gases+and+liquids

The behavior of the three phases of matter will be discussed here, starting with gases.


 * Gases**

The properties and behaviors of a gas are generalized in the Kinetic Molecular Theory and the gas laws.

However, there are a few more important terms that need to be understood. Gases are considered //fluids// because they can flow from one area to another (like liquids) and are compressible.

When a gas is concentrated in one area (e.g. when perfume is spilled in one corner of a room), it will spread out and eventually evenly mix with the other gases (e.g. air) in the room until it is homogeneous. This is called //diffusion//, which is conceptually similar to dilution.



Gases will travel at different speeds, depending on the kinetic energy of a gas (mass and velocity) and its temperature. In order to eliminate any interference (say, intermolecular forces) from other gases, we use the measurement of //effusion//.



The rate of effusion for a gas can be determined using **Graham's Law**. Without going into the math, this effusion rate is essentially inversely proportional to the mass of the gas molecule. The heavier the moecule, the slower the rate of effusion.

Another way to calculate the speed of a gas molecule is to use **root mean square speed (rms speed)**.



In this equation, the velocity (//v// rms, in meters per second) is related to the Kelvin temperature (T) and the molecular mass (M m ) of the gas. The molar gas constant R is equal to 8.314 J/mol K. Here is an example - let's compare the rms speed of helium to that of radon at room temperature (298 K).

for **helium**:

//v// rms = (3 x 8.314 x 298/4.00) 1/2

= = = 43.1 m/s

for **radon**:

//v// rms = (3 x 8.314 x 298/222) 1/2

= = = 5.79 m/s

As you can see, the lighter the gas is, the faster it will travel.

**Liquids**

The liquid phase is unique in that the molecules are closely packed and yet are able to flow. They will typically have much higher densities than that of gases, but not as dense as solids. The study of the motion of liquids is called fluid dynamics.

At the surface of a liquid, there is an exchange of molecules between the gas phase and the liquid phase. This is called //vapor pressure//. The vapor pressure of a liquid is dependent on the temperature and the concentration of the vaporized liquid above the surface.



Liquids also have the ability to flow. This flow can be driven by factors such as gravity, pressure, or dilution. The //viscosity// of a liquid is a measure of its ability to flow, or its resistance to flow. The "thicker" a liquid is, the greater its viscosity.

//Surface tension// is another property of fluids. It is a measure of the resistance of a liquid's surface to an external force. Simply put, it means the molecules will "stick together" and resist being separated when an object enters the liquid.



Surface tension is the property that causes water to form droplets, and to bead up on a waxy surface. It allows a paper clip to float (even though the metal paper clip is more dense than water), and a water strider to "swim" on the surface of a pond.




 * Solids**

Molecules or atoms that make up a solid are typically held together with strong intermolecular forces or metallic bonds. These particles are closely packed and have high densities relative to liquids and gases.

There are two categories of solids. If the arrangement of particles is random and disorganized, the solid is //amorphous//. If it is structured and patterned, it is a //crystalline// solid.



The three phases of matter are dependent on temperature and pressure, and can be visualized in a phase diagram.