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 and
boiling 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?
Density and viscosity
Density is a measure of the mass of a substance per
unit volume. The density of a solid is generally higher than that of a liquidbecause the particles are held much more closely together and therefore there
are more particles packed together in a particular volume. In other words, thereis a greater mass of the substance in a particular volume. In general, density
increases as the strength of the intermolecular forces increases.
Viscosity is a measure of how resistant a liquid is to
flowing (in other words, how easy it is to pour the liquid from one container toanother). Viscosity is also sometimes described as the 'thickness' of a fluid.
Think for example of syrup and how slowly it pours from one container intoanother. Now compare this to how easy it is to pour water. The viscosity of
syrup is greater than the viscosity of water. Once again, the stronger theintermolecular forces in the liquid, the greater its viscosity.
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.
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:
Density = Mass/Volume
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, 10 ml and 100 ml 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 10 ml 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.
