Density

in everyday terms think of it as how crowded the particles are that make up
matter… like if there are to many people packed together on a bus we could say it
was “densely packed”

in science DENSITY is mass per unit volume



the particle model tells us that different substances have different-sized particles
the size of the particles determines how many particles can fit into a given space
each substance has its own unique density based on how close together the
particles can fit together
→ the more empty space between particles the less dense something will be
→ the less empty space (or more tightly packed) between particles the more dense
something will be
or D = mass of substance
volume of substance
Density of Solids, Liquids and Gases

the closer together the particles of a substance are the denser the substance will be
o particles of a gas are farther apart than particles of a liquid
o particles of a liquid are farther apart than particles of a solid

imagine you have 2 jars of the same size – one is filled with water (a liquid), the
other with water vapour (a gas)
o according to the particle model the gas particles will be very far apart and the
liquid particles will be closer together
o because there is more space between the gas particles it is LESS DENSE
o because the water particles are closer together it is MORE DENSE
therefore gases are less dense than liquids


now think about solids – the particle model tells us that the particles of a solid are
very very close together, therefore a solid will be more dense than a liquid

look at table 1.5 on page 53: we can compare the densities of materials to tell if
something will sink or float when placed in a fluid
o e.g. density of water is 1 g/ml (1 g of mass per 1 ml of volume) so any fluid or
solid material with a density of >1 will sink and any fluid or solid material with a
density of <1 will float

if the density of water is 1.0 g/ml and the density of sea water (salt water) is
about 1.03 g/ml, in which water would you float the easiest… and why?

look at figure 1.29 – what is the density of mercury? Is the density of the iron bolt
greater or less than the density of mercury?
look at figure 1.30 – what is the density of water? Based on this picture, what do
you know about the density of wood?

DENSITY = Mass per unit Volume



we can measure the density of a substance
1st we need to know the volume of the substance (how much space it takes up)
o the volume of a liquid can be measured using a measuring cup
o the volume of a gas can be measured by determining the volume of the
container holding it
o the volume of a solid can be measured using a mathematical formula
e.g. for a rectangular shaped object V = length x width x height
o the volume of an irregularly shaped object can be measured by displacing a
volume of water… as shown in figure 1.33
nd
2 we need to know the mass of the substance
o mass is a measure of the amount of matter in a substance
o mass and weight are NOT the same thing
 weight is based on the force of gravity on an object
 mass is constant, regardless of gravity
o here on earth when we weigh something on a scale the number we get is the
objects mass
 so if you weigh yourself on a scale and find out you ‘weigh’ 100
pounds that is actually your mass… if you go into space your
weight would show up as a lot less (less gravity) but your mass
would still be 100 pounds

density of liquids and gases is normally measured in g/ml and for solids as g/cm 3

density is expressed as a decimal which is converted from a fraction:
e.g. a substance with a mass of 3 g and a volume of 10 mls
Density =
3g
10 ml
= 0.3 g/ml
e.g. a substance with a mass of 5 g and a volume of 30 mls
Density =

5g
30 ml
= 0.16 g/ml
as long as the temperature and pressure do not change the density of an object will
not change
o why would density change if temperature changed → because if
temperature increases, particles move faster and get farther apart, and
then density would decrease; or the reverse is also true
o why would density change if pressure changed → if the pressure
decreases then the particles of gas can move farther apart (they are less
contained), and if they are farther apart we know that the density of the
substance will decrease; or the reverse is also true.