First, molecular physics studies the structure of molecules and substances in general, its weight and size, and the interaction of its components - microscopic particles (atoms).This topic includes the study of the relative molecular mass (the weight ratio of a single molecule / atom of substance to a constant value - the mass of one carbon atom);the concept of amount of substance, and the molar mass;expansion / contraction of materials by heating / cooling;the velocity of the molecules (molecular-kinetic theory).The molecular-kinetic theory based on the study of individual molecules of matter.A behavior in the subject material at different temperatures is considered a very interesting phenomenon - many known that the substance expands when heated (the distance
between molecules is increased), and contracts when cooled (distance between molecules is reduced).But what is interesting - the transition of water from liquid state to a solid phase (ice) water expands.This is ensured by the polar structure of molecules and the hydrogen bonds between them, still so obscure to modern science.
Also, molecular physics, there is the concept of an "ideal gas" - a substance that is in gaseous form, and has certain properties.An ideal gas is very flat, ieits molecules do not interact.In addition, the ideal gas obeys the laws of mechanics, while the real-world gases do not have such properties.
From section of molecular physics, a new direction - thermodynamics.This section examines the physics of the structure of matter and the influence of an external factor such as pressure, volume and temperature, without considering the microscopic picture of a substance, and considering the communication therein in general.If you read books on physics, you can come across a special plots of these three variables in relation to the state of matter - they depict isochoric (volume unchanged), isobaric (pressure consistently) and isothermal (temperature is unchanged) processes.The thermodynamics also includes the concept of thermodynamic equilibrium - when all three of these values ​​are constant.Very interesting issue that affects the thermodynamics - why, for example, water at a temperature of 0 ° C can be located both in liquid and in solid state.