EFFECT OF PRESSURE
We have discussed the effect of energy on the molecules to change compound have one physical state to another. Since basically the effect is to allow the molecules to move away from each other, then decreasing the pressure should also ease the change in physical state. We can illustrate this from the results for water.
 Phase diagram |
This plot shows the transition temperature between
To discuss a state transition it is best to think of the liquid state as an intermediary between the orderly (solid) state and the chaos (liquid) state. For example it is easier to understand why an increase in pressure will make it easier (meaning less energy required) to freeze water. It is harder to see how melting is easier with increase in pressure. Similarly it is easier to understand why water boil at lower temperature with lower pressure.
Just because the water melt or boil do not make that temperature the melting or boiling point. This is because melting and boiling points are defined as the points for melting and boiling at 760 torr pressure. So the melting point for water is 0�C, and the boiling point of water is 100�C.
Note: 1 torr = 1mmHg. Torr is more exact. The value for 1 atmospheric pressure at the top of Mount Everest is slightly different from that in Beijing. However for concept discussion I would used the term 1 bar for ease of writing. 1 bar is also an exact value used widely by the engineers. It is slightly more than 760 torr.
Tutorial 5
- Water can melt and boil at different temperature depending on the pressure. How many melting and boiling points are there for water?
- Give two other words for chaos. Answer
INTERMOLECULAR INTERACTION BETWEEN IONIC MOLECULES
Ionic molecules will form crystalline compounds. In crystallography we need to know certain terms. A beautiful crystal is formed by the repetition of a basic unit known as a unit cell. The cell repeats itself to give a crystal lattice. The different entities in the basic molecule are known as the asymmetric units. Example for calcium carbonate the asymmetric units are calcium and carbonate. Because of the systematic repetition, crystals have orderly structure and so it can reflect and refract light to give you beautiful gems.
Of course the same ionic molecule can form different types of crystal. Calcium carbonate can be found as limestone or pearl. Each type of crystal is known as a phase. So diamond is one phase of solid carbon and graphite another phase.
Needless to say no one will try to melt or boil a rock, unless you have the temperature of a volcano. Lava is a molten mixture of ionic compounds. So ionic compounds do not melt or boil. The only event that can take place when you heat an ionic compound is for the molecule to decompose. For example if you heat calcium carbonate, the carbonate will decompose to give carbon dioxide and the end product is calcium oxide at about 811�C. Decomposition is a chemical change (that is a new product is formed) and not a physical change.
This lesson will not be complete if we do not know about metal crystals. Some of the most frequently encountered chemicals are iron (knives, nails, etc), copper (electrical wires), zinc, tin, etc. Here the atoms are bonded to each other by metallic bonds. Such bonds are more difficult to apprehend but can be explained using molecular orbital bond theory, where the d-orbitals in the atom played a major part. Because there is only one type of asymmetric unit (that means one size), it can be packed very closely; or in crystallographic term "close-packed" structures. The better known close-packed are cubic close-packed and hexagonal close-packed. Of course metallic compounds can be melted, but only at very high temperature, and no one will try to vapourise metallic compounds.
Tutorial 6
Why are iron and copper good conductors for electricity and heat? Answer
