MadSci Network: Chemistry |
Greetings.
All matter has the ability to be a solid, a liquid, a gas, or ionized
plasma. What stage the mass is in depends on its heat.
For example, water at an average heat (temperature) of -10C is ice. At 40C
it is water, and above 100C it is steam. This is, of course, assuming that
pressure is the standard (1 atm) and volume is constant.
Each substance is different however, as you have pointed out. Gold is
solid at room temperature, mercury is a liquid, and neon is a gas. The
reason for the difference is that they all have different elemental
properties. For example, neon is a noble gas. That is, it has all of its
outer shell electrons filled. This is demonstrated on the periodic table
by its representation in the last (VIII A) group. As it has its outer
shell filled, it does not react easily with anything else (it has no reason
to). Therefore, the forces between individual atoms of neon are almost
nil. Since there are no forces, they are free to move around and,
therefore, are a gas. However, if you take a lot of heat away from the
atoms, they lose kinetic energy and begin to come together. This, if it
occured, would make it a liquid. However, because of its noble gas status,
it is extremely difficult to make neon a liquid.
Water is liquid at room temperature. Why? There's an intermolecular force
called hydrogen bonding which causes this. The slightly negatively
charged oxygens with two lone pairs of electrons, are attracted to the
slightly positive hydrogens of another water molecule. This attraction
causes them to form a liquid at room temperature. It is only after adding
enough kinetic energy (heat) to break the hydrogen bonds that they can
separate and become a gas. Similarly, removing heat can form them into a
solid.
Through this, I hope you can see that a substance's melting and boiling
points are dependent upon its physical properties. I've only shown some of
the more simplistic examples here. Imagine trying to sort out the
intermolecular forces between molecules with thousands of atoms!
As to your other question: What is the state of matter of a substance
with the melting point at -78 C and the boiling point at 10 C? At room
temperature (usually taken to be about 20C-25C) that matter would be above
its boiling point and, therefore, would be a gas.
I hope this has answered your question. Feel free to E-mail me with any further questions.
Try the links in the MadSci Library for more information on Chemistry.