Density Chapter 4 Physical Science

Tuesday, October 2, 2011  Supplies: A pencil and science notebook  Standards:  8a) Density is mass per unit volume  8b) Calculate the density of substances from measurements of mass and volume  9f) Apply simple mathematic relationships to determine a missing quantity

Warm-up  Given what you know about energy in solids, liquids, and gases, how and why do you think a lava lamp works.

Agenda  Pass back homework  Density Lecture  Ticket-out

What is density  Population density?  Forest density?  Density of stars?  Air quality?

Density in Physics  Density is a property of matter, like color or size.  Density is mass per unit volume.  Given a cubic centimeter of  Steel  Water  Air  Which has more mass?

Write it  D=m/v  Solve for m  Solve for v

Solve it  A stainless steel sphere has a mass of 25 grams and a volume of 3.2 cm 3. What is the density of the sphere?  A cube has a mass of 7.8 grams. The length of the sides of the cube is 1.2 cm. What is the density of the cube?

Density is a ratio  Units are g/cm 3  Ratio means  1 g/cm 3 = 1000 kg/m 3  Steel at 8.7 g/cm 3 is 8,700 kg/m 3 (multiply by 1000 and change units.)  Oak at 600 kg/m 3 is 0.60 g/cm 3 (divide by 1000 and change units.)  How does the density of oak compare to the density of water?

Solve it, them compare  Honey’s density is 1.4 g/cm 3  Write this as kg/m 3  Sandstone’s density is 20,280 kg/m 3  Write this as g/cm 3  Compare answers with with someone nearby.

Density Samples

Determining Density  Use a balance to find the mass  Use a graduated cylinder to find the volume (not a beaker – why?)  D = mass/volume

Using a graduated cylinder  Read the mark at eye level  Read volume at center of meniscus (ask your teacher!)  1 mL = 1 cm 3

Volume of solids  Length X width X heights gives cubic centimeters  What is the volume of a box that is 10” long, 5” wide, and 3” high?  What is the volume of a box that is 2’ long, 1’ wide, and 9” high?

More volumes, use 3.14 for π  Sphere: V=(4/3)πr 3  What is the volume of a sphere with a radius of 2 cm?  Cylinder: V=πr 3 h  What is the volume of a cylinder that is 10 cm tall and has a radius of 3 cm?

Irregular shapes  Volume by displacement of fluid  Remember: 1 mL = 1 cm 3  Fill graduates cylinder with fluid to known level (50 mL)  Carefully slide object down the side of the cylinder  Compute the increase in the fluid height  82 mL - 50 mL = 32 mL  32 mL = 32 cm 3

Compute this  I fill a graduated cylinder with 50 mL of water.  I slip a stone into the cylinder and the water raises to 62 mL.  What is the volume of the stone?

Why density varies  Mass of each atom or molecule that that makes up the substance – a calcium atom has more ‘stuff’ in it than an oxygen atom.  How tightly the atoms are packed -- the atoms are closer together in diamond that in graphite, but both are carbon.  (Except for water) solids are more dense than liquids  Liquids are more dense than gases

Ticket Out: Answer this  Which has more mass, a pound of bricks or a pound of feathers?  Which is more dense, a pound of bricks or a pound of feathers.