<< Chapter < Page Chapter >> Page >

The properties of matter

Let us now look at what we have learned about chemical bonds, intermolecular forces and the kinetic theory of matter, and see whetherthis can help us to understand some of the macroscopic properties of materials.

  1. Melting point
    Melting point
    The temperature at which a solid changes its phase or state to become a liquid . The process is called melting and the reverse process (change in phase from liquidto solid) is called freezing .
    In order for a solid to melt, the energy of the particles must increase enough to overcome the bonds that are holding the particlestogether. It makes sense then that a solid which is held together by strong bonds will have a higher melting point than one where the bonds are weak, because more energy (heat) is needed to breakthe bonds. In the examples we have looked at metals, ionic solids and some atomic lattices (e.g. diamond) have high melting points, whereas the meltingpoints for molecular solids and other atomic lattices (e.g. graphite) are much lower. Generally, the intermolecular forces between molecular solids are weaker than those between ionic and metallic solids.
  2. Boiling point
    Boiling point
    The temperature at which a liquid changes its phase to become a gas . The process is called evaporation and the reverse process is called condensation
    When the temperature of a liquid increases, the average kinetic energy of the particles also increases and they are able to overcomethe bonding forces that are holding them in the liquid. When boiling point is reached, evaporation takes place and some particles in the liquid become a gas. In other words, the energy of theparticles is too great for them to be held in a liquid anymore. The stronger the bonds within a liquid, the higher the boiling point needs to be in order tobreak these bonds. Metallic and ionic compounds have high boiling points while the boiling point for molecular liquids is lower.The data in [link] below may help you to understand some of the concepts we have explained. Not all of the substances in the table aresolids at room temperature, so for now, let's just focus on the boiling points for each of these substances. What do you notice?
    The melting and boiling points for a number of substances
    Substance Melting point ( 0 C ) Boiling point ( 0 C )
    Ethanol (C 2 H 6 O) - 114,3 78,4
    Water 0 100
    Mercury -38,83 356,73
    Sodium chloride 801 1465
    You will have seen that substances such as ethanol, with relatively weak intermolecular forces, have the lowest boiling point, while substances withstronger intermolecular forces such as sodium chloride and mercury, must be heated much more if the particles are to have enough energy to overcome theforces that are holding them together in the liquid.

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 ( 0 C)
Methane CH 4 -161.6
Ethane C 2 H 6 - 88.6
Propane C 3 H 8 -45
Butane C 4 H 10 -0.5
Pentane C 5 H 12 36.1
Hexane C 6 H 14 69
Heptane C 7 H 16 98.42
Octane C 8 H 18 125.52

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Siyavula textbooks: grade 10 physical science. OpenStax CNX. Aug 29, 2011 Download for free at http://cnx.org/content/col11245/1.3
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 10 physical science' conversation and receive update notifications?

Ask