DensityChapter 3

Scientific Measurements

I CAN

• I CAN define DENSITY and explain how it is calculated and determine the volume both a regular object (mathematically) and and irregular object (Archimedes' Principle).

DENSITY

• Which weighs more? A pound of lead or a pound of feathers?

• Neither…they each weigh a pound!

• Which takes up more space? Why?

• The feathers do because they are not as tightly packed as the particle in the lead.

What is DENSITY?

• Density COMPARES the MASS of an object to its VOLUME.

• Density is a UNIQUE PROPERTY of a substance; it can be used to help IDENTIFY UNKNOWN SUBSTANCE.

• DENSITY is defined as MASS PER UNIT OF VOLUME.

Calculating Density

• The mathematical equation for finding density is:

DENSITY = MASS VOLUME

-any MASS and VOLUME unit can be used, but commonly in science GRAMS are used for mass and MILLILITERS (ml) and CUBIC CENTIMETERS (cm3) are used for volume.

• mass and volume units cannot cancel each other; the final unit for density is the MASS UNIT over the VOLUME UNIT.

• For example g/ml or kg/l

• Pure water has a density of 1 g/ml.

• When we compare a substance’s density to that of water, we refer to the value as SPECIFIC GRAVITY.

• If an substance’s density is LESS THAN that of water, it will FLOAT in water; if MORE THAN water’s density, it will SINK!

Practice Problems

• What is the density of a substance if 25 grams has a volume of 5.0 ml?

• Density = Mass = 25 grams = 5g/ml

Volume 5.0 ml

• What is the density of a substance if 60 grams has a volume of 100 mL?

• Density = Mass = 60 grams = 0.60g/ml

Volume 100 ml

Mass and Volume

• Finding the MASS of an object is easy….place it on a balance and get the value.

• But what about VOLUME? Volume can be found in two ways:– A. Mathematically– B. Archimedes Principle

Calculating Volume

• The volume of regular objects, such as cubes or boxes is found by this formula:

– VOLUME = Length X Width X Height

– What would the units be? – Since you are multiplying the same unit by

itself three times, the final unit is CUBED!

Sample Problem

• A student measure a cube of wood and gets the following results:

• L= 5 cm• W = 4 cm• H = 3 cm

• What is the volume of the cube?• V = L X W X H• V = 5cm X 4 cm X 3 cm = 60 cm3

Archimedes’ Principle

• Archimedes’ Principle says that an object will displace a volume of water equal to its own volume.

• Used to determine the volume of oddly shaped object, such as a stone.

Using Archimedes’ Principle

• 1. Fill a graduated cylinder with enough water to cover

the object. • 2. Record the INITIAL water level.

• 3. Carefully add the object and

record the FINAL water level. • 4. The difference between the two

numbers is the object’s volume.

Practice Problem

• A student is asked to find the volume of a 44 g stone in lab. She fills a graduated cylinder to the 50 ml mark. She carefully adds the stone, then records the final volume of water, which is now 62 ml.

• What is the stone’s volume?

• Volume = 62 ml – 50 ml = 12 ml

• What is the stone’s density?

• D = M / V = 44g / 12 mL = 3.7 g/mL

Other Calculations

• How can the density equation be used to find other values, such as Mass or Volume?

• D = M / V V = M / D M = V x D

Practice Problems

• A certain mineral has a D = 2.1 g/cm3

What is the MASS of a sample having a V= 8cm3?

• A piece of wood has a D=2.3 g/mL

A piece of this wood having a M=22.6 g would have a Volume of?