April 05, 2013
Density
Using everyday words, density can be described as the “crowdedness” of
the particles that make up matter. In scientific terms, density is the
amount of a substance that occupies a particular space. ‘When you
describe a substance as being “heavy” or “light,” you are referring to the
property of density.
According to the particle theory, different substances have different-sized
particles. The size of the particles determines how many particles can “fit
into” a given space. Therefore, each substance has its own unique density,
based on particle size. Furthermore, the particle theory suggests that there
is empty space between the particles.
April 05, 2013
Density of Solids, Liquids, and Gases
According to the particle theory, gas particles have
more space between them than do liquid particles.
Therefore, the water vapour in the container would
have fewer particles than the liquid water. It would be
reasonable to conclude that the density of the water
vapour is less than the density of liquid water.
How are density and state of matter related to the physical properties of a
substance? Solid objects can move easily through liquids and gases. According
to the particle theory the fluid properties of water and of air allow water
particles and air path des to move out of the way of the firmer, non-fluid bodies
of marine animals.
April 05, 2013
When an object moves through a fluid, it pushes particles apart and
moves between them. Particles in a solid cannot be pushed apart.
Attractive forces among the particles of a solid are stronger than those
between fluid particles and thus the particles in a solid cannot be pushed
apart.
If you were to step onto the surface of a lake, the water would not support
your foot. Instead, your foot would go right through the water. In fact, you
would continue to fall through the water, pushing the water particles out of
the way. Liquids cannot support objects in the same way that solids can,
because the particles of a liquid move apart easily, allowing a dense, solid
object, such as your foot, to pass through the liquid. The attractive forces
between liquid particles are not strong enough to prevent your foot from
pushing them apart.
April 05, 2013
Similarly, you cannot walk on air, because gases are even less dense than
solids or liquids. When you move through air, you are moving through mostly
empty space. You do not have to move as many particles of air out of the way
as you do in. This explains why running through air is much easier and faster
than through water. In general, gases are less dense than liquids.
F
Fill in the table below with the following words:
State of Matter
Solid
Particle Spacing
tight
Density
high
Liquid
medium
medium
Gas
far apart
low
April 05, 2013
As temperature increases, a substance will change from solid, to liquid, to gas.
The particle theory states that the particles of a substance spread out as they
gain energy when heated. Thus, the particles take up more space, which means
that the density of the substance decreases. It is almost always true that, for each
pure substance (for example, gold), the density of its solid state is greater than
the density of its liquid state. The substance’s solid state and liquid state are, in
turn, denser than its gaseous state.
In some cases, the densities of two substances can be so different that the liquid
state of one is denser than the solid state of the other! Many solid metals, such as
copper, nickel, and silver, can float on mercury, one of the densest substances
known.
April 05, 2013
Density: How are Mass and Volume Related?
You could determine a substance’s density if you
knew how much of the substance occupies a certain
space.
Mass is the amount of matter in a substance.
Volume is a measurement of the amount of space
occupied by the substance.
As you know, the volume of a liquid can be measured
using a measuring cup or a graduated cylinder, for
example. The volume of a gas can be determined by
measuring the volume of the container that holds it.
April 05, 2013
The density of a substance can be determined by calculating its mass-to-volume
ratio. You can do this by dividing the object’s mass by its volume. Therefore, the
formula for density is:
Density (D) = Mass (m)
Volume (v)
For example, the density of an object having a mass of l0g and a volume of 2 cm3
is 5 g/cm3. The density of solids is usually given in g/cm3 (grams per cubic
centimetre).
The density of liquids and gases is often given in g/L (grams per litre) or g/mL
(grams per millilitre).
April 05, 2013
Using pure water as an example, you could express its density as either 1 g/cm3
or as 1 g/mL (1 cm3 = 1 mL).
As long as the temperature and pressure stay the same, the mass-to-volume ratio,
or density, of any pure substance is a constant, which means it does not change.
According to the particle theory; the size of the particles of a substance does not
change when the mass or volume of the substance changes. A certain number of
particles of a particular size will always occupy a certain space. As the number of
particles increases from substance to substance, the space required for those
particles must also increase. Therefore, density is a property of matter that is
unique to a specific pure substance.
April 05, 2013
Review Questions.
1.
Which of the three states of matter generally has the greatest density? The least?
2.
If a cube of metal with a side measuring 5 cm has a mass of 70 g, what is its density? (Show all calculations.)
3.
Using the table, what is the metal? How do you know?
April 05, 2013
Review Questions.
4.
Name one liquid and one solid that will float on water.
5.
Which is the densest material, as listed in the table? Which is the least dense?
6.
What happens to density when the volume of a substance increases? (Use the formula, D = m/v, to help figure this out.)
April 05, 2013