Introduction: Pure Substance can Exist Mainly in Three Different PhasesObserving GasesPressureThe Gas LawsGas DensityDiffussion and EffusionKinetic Theory of GasesThe Kinetic EquationConcept of Temperature from Kinetic TheoryMore about Pressure of a Gas from Kinetic EquationGas Laws from Kinetic TheoryVelocity of MoleculesCharacteristic Features of Speed Distribution Law for Three DimensionsReal GasCauses of DeviationExplanation of PV-P Curve, (Amagat’s Curve) by Van der Waals Equation: Van der Waals Equation under different ConditionsAmagat Curve of a Gas at Various Temperatures and Concept of Boyle TemperatureBoyle Temperature from Van der Waals EquationExperimental P-V Isotherm of a Real GasContinuity of StateDerivation of Critical ConstantsAlternative Derivation of Critical ConstantsReduced Equation of State from Van der Waals EquationCompressibility FactorCollision Number (Zi), Collision Frequency (Zii) And Mean Free Path (λ)More About Effusion