The gas theory works on the following assumptions:

• Particles are constantly moving in straight lines
• There is elastic collision - no loss of energy - when particles collide
• The particles' volume can be ignored
• Particles don't attract one another (because they are so far away from each other)

The ideal gas law can be explained with the equation:

Pv = nRT

Where:

• P is pressure
• V is volume
• n is the number of particles
• R is a number known as the gas constant (The value for R is often, but not always, 8.314 joule kelvin^-1 mole^-1)
• T is Temprature

Pressure is caused by the collision of molecules against the walls of the container. These collisions can be thought of as a force exerted upon an area of the wall. We can express this using the equation: 

P = F / A

Where P stands for pressure, F stands for force and A stands for area. Force is measured in units called newtons (N) and area is measured in square metres (m²). The unit of pressure then can be stated as N/m², which is called the pascal (Pa) after the seventeenth-century philosopher and scientist Blaise Pascal. In the imperial system, pressure is given in pounds per square inch (lb/in² or psi). This unit is still used in pressure gauges that measure the air pressure in car tires.

pressure is measured using a barometer. If the pressure becomes stronger than the container is able to withstand, there will be an explosion. 

The amount of space a gas takes up is its volume. Volume is measured in units called capacity units. The most common examples of capacity units are millilitres (mL) and litres (L). Very small volumes can be measured in microlitres (μL) while very large volumes can be measured in kilolitres (kL).

The number of gas moleculesis measured in moles. The number of molecules in 1 mole is known as Avogadro’s Number, and is 6.02x10²³.

Temprature is measured in celsius, fehrenheit, or Kelvin. We are all familiar with the Celsius scale, which was named after Anders Celsius, a Swedish astronomer and physicist. We use this scale in day-to-day life for measuring the temperature outside, the temperature of swimming pool water, and our own body’s temperature. Both the Celsius scale (used in the metric system) and the Fahrenheit scale (used in the imperial system) are based on water because originally people were interested in knowing when water changes state (e.g., its freezing point and boiling point). These scales work well for many liquids and solids, but it turns out that neither of the scales works well for describing the behaviour of gases.

So why do the Celsius and Fahrenheit scales not work well for describing gases? If you remember from above, kinetic energy is related to temperature. A substance at a temperature of 0˚C does not mean it has zero kinetic energy. As we will see below, the particles of a substance at 0˚C still has a considerable amount of kinetic energy. 

What is needed for gases is a temperature scale in which zero means the particles are not moving at all (i.e., have zero kinetic energy). 

A temperature scale was created specifically for this purpose. It is called the absolute temperature scale or the Kelvin scale, named after its creator, Lord Kelvin of England. Absolute zero, or 0 K, is the lowest temperature. (Note that
there is no degree sign when using the Kelvin scale). Thus, the freezing point of water can be written as 0˚C or 273 K. To convert ˚C to K add 273 to the number in ˚C (e.g., 3 ˚C would be 3 + 273 = 276 K).

All of the energy of a gas is in the form of kinetic energy. Any change in kinetic energy is accompanied by a change in temperature.

Boyle's law = If we hold nRT constant, and if we increase pressure P, we will decrease volume v proportionately, and vise versa

Charles's law = heating gas causes it to expand, and vise versa. Heat makes molecules move faster, and faster molecules hit container boundaries with more force causing boundaries to expand.

In order to make a hot air balloon rise, heat is added to the air inside the balloon. This is because heat causes molecules in balloon to move apart, which makes balloon less and less dense; eventually the balloon's density decreases so much it becomes lighter than air resulting in the rising of the balloon.

Gay-lussac's law = if you keep the volume of a gas constant (such as in a closed container), and you apply heat, the pressure of the gas will increase.

Avragadro's law = if the amount of gas increases and the pressure stays the same, then its volume will increase too.