stayed in separate layers based on sugar content.

ApplicationWhat’s Happening: think about ocean and continental crusts, apply what happened with the drink to what you know about these two crusts…

Ocean floor crust is heavier and more dense than continental crust (because of the water

Density Homework – due Thursday