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Are there any other states of matter besides solids, liquids and gases?

    Yes there are two other states of matter; glasses and plasmas. We will discuss these later, but glass is a sort of solid-liquid and a plasma is a high temperature gas in which the atoms have been ionised. We are all familiar with glass but plasmas are not so common on Earth though interestingly most of the matter in the Universe is in this state; this is because stars are plasmas - very high temperature spheres of gas. Nearer to home, the strip lighting tube in the kitchen contains gas which, when a voltage is applied by switching it on, becomes a plasma.

What is the essential difference between the different states of matter?

    If you look at a general table of densities you will find that solids are more dense than liquids which are in turn more dense than gases. In round figures (though there are exceptions)

  • gases have typical densities of 1 kg.m-3
  • liquids have typical densities of 103 kg.m-3
  • metallic solids have typical densities of 104 kg.m-3.
  • The non-metallic solids have densities which are higher than liquids though not by very much (about a factor of 2).

    Clearly for any given state of matter, those elements with higher atomic numbers are more dense than the lighter elements.

    Similarly, for a given element the densities of that element in the gaseous, liquid and solid states increase in a similar way (though water is an interesting exception in that solid water (ice) is less dense than liquid water; hence it floats)

Does this mean that the atoms in different states of matter have different separations?

    Yes, and we can quite easily calculate what the typical separations are.

    If we divide the mass density (kg.m-3) by the atomic mass (kg) then we get the number density (m-3), that is, the number of atoms per unit volume. If we invert this number and then take the cube root we get a length (m) which we can consider as being representative of a typical distance between atoms.

    For example the density of water is 103 kg.m-3, the molecular weight of H2O is 18 proton masses which gives a number density of 103 / (18 x 1.6x10-27 ). Taking the cube root of the inverse gives approximately 10-10 m. If we use this figure to scale the densities for the different states of matter this leads to numbers like 10-9 , 10-10 and 5x10-11 m for typical separations of atoms in gases, liquids and solids respectively.

    We see from this that that the matter in gases is further apart than it is in liquids which in turn is further apart than matter in solids. To scale these numbers it is useful to note that the diameter of an atom is of order 10-10m (remember that all these numbers are approximate). Hence in solids the atoms are well packed together; in gases they are well separated.

What are the forces involved in determining the separation of matter?

    There are four "real" forces in nature, three of which are involved in the binding of matter. They are gravitation, electromagnetic, and (the strong) nuclear forces.

    Gravitation and nuclear are attractive forces which affect masses and nucleons respectively. Electromagnetic forces affect charges and are therefore repulsive as well as attractive.

    With so many attractive forces one could well ask the question as to why all the matter in the Universe has not coalesced into itself. Clearly there must be other repulsive forces. One of these is the "pseudo" centripetal force. Another is pressure and a third is simply the state of the Universe.

    To illustrate this just consider the following systems which are stable. The planets around the Sun stay in stable orbits as a consequence of the balance between gravitational forces and centripetal forces. The Sun, as many other stable stars, does not fall in on itself as a result of the gravitational forces because it is kept up by its internal pressure (it is a hot gas and the particles are moving around with large velocities) The Universe is not falling in on itself because the space containing it is expanding; it is not actually stable but things are just happening rather slowly. (At a time when it was thought that the Universe was in fact stable, Einstein actually had to introduce a new Cosmological force which opposed gravity). Finally, you are stable sitting in your chair because your weight is counteracted by the repulsive forces between the atoms in you and those in the chair.

    So stability is established by balancing opposite forces.

Is some matter more stable than other matter?

Yes; here are some examples.

  • An object falling downhill is more stable at the bottom of the hill than at the top.
  • The inert gases (Helium, Neon, Argon, Krypton and Xenon), are less reactive than other gases; they are more stable. They are satisfied with their quota of electrons and do not feel the need to share them.
  • Molecular hydrogen is more stable than atomic hydrogen. Each atom of hydrogen gas has one electron, but by sharing their combined two in a molecule each atom behaves as if it had two which is the number a Helium atom has. It is therefore more stable.
  • The helium nucleus is more stable than the heavy hydrogen nucleus (one proton and one neutron)
  • Water is more stable than steam
Can you think of any others?

What happens when a system becomes more stable?

    As we saw above, when an object falls downhill it is in a more stable position at the bottom of the hill than when it was at the top. We can say that it is more bound to the Earth at the bottom of the hill, that is, its binding energy has increased as a result of its falling. If free to move to a more stable state, energy is released in so doing; potential energy is converted into kinetic energy. This is how a hydroelectric system works. Water is released from a reservoir in the hills, flows down a tube, gains velocity, drives a turbine which generates electricity. A physical hill is simply an example of a gravitational field. The height of the hill is a measure of the gravitational potential; the slope (or gradient) of the hill a measure of the gravitational field strength.

    Any system which rearranges itself into a more stable situation will release energy and if the force fields in which they move are stronger then greater amounts of energy are released. Are there other kinds of systems besides gravitational ones. Click here to find out.

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The University of Birmingham 

Physics and Astronomy Department, The University of Birmingham