#phase change

the dress is neither blue and black nor white and gold

it is brittany

there is only brittany (◕‿◕✿)

We’ve spent some time going over the basics of heat enginesandrefrigerators. These are machines which manipulate a working fluid through some sort of cycle in order to move heat from one location to another. We have some idea of the overall way they work, but to really understand the details of what’s going on, we’re going to have to know a little more about how this working fluid behaves.

For right now, we’ll restrict our discussion to materials which qualify as pure substances. A pure substance has the same chemical composition throughout - all its molecules are the same. We’re all familiar with phases of matter - solid, liquid, and gas are the ones we see on a daily basis. Matter in solid form has its molecules arranged in a regularly structured lattice. In a liquid, molecules have about the same distance from each other as they do in a solid, but they’re not held in a structure and can move around each other freely, although their close spacing means that intermolecular forces play a large part in their motion. In a gas, molecules are widely spaced and careen about as they please, interacting with each other only through collisions.

For thermodynamics-related purposes, we’re really interested most in liquids and gases. Controlling the transition between these two states is the key to moving a lot of energy around. So let’s define some terms.

Acompressed liquidorsubcooled liquid is one that is in no danger of vaporizing.

Asaturated liquid is one that is about to vaporize.

Asaturated vapor is one that is about to condense.

Asuperheated vapor is one that is not in any danger of condensing.

Distinguishing between these is important from a thermodynamics because a fluid will have different properties related to its ability to absorb and release energy in each state. In coming articles, we’ll look a little more closely at the exact process of transition between phases.