|MadSci Network: Chemistry|
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.