Gas molecules are made up of atoms that have split into separate particles, called electrons and nuclei. The nuclei remain intact, but the atoms have split into these two components.
When gas molecules collide with one another or with other particles, their kinetic energy (motion) changes. When this happens, they can stick to each other or apart from each other.
You can test this yourself: Put a glass of water out on a humid day and watch what happens. You will see that the water evaporates and particles stick to each other, forming clouds. This is because of the interaction of water molecules with air molecules.
This experiment shows how important it is to control the sample size when doing physics experiments with gases. If you use too much water, you may not get any clouds due to all the sticking between molecules.
Molecules with larger mass have higher temperature
Temperature is one of the most important properties of any gas, and for many gases, it is the only property that matters. At any given temperature, a gas will have a particular volume and pressure, which are determined by its molecules.
As we noted above, molecules of different masses have different kinetic energies. Because of this, molecules of different masses will have different temperatures at which they vibrate the most.
Smaller molecules tend to have higher average kinetic energies due to their smaller mass. Therefore, a sample of gas that contains molecules of different masses will have a distribution of temperatures at which these molecules vibrate.
This fact is very important when analyzing gases for applications like air conditioning or air purification. In order to effectively reduce all contaminants in the gas, one must first determine the average temperature at which all particles in the sample vibrate and then adjust settings to address that temperature.
A gas is composed of small particles
A gas is the phase of matter characterized by atoms and molecules that are free to move. This is in contrast to solids, where atoms and molecules are bound to one another, and liquids, where they are free to move independently.
Gases consist of particles called atoms and molecules. Atoms are the building blocks of matter, which consist of protons, neutrons, and electrons.
Atoms can be an element or a compound. An element is a atom with a specific number of protons in its nucleus. A compound atom is composed of two or more different elements that bond together.
Molecules are made up of two or more atoms that bond together. Gas particles can be different sizes due to the difference in size between atoms and molecules.
The distance between particles is extremely small
The distance between particles in a gas is extremely small. To model this situation, scientists use a set of numbers called the quantum theory.
In this model, the space between particles is considered empty. Particles only interact by collision, and even then, it is only when they collide at the edge of this void that they interact.
This is why heat does not conduct through a gas; there is no medium for the heat to travel through! The same goes for why a gas does not conduct electricity; there is no path for the electrons to travel along.
Because of the way gases behave, it is very difficult to determine the average mass of a particle in a gas. Even if you could determine this number, it would change constantly as particles enter and exit the volume of the gas.
The mass of the molecules determines their velocity
As molecules move through a gas, their motion is determined by interactions with nearby molecules. The speed of a molecule is also called its velocity.
The speed of a molecule is determined by the mass of the molecule and the temperature of the gas. At a given temperature, heavier molecules will have a higher speed.
At lower temperatures, molecules move more slowly and are said to be slower. At higher temperatures, molecules move faster and are said to be faster.
This is because, at higher temperatures, the kinetic energy of the molecule increases, making it move faster. The opposite is true at lower temperatures; kinetic energy decreases, making molecules slow down.
By testing the speed of different mass molecules at a given temperature, you can determine their average velocity.
The temperature of a gas is determined by its molecular motion
Along with the average velocity of the molecules, the temperature of a gas is determined by the average kinetic energy of the molecules. The hotter a gas is, the more kinetic energy its molecules have.
The temperature of a given sample of gas is independent of its composition, whether it consists of oxygen and nitrogen or neon and helium. This is true because temperature is a property of a mass distribution and not just one specific substance.
Heating a pure noble gas such as neon will not change the temperature of the gas much. This is because noble gases have very little molecular motion so they take a lot of heat to get moving!
Raising the pressure of a given volume of gas increases the molecular motion and thus the temperature. For example, if you double the pressure in a given volume of gas, you will double the molecular motion and thus double the temperature.
Greater the number of molecules, greater is the pressure
A gas containing molecules of different masses exerts a pressure on the walls of its container. The greater the number of molecules in the container, the greater will be the pressure on the walls.
This fact is related to statistical mechanics and can be proved with it. The proof involves calculating average kinetic and potential energies of all molecules in the container, taking into account their velocities and directions, and comparing this energy with that of empty space—zero-point energy.
You can prove this fact for any number of molecules in a closed system—a system isolated from external influences. For example, a bottle containing a certain number of water molecules will have a constant internal pressure if there are no outside influences acting on it (such as external atmospheric pressure).
This fact is related to statistical physics and was first established by Russian scientist Ivan Pavlovych Demyanov (1850–1902).
Heavier molecules travel slower than lighter ones
In a gas, molecules travel at various speeds depending on the temperature of the gas. As molecules in a gas warm up, they gain kinetic energy, making them faster. If a gas is cooled, the molecules lose kinetic energy, making them slower.
The speed of a molecule is determined by its temperature, not its mass. Heavier molecules have more internal friction due to their higher mass, which means they take longer to accelerate or decelerate. This fact makes it seem like heavier molecules travel faster, but only because they are already moving faster internally.
When placed in the same amount of space and time, lighter molecules will travel faster than heavier ones. The relationship between mass and speed is dependent on the properties of a molecule and does not change with size. Smaller molecules just have more of them.
Gas behaves as an ideal gas under normal conditions
Another important concept related to averages is the ideal gas law, which describes a relationship between pressure, volume, temperature, and number of moles of gas.
The ideal gas law was first formulated by Jacques-Louis Moreau in 1834 and then later by John Dalton. The formula for the ideal gas law is P ∝ nRT, where P is pressure, n is number of moles, T is temperature, and R is a universal constant. This shows that as any of these four variables change, the other two change in proportionate ways.
Ideal gases assume that molecules of the gas are non-interacting; that is, each molecule behaves as if it were in a vacuum. Because of this assumption, the number of molecules in a given volume of gas is constant no matter what kind or how many molecules are present.
