It should be clear now that we can explain a lot of
the
macroscopic properties of matter (i.e.
the characteristics we can see or observe) by understanding their
microscopic structure and the way in which the atoms
and molecules that make up matter are held together.
Exercise: forces and boiling point
The table below gives the molecular formula and the boiling point
for a number of organic compounds called
alkanes (more on these compounds in grade 12). Refer
to the table and then answer the questions that follow.
| Organic compound |
Molecular
formula |
Boiling point (
) |
| Methane |
|
-161.6 |
| Ethane |
|
-
88.6 |
| Propane |
|
-45 |
| Butane |
|
-0.5 |
| Pentane |
|
36.1 |
| Hexane |
|
69 |
| Heptane |
|
98.42 |
| Octane |
|
125.52 |
Data from: http://www.wikipedia.com
- Draw a
graph to show the relationship between the number of carbon atoms in eachalkane and its boiling point. (Number of carbon atoms will go on the x-axis andboiling point on the y-axis).
- Describe what you see.
- Suggest a reason for what you have observed.
- Why was it enough for us to use 'number of carbon atoms'
as a measure of the molecular weight of the molecules?
Investigation : determining the density of liquids:
Density is a very important property because it helps
us to identify different materials. Every material, depending on the elementsthat make it up and the arrangement of its atoms, will have a different
density.
The equation for density is:
Discussion questions:
To calculate the density of liquids and solids, we
need to be able to first determine their mass and volume. As a group, thinkabout the following questions:
- How would you determine the mass of a liquid?
- How would you determine the volume of an irregular
solid?
Apparatus:
Laboratory mass balance,
and
graduated
cylinders, thread, distilled water, two different liquids.
Method:
Determine the density of the distilled water and two
liquids as follows:
- Measure and record the mass of a
graduated
cyclinder.
- Pour an amount of distilled water into the cylinder.
- Measure and record the combined mass of the water and
cylinder.
- Record the volume of distilled water in the cylinder
- Empty, clean and dry the graduated cylinder.
- Repeat the above steps for the other two liquids you
have.
- Complete the table below.
|
Liquid |
Mass (g) |
Volume (ml) |
Density (
) |
| Distilled water |
|
|
|
| Liquid 1 |
|
|
|
| Liquid 2 |
|
|
|
Investigation : determining the density of irregular solids:
Apparatus:
Use the same materials and equpiment as before (for
the liquids). Also find a number of solids that have an irregular shape.
Method:
Determine the density of irregular solids as follows:
- Measure and record the mass of one of the irregular
solids.
- Tie a piece of thread around the solid.
- Pour some water into a
graduated cylinder and
record the volume.
- Gently lower the solid into the water, keeping hold of
the thread. Record the combined volume of the solid and the water.
- Determine the volume of the solid by subtracting the
combined volume from the original volume of the water only.
- Repeat these steps for the second object.
- Complete the table below.
|
Solid |
Mass (g) |
Volume (ml) |
Density (
) |
| Solid 1 |
|
|
|
| Solid 2 |
|
|
|
| Solid 3 |
|
|
|
