Density

Chapter 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 object to its VOLUME .

the MASS of an • 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 (cm 3 ) 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 MORE THAN

in water; if 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 cm

3

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

• • • record 1. Fill a graduated cylinder with enough water to cover the object. 2. Record the INITIAL water level. 3. Carefully add the object and 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= 8cm 3 ?

• 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?