What is Density?

What is Density?
What is Density?
• Biology – How many individuals per unit
area?
• Chemistry – How many particles per unit
area?
Units for Density
• Density = mass/volume or D = m/v.
• Density is the mass of an object divided by the volume
• Units mass = gram (g)
• Units volume = centimeters cubed cm3 or ml
So, the overall units for density are g/cm3 or g/ml
Density and Phases of Matter
• Phases of matter are solids, liquids, gasses
• Solids are the most dense objects because they have more
particles close together and very little movement.
• Liquids are next because the particles have more movement
and are moving (flowing) around. The least dense liquid will
collect on top of the most dense object.
• Gasses are least dense because the particles are moving
quickly past each other and are spaced far apart.
Place the following substances in order from least dense to
most dense.
•
•
•
•
•
Gases @ 293K or 20oC
Unit is g/cm3
Hydrogen
Helium
Nitrogen
Air
Carbon Dioxide
Place the following substances in order from least dense to
most dense.
Liquids
Unit is g/ml
•
•
•
•
•
Methanol
Water
Formaline
Carbon Tetrachloride
Sulfuric acid
Place the following substances in order from least dense to
most dense.
Solids
Unit is g/cm3
•
•
•
•
•
Aluminum
Tin
Iron
Nickel
Gold
Regular Shaped Object
• These are objects with fixed shapes, having defined
edges like a rectangle or a square
• To find the mass use a triple beam balance or an
electronic balance using grams as the unit.
• To find the volume you have to measure 3 sides of the
object: length, width, height and multiply the numbers
together. L x W x H = _____ cm3. Note that cm3 is the
volume unit for a regular shaped object
• Density unit for a RSO will be g/cm3
Practice
A student in lab measured the mass of
a rectangle to be 23.7 g. The length was
21.0 cm. The width was 7.0 cm. The
height was 5.0 cm. What is the density
of the object? Show all work and
units!!!
Answer
V = L x W x H =
21.0cm x 7.0cm x 5.0cm = 735.0 cm3
or 7.4x102 cm3
D = Mass = 23.7 g
= 0.032 g/cm3
Volume 7.4x102 cm3
2 Sig Figs
Irregular Shaped Object
• These are objects with no fixed shape and do not have defined edges
like a rock or a cylinder.
• To find the mass use a triple beam balance or an electronic balance
using grams as the unit.
• To find the volume you have to use the Water Displacement
Method. Using a graduated cylinder place, enough water in the
cylinder to cover the object. Place the ISO in the cylinder. The water
level will rise. The difference between the starting volume and the
ending volume will be the volume of the object.
• So, use this equation:
ending volume (EV) – starting volume (SV) = volume of object in ml.
• Density unit for an ISO will be g/ml
Practice
Malia found a 10 g diamond in a mine in
West Virginia. She wanted to know the
volume of the diamond so she place 50 ml
of water in a 100 ml graduated cylinder and
after placing the diamond in the cylinder
the water level rose to 75 ml. What is the
density of the diamond? Show all work and
units!!!
Answer
V = EV-SV = 75ml – 50ml = 25ml
D = Mass = 10 g = 0.4 g/ml
Volume 25 ml
1 Sig Fig
Liquid
• Liquids have no fixed shape because they take the shape of the
container.
• To measure the mass of a liquid, first mass an empty graduated
cylinder in grams then pour some liquid into the cylinder and mass
again. The difference between the ending mass and starting mass is
the mass of the liquid.
• So, use this equation:
ending mass (EM) – starting mass (SM) = mass of object in g.
• To get the volume just read the meniscus on the graduated cylinder.
• Memorize: Water has a density of 1 g/ml no matter if the water is in
China or the USA, pure water (H2O) has the same density
everywhere. 1g = 1ml of H2O, 50g = 50ml H2O, 100g = _____ml H2O
etc…
Practice
Jacob massed an empty graduated
cylinder to be 58.0 g. He then put 28.0
ml of rubbing alcohol in a graduated
cylinder. He then measured the object
and the graduated cylinder to be 78.0 g.
What is the density of the rubbing
alcohol? Show all work and units!!!
Answer
M = EM-SM =
M = 78.0g – 58.0g = 20.0g
D = Mass = 20.0 g = 0.714 g/ml
Volume 28.0 ml
3 Sig Figs
Some questions ask you to find
mass or volume, so you will need to
rearrange the original equation
D = Mass
Volume
M=VxD
V = Mass
Density