Transitions Among the Three States of Matter

 

 

 

The three states of matter can undergo transitions from one to another.

 

Ø  The major difference between the states is the freedom of movement.

 

Ø  The molecules of gases have great freedom of movement with great distances between them and a very slight attraction between them.

 

Ø  In liquids the molecules also have freedom of movement but the distances between them are greatly reduced and the attraction between them is greater.

 

Ø  In the solid state, the forces between the molecules, atoms, or ions hold them in relatively fixed positions with the only movement being some oscillations of the particles.

 

 

The physical properties of liquids

 

  1. At a constant temperature, the volume of a liquid is independent of any pressure exerted upon it.

     

  2. The molecules of a liquid are restricted by the strong attractive forces that hold them together, but they will speed up with rising temperature as their kinetic energy increases ( K.E. = ½ mv2 ) and the kinetic energy can become great enough that the molecules separate and move out of the liquid state.

     

  3. The density of a liquid is directly proportional to its molecular weight and inversely proportional to its molecular volume.

     

  4. Surface tension results from unbalanced attractive forces. Molecules in a liquid that are surrounded by other molecules have a net force of attraction = zero. Those at the surface are subjected to a net downward force, thus a drop of a liquid takes on a spherical shape.

     

a.       An increase in temperature increases the molecular distances and a reduction in surface tension.

 

  1. Viscosity (resistance to flow) is due to molecular attraction. Increasing temperature can cause a reduction in viscosity.

     

  2. Evaporation of a liquid occurs when molecules with higher K.E. are traveling upward and will sometimes escape.

     

a.       Not all molecules in a liquid have the same K.E. (velocity).

 

b.      The loss of more energetic molecules results in a cooling of the liquid and heat must be added to maintain the temperature.

 

c.       When the number of molecules becoming vapor equal the number of molecules returning to the liquid in a closed container, equilibrium is reached and the vapor pressure of the liquid for that temperature has been reached.

 

d.      Increasing the temperature increases the vapor pressure

 

e.       There are more vapor molecules hitting the walls of the container because more were vaporized by the higher temperature.

 

f.       The vapor molecules are traveling at faster speeds due to more K.E.

 

g.      The heat of vaporization is the number of calories required to vaporize 1 g. of a liquid at its normal boiling temperature.

 

  1. The boiling point of a liquid is different from evaporation of a liquid. Evaporation takes place whenever a liquid is exposed to the atmosphere as vapor molecules diffuse from the liquid through the atmosphere. Boiling occurs when the vapor from a boiling liquid escapes with sufficient pressure to push back and displace any gas present, rather than diffusing through it.

     

a.       When a liquid reaches its boiling point, the vapor pressure becomes equal to the pressure of the gas above it and the vapor molecules displace the molecules above the liquid.

 

b.      Bubbles of vapor will also form within the liquid and not just occur at the surface as in evaporation.

 

c.       Changing the pressure of the gas above the liquid will change the boiling point of the liquid. A higher pressure raises the boiling point to a higher temperature (B. P. is when the vapor pressure = the opposing pressure)

 

  1. If a liquid is heated inside a sealed flask with no other substance present, the liquid becomes less dense while the vapor becomes more dense as its pressure increases. These two things keep progressing as the temperature rises until the critical temperature is reached and the two densities are equal and the two phases become identical. (The critical temperature is the highest temperature at which a substance can exist in the liquid state.)

     

    1. The critical pressure is the pressure exerted by a liquid at the critical temperature.

       

    2. The volume occupied by a unit quantity of liquid at the critical temperature is called the critical volume.

       

 

 

 

 

Gas-Liquid Transitions

 

 

 

1.      The liquefaction of a gas can be done by either lowering its temperature or increasing the pressure acting on it, provided the temperature is below the critical temperature.

 

a.       Any gas can be liquefied by lowering its temperature.

 

b.      A gas can be liquefied by increasing pressure only if the temperature is below the critical temperature.

 

The SolidState

 

1.      Matter in the solid state exhibits hardness, rigidity, and a characteristic shape.

 

 

 

2.      There is oscillation of the molecules in a solid and if the oscillation becomes great enough, sublimation can occur (iodine and camphor).

 

a.       At that point the vapor pressure of the solid equals the opposing pressure.

 

b.      Lowering the pressure lowers the temperature of sublimation. If the sublimation temperature is higher than the boiling point, the substance will boil first.

 

c.       If dry ice is subjected to high pressure and the temperature rises above -56oC it will melt instead of subliming. In cylinders, CO2 is in the liquid state.

 

 

 

3.      The attractive forces between the molecules hold strongly in particular directions and hold them in definite spatial relationships to each other causing the hardness and characteristic shape.

 

a.       The shape depends upon the characteristic shape of the individual crystals of a solid.

 

b.      When there is no crystalline shape, the material is amorphous.

 

 

 

4.      Different forms of the same solid substance are called allotropes.

 

a.       White phosphorous is an unstable allotrope of red phosphorous. It reacts more violently and changes into red.

 

b.      Diamond and graphite are allotropes of carbon. Diamond has strong bonds in three dimensions while graphite has strong bonds in two dimensions and weak bonds in the third dimension.

 

Melting Points

 

1.      The melting point of a solid is reached when the kinetic energy overcomes the forces that have been holding the molecules in fixed positions. The freezing point holds true in the opposite direction. Freezing point = melting point

 

 

 

2.      Heat of fusion is the temperature at which a solid will melt and when a substance is being heated the temperature will rise until the heat of fusion is reached. Then it remains constant until the phase change is complete.

 

a.       Water is 80 cal/g. for fusion and 540 cal/g for vaporization (greater distances of separation require much more energy).

 

 

 

3.      Pressure has very little effect upon melting points. Very  high pressure however will raise the melting point of ice several degrees.