2.2 The kinetic molecular theory  (Page 2/2)

Taking water as an example we find that in the solid phase the water molecules have very little energy and can't move away from eachother. The molecules are held closely together in a regular pattern called a lattice . If the ice is heated, the energy of the molecules increases. This means that some of the water molecules are ableto overcome the intermolecular forces that are holding them together, and the molecules move further apart to form liquid water . This is why liquid water is able to flow, because the molecules are more free to move than they were in the solid lattice. If themolecules are heated further, the liquid water will become water vapour, which is a gas. Gas particles have lots of energy and are far away from each other.That is why it is difficult to keep a gas in a specific area! The attractive forces between the particles are very weak and they are only loosely heldtogether. [link] shows the changes in phase that may occur in matter, and the names that describe these processes.

Summary

• There are three states of matter: solid, liquid and gas.
• Diffusion is the movement of particles from a high concentration to a low concentration. Brownian motion is the diffusion of many particles.
• The kinetic theory of matter attempts to explain the behaviour of matter in different phases.
• The kinetic theory of matter says that all matter is composed of particles which have a certain amount of energy which allows them to move at different speeds depending on the temperature (energy). There are spaces between the particles and also attractive forces between particles when they come close together.
• Intramolecular force is the force between the atoms of a molecule, which holds them together. Intermolecular force is a force between molecules, which holds them together.
• Understanding chemical bonds, intermolecular forces and the kinetic theory of matter can help to explain many of the macroscopic properties of matter.
• Melting point is the temperature at which a solid changes its phase to become a liquid . The reverse process (change in phase from liquid to solid) is called freezing . The stronger the chemical bonds and intermolecular forces in a substance, the higher the melting point will be.
• Boiling point is the temperature at which a liquid changes phase to become a gas. The reverseprocess (change in phase from gas to liquid) is called condensing . The stronger the chemical bonds and intermolecular forces in a substance, the higher the boilingpoint will be.
• Density is a measure of the mass of a substance per unit volume.
• Viscosity is a measure of how resistant a liquid is to flowing.

End of chapter exercises

1. Give one word or term for each of the following descriptions.
   1. The property that determines how easily a liquid flows.
   2. The change in phase from liquid to gas.
2. If one substance A has a melting point that is lower than the melting point of substance B, this suggests that...
   1. A will be a liquid at room temperature.
   2. The chemical bonds in substance A are weaker than those in substance B.
   3. The chemical bonds in substance A are stronger than those in substance B.
   4. B will be a gas at room temperature.
3. Boiling point is an important concept to understand.
   1. Define 'boiling point'.
   2. What change in phase takes place when a liquid reaches its boiling point?
   3. What is the boiling point of water?
   4. Use the kinetic theory of matter and your knowledge of intermolecular forces to explain why water changes phase at this temperature.
4. Describe a solid in terms of the kinetic molecular theory.
   
5. Refer to the table below which gives the melting and boiling points of a number of elements and then answer the questions thatfollow. ( Data from http://www.chemicalelements.com )

   Element	Melting point	Boiling point ( ${}^{°}\mathrm{C}$ )
   copper	1083	2567
   magnesium	650	1107
   oxygen	-218,4	-183
   carbon	3500	4827
   helium	-272	-268,6
   sulphur	112,8	444,6

   1. What state of matter (i.e. solid, liquid or gas) will each ofthese elements be in at room temperature?
   2. Which of these elements has the strongest forces between its atoms? Give a reason for your answer.
   3. Which of these elements has the weakest forces between its atoms? Give a reason for your answer.