Return to Chemistry IndexDensity - Qualitative and Quantitative

Bahner, Stan Whitney Young High School

997-3739

Objectives: (In the words of the student)I will define density in words and mathematically. I will measure the mass and volume of three objects and calculate the density of the material in each.Apparatus Needed1. six film containers filled with materials having a range of densities. (For example: loose tissue paper. bits of styrofoam torn from a styrofoam cup, saw dust, sand, small nails packed tight, quarters packed tight). Label the containers a, b, c, d, e and f. 2. six larger sized objects covering a range of densities. Label the objects A, B, C, D, E and F. a brick a good sized chunk of lead or steel (tennis ball sized to baseball sized) a full gallon container of ditto fluid, paint thinner, or antifreeze a gallon milk container filled with water a two liter pop bottle filled with water a foot long piece of 2x4 lumber 3. geometric objects having a range of densities (cubes, rectangles, triangles, cylinders, trapezoids made of oak, walnut, pine, maple, birch, plastic, aluminum, brass, etc.) Some of the objects have holes drilled in them. 4. plastic game pieces from a Superfection game that have been filled with plaster mixed with lead bb's to make a range of densities. These were then sealed with polyurethane floor sealer. Thus they can be immersed in water and their volumes determined using Archimedes principle. 5. telescoping pair of boxes cut from bottoms of two half-gallon orange juice containers 6. rulers 7. calculators 8. balances (Cent-o-gram single pan preferred) 9. (if you wish to use Archimedes' method to have students check their calculated volumes.) a. 250 ml beakers b. thread or kite stringRecommended StrategiesSet-up: Arrange the apparatus on lab tables. Pre-lab: Discuss the need for a term "density" in addition to the term "heavy". Common usage lets the word "heavy" have both meanings. For example when handed a brick and then a smaller but heavier piece of iron or lead, one typically says "Wow! That's really heavy" It would be more precise to say "Wow! That's really dense." Students shouldn't be encouraged to use this odd sounding usage but only to recognize the difference between the two meanings which "heavy" commonly has and to be able to use the term density as a synonym of one of the meanings. Discuss the trick question "Which is heavier, a pound of feathers or a pound of steel?" Use the various objects to discuss "density". Use the telescoping juice container halves to explain that when you push them together the same mass is in a smaller volume and thus the density increases. Explain that floating is best explained using the term density. Objects less dense than water float. Helium is less dense than air and thus helium filled balloons float. Define density as mass divided by volume, D = M/V. Give the formulas for the volume of a rectangular solid [v = l x w x h], solid triangle [(v = 1/2 b x h)T] (T = thickness), cylinder V = 3.14 r^{2}l.

(pi times the radius squared times length.) Explain that when you use

metric units the density of water comes out to be 1.0 g/cm^{2}. Thus water

is the standard of density against which other substances are measured.

Discuss strategies for obtaining the volume of a complex geometric

object. For example a wood trapezoid with a hole drilled in it can be

seen as a rectangular solid plus a triangular solid minus a cylinder.

By measuring the dimensions of the object and using the formulas, the

volume can be calculated.

If you wish to have the students check their calculated volumes by

the Archimedes' method set up a balance and 250 beaker with object

suspended in water on a string. The Cent-o-gram single pan balances

have a solid platform under the weighing pan which can be raised for

this purpose. Also there is a double hook above for the string. If you

are using double pan balances you will have to suspend the object from

a ring stand. Explain Archimedes' principle, EUREKA story, etc. and how

the buoyant force is equal to the volume. (Many students already know

the Eureka story about the king's crown. Ask a student to tell the

story.)Student activities:

1. Have students rank the film containers putting the most dense first.

Use the form D_{a}>D_{d}>D_{c}, etc.

2. Have the students rank the larger objects putting the most dense

first. Since there are two different size containers of water, it

should help drive home the point to the students that density is a

property of the substance. Therefore they can put an = between

the two water containers rather than a ">" symbol.

(You may wish to have students experience the density of mercury by

having them lift a bottle of mercury. The density of mercury is

VERY SURPRISING and students have a tendency to let their hand

drop if you hand it to them. You must have a GUARANTEED SECURE way

to assure that the mercury won't be spilled. Make sure the mercury

is in a strong unbreakable bottle. It is usually supplied in thick

plastic bottles. Try tying the bottle to a lab table (to the gas

pipe, etc.) with a short rope. Sit next to the bottle and directly

supervise the students coming one by one to lift it from the

table. As soon as each student has experienced the density of

mercury put the bottle away in a carrying case and put it away for

proper storage.)

3. Have each student choose two simple geometric shapes and a complex

geometric shape to calculate the volume for each. Measure

dimensions to the nearest 0.1 cm. Mass the object and calculate the

density.

4. Have students check their volume calculations by comparison to the

volume measured by the Archimedes method.