Today is Thursday (!),February 12th, 2015

Pre-Class:Choose one:

Why does wood float?

or

Which is heavier, a kilogram of lead or a kilogram of

hydrogen gas?http://fineartamerica.com/images-medium/the-schooner-margaret-todd-bar-harbor-maine-bob-orsillo.jpg

Stuff You Need:Calculator

In This Lesson:Density

(Lesson 5 of 6)

Today’s Agenda

• Density!

• Where is this in my book?– P. 89 and following…

By the end of this lesson…

• You should be able to calculate density of a given substance.

Density

• The density of an object (that’s one of its intensive properties, a physical property) is the relationship between its mass and volume.

• Density = mass / volume– Mass = volume * density– Volume = mass / density

M

D V

Density

• To put it another way, if two objects take up the same amount of space but have different masses, one must be more dense than the other.

• Alternatively, if two objects have the same mass but different densities, their volumes must be different.– I think you get the idea…

Density Facts

• Density is always expressed in terms of [mass]/[volume].– For us, it will typically be g/cm3 or g/mL.– Don’t forget the units!

• Water has a density of 1 g/cm3.– Funny how water always has nice round numbers,

huh?• 1 cm3 of water has a mass of 1 g.

Buoyancy

• To see if something is buoyant (can float), you need two densities:– The object’s.– The fluid’s.

• Object density > fluid density? Object sinks.• Object density < fluid density? Object floats.• Object density = fluid density? Object “kinda” floats.– For water, since its density is 1 g/mL, anything higher than 1

will sink; lower will float.• Dense water? Try the Dead Sea (video).

Buoyancy

http://i.telegraph.co.uk/multimedia/archive/02109/CostaConcordia-WEB_2109037b.jpg

http://cdn.theatlantic.com/static/infocus/costa011612/c01_36991800.jpg

Less Dense Than Water More Dense Than Water

The Costa Concordia, which wrecked off Italy’s coast in January 2012.

Density and Problem-Solving

• Here’s a challenge question for you:– Imagine you’re a king (or queen), and you have a royal

crown-maker.– Problem is, the dude looks really suspicious.

• Has funny eyebrows or something.

– So you suspect that maybe he’s not making your crowns out of solid gold like you said. Instead, he might be cutting the gold with something gold-like, but cheaper.

– How do you prove him innocent or guilty without destroying one of your crowns?

– Discuss!

Density and Problem-Solving

• Turns out this was a real problem faced by this guy:

• His name was Archimedes andhe solved this problem.

• What’d you come up with?

http://charlespaolino.files.wordpress.com/2010/07/archimedes_16201.jpg

How Archimedes Solved The Problem

• Archimedes noticed the property of displacement (how much matter is moved) when he got into his bathtub.

• He realized this is a way tomeasure volume.

• He was so excited by hisdiscovery (early science dork)that he supposedly jumped outof his bathtub and ran nakeddown the streets of Sicilyshouting “Eureka!” (“I have it!”)

Let’s try some examples ourselves:

• Grab your calculators, grab the whiteboards, [don’t] grab your partners, and let’s try some density problems right now.

http://www.algebralab.org/practice/practice.aspx?file=word_density.xml

Density Problem #1

• What is the density of a piece of wood that has a mass of 25.0 grams and a volume of 29.4 cm3?• 0.850 g/cm3

Density Problem #2

• A piece of wood that measures 3.0 cm by 6.0 cm by 4.0 cm has a mass of 80.0 grams. What is the density of the wood?• 1.1 g/cm3

• Would it float in water?– No! It’s more dense than water, so it would sink.

Density Problem #3

• A group of gold-colored beads was measured to have a mass of 425 grams. By water displacement, the volume of the beads was calculated to be 48.0 cm3. Given the following densities, identify the metal: (1 mL = 1 cm3)A. Gold: 19.3 g/mLB. Copper: 8.86 g/mLC. Bronze: 9.87 g/mL

Density Problem #3

• A group of gold-colored beads was measured to have a mass of 425 grams. By water displacement, the volume of the beads was calculated to be 48.0 cm3. Given the following densities, identify the metal: (1 mL = 1 cm3)A. Gold: 19.3 g/mLB. Copper: 8.86 g/mLC. Bronze: 9.87 g/mL

Density Problem #4

• If I throw a plastic ball into a pool for my dog to retrieve, what volume must the ball be to float? The mass is 125 grams and the density should be 0.500 g/mL.• 250 mL• [density = mass/volume]

Density Problem #5

• CHALLENGING: If the ball happens to spring a leak, how many mL of water can the ball take in before it sinks? [remember, ball is 125 g and we designed it to be 250 mL]– Greater than 125 mL

Density Problem #6

• CHALLENGING: What is the mass of a cylinder of lead that is 2.50 cm in diameter and 5.50 cm tall? The density of lead is 11.4 g/mL and the volume of a cylinder is V=πr2h.– Volume of the cylinder is 8.59π mL (≈27 mL)– Mass of cylinder = 308 g• [Density * Volume = Mass]

Density Problem #7

• The volume of a solution is shown by the graduated cylinder below. If the mass is 60.75 grams, what is the density?• 1.350 g/mL

http://www.algebralab.orit to be 250 mL].g/practice/practice.aspx?file=word_density.xml

Density Problem #8

• An ice cube measuring 5.80 cm by 5.80 cm by 5.80 cm has a density of 0.917 g/mL. What is the mass?• 179 grams

Density Problem #9

• Gasoline is a non-polar liquid that will float on water. 450 grams of gasoline is spilled into a puddle of water. If the density of gasoline is 0.665 g/mL, what volume of gasoline is spilled?• 677 mL

Density Problem #10

• The density of aluminum is 2.70 g/mL. If the mass of a piece of aluminum is 244 grams, what is the volume of the aluminum?• 90.4 mL

DENSITY CHALLENGER

• Also known as “The Density Boss.”• A little aluminum boat (mass of 14.50 g) has a

volume of 450.00 cm3. The boat is placed in a small pool of water and carefully filled with pennies. If each penny has a mass of 2.50 g, how many pennies can be added to the boat without it sinking?• 174 pennies

Now it’s your turn to work alone…

• Try the Density Practice Problems worksheet.

So now then…

• Let’s put this to the test in a virtual lab called Density Laboratory.

• It’s found on ExploreLearning.com.• Log-in information will be on the next slide or

(recommended) posted on my teacher site on the Chemistry Links.

• Complete the accompanying worksheet.• [Log-in Instructions]

Density to Water

• You may have noticed that there is a relationship between floating/sinking and the density of the water.

• The specific gravity of a substance is the ratio of that substance’s density to the density of water.– Specific gravity greater than 1? It’s more dense than

water.• Liquids are usually measured with a hydrometer.

Density Lab

• We’re going to finish our lessons on density with a look into a less electronic exercise.

• The Density Lab!

Closure and Homework

• Tonight for homework, make sure your Coca-Cola lab is done. Make sure your density virtual lab is done. Make sure your density worksheets are done.

• Then read pages 89-92.• Do questions #85-87 on page 97.