Changes of State and Potential & Kinetic Energy

Substances change between solid, liquid and gas states.  When a substance changes its state, potential and kinetic energy interact.  What does this mean?


Kinetic energy is the easiest to understand. It is the energy of motion of any moving object, from a molecule to a planet.  Energy is defined in physics as “the ability to do work.” Work is defined as moving an object.  A moving object obviously has the ability to move another object just by bumping into it.  Since all molecules above Absolute Zero (0°K = -273° C or -459°F) are moving, they all have some kinetic energy. The total kinetic energy of molecules is commonly known as heat.  The average kinetic energy of the molecules is the temperature.  Heat and temperature are not the same thing.  The temperature does not depend on the amount, but the heat does.  A cup of hot water may be the same temperature as a bucket of water, but the bucket has more heat and more energy.

You can change the temperature of something by bringing it into contact with something that is hotter.  An example is putting a pan on a hot stove.  The faster moving molecules of the stove collide with the cooler ones in the pan.  This speeds up the pan molecules (the pan gets warmer), which in turn heats up whatever is in the pan. Of course, the faster molecules lose speed themselves in the process.

Potential energy seems a little more complicated, perhaps because you don't necessarily see anything happening.  Remember, energy is the ability to move something.  Potential energy is the energy possessed by something because of a force acting on it.  The force can be of any kind -- magnetic, electric, gravitational, or nuclear.  A rock at the top of a hill has potential energy because the force of gravity acts on it.   In looking at states of matter we will be considering the intermolecular bonds.  These bonds have potential energy because intermolecular forces are acting on the molecules.


What does this have to do with what you’re studying?!

All molecules are moving in a substance with a temperature above -273°C; therefore, they have kinetic energy.  As a liquid slowly cools to form a solid, the temperature slowly drops.  In other words, the molecules are moving more slowly and they have less kinetic energy.  At the freezing point, some of the kinetic energy the molecules have is converted into potential energy.  This potential energy takes the form of a bond that is created between molecules once they settle into the lattice pattern of a solid.  This bond is the same as the force of attraction between Molly and Marvin molecule in the Eureka video – it is the invisible “spring” between them.  As more and more molecules settle into the latticework when the whole substance freezes, more and more kinetic energy is converted into potential energy.  However, because the total amount of energy remains the same, the temperature of the substance remains the same throughout the freezing process.  This is why a graph showing time vs. temperature will have a plateau (or a flat part) in the graph at the freezing point temperature.  Once the substance is completely frozen, it continues to lose heat if there is still something cooling it down.  In the freezing point lab, the room temperature (~20°C) is far below the PDCB freezing point temperature (~52°C), so the PDCB continues to cool as a solid long after it has frozen.