... and the Particle Theory

Substances contain energy. They can have more or less energy, depending upon how warm they are. But if too much energy is added to a substance ... more than it can contain ... the substance has to change its form. Let's have a look at a substance close up, where we can see its molecules, and we'll explain.

The molecules in a solid are locked in place by invisible forces. These particles are not free to move around ... all they can do is vibrate. The more energy they have, the more they vibrate.

If you apply more and more heat to a solid, the particles vibrate faster and faster. Eventually the particles will have enough energy to break free of the forces holding them in line ....

When that happens, the solid turns to a liquid! The particles are still held by forces that keep them grouped together, but now they are free to vibrate and move around  within the substance. As you apply more heat energy, the particles move around faster.

As a liquid, the substance can hold a lot more heat. The more heat you add, the faster the particles move around. That's why liquids have a higher specific heat capacity than solids.

Eventually, however, you will have made the liquid very hot, and those liquid particles will be moving around really fast! Every once in a while, a very energetic particle will get enough energy to escape completely from the liquid ...

... and particle by particle, the liquid molecules will escape from the liquid completely. This process occurs when you boil a liquid ... the liquid evaporates. This means that the particles escape, and are no longer part of the liquid substance ... they are free to wander around on their own. Particles that can do this are called a gas.

Substances in gaseous form are made of particles that are free to move wherever they want, as fast as they want. The more heat you apply, the faster they move ... there's no holding them back! Gases have the highest specific heat capacity of any substance. You can pump as much heat into a gas as you want, and it will store that heat as the kinetic energy (movement) of its particles.

Is that the end? Well, no. If you continue to heat a gas, and keep it contained while you heat it, eventually the molecules will start to break apart. At temperatures in the tens of thousands of degrees, even the atoms will eventually come apart, and the hot gas becomes a sea of electrons and nuclei. Then the substance is called a plasma. But that's for another page!

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