Change of State - An Introduction

Greetings,

In addition to chemical changes, matter changes state.  During a state or phase change matter only assumes a different form: The chemical composition does not change.  However, there is always a heat energy flow associated with a change of state.  Depending on the phase change, a weakening or strengthening of intermolecular forces occurs.  In the case of ionic compounds, the actual overall ionic bond strength changes.

Molecular Level Considerations

At the molecular level, different states exhibit different degrees of  atomic/ionic/molecular motion.  An increasing state of molecular motion results from an increase of average particle-kinetic-energy.  Increasing kinetic energy results from an increased temperature, due to direct heating or an increase of surrounding pressure.  Decreasing temperature has the opposite effect.

States and Particle Motion

Starting with a solid, a dramatic increase of molecular motion occurs in going from the solid state to the liquid and to the gas state.  Accompanying increasing molecular motion is a dramatic decrease in intermolecular forces.  Consequently, the different states of matter have different structures at the molecular level.

The following diagram shows all six changes of state and their labels.  The increasing color-intensity of the arrows indicates increasing energy changes.

 

A future post will discuss details of energy and temperature changes occurring during changes of state.

That's all for now!

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