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Data from: http://www.wikipedia.com

  1. 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).
  2. Describe what you see.
  3. Suggest a reason for what you have observed.
  4. 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:

  1. Measure and record the mass of a 10 ml graduated cyclinder.
  2. Pour an amount of distilled water into the cylinder.
  3. Measure and record the combined mass of the water and cylinder.
  4. Record the volume of distilled water in the cylinder
  5. Empty, clean and dry the graduated cylinder.
  6. Repeat the above steps for the other two liquids you have.
  7. Complete the table below.
Liquid Mass (g) Volume (ml) Density ( g · m l - 1 )
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.

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Source:  OpenStax, Siyavula textbooks: grade 10 physical science. OpenStax CNX. Aug 29, 2011 Download for free at http://cnx.org/content/col11245/1.3
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