B.1.16 – Temperature & Phase Change

Defined Words: temperature, gas, liquid, solid, plasma


Everything is moving all the time! We’ve created a measurement called temperature to describe the overall movement energy of the particles in a substance. All atoms and molecules are constantly in motion, even in their solid forms. Only when atoms reach “absolute zero” (-273.15 °C) does the movement stop entirely. Humans still haven’t created an absolute zero environment, but we do already have the record for the coldest spot in the known universe. Humans break all sorts of records!

The more energy a substance has, the faster the molecules of the substance can move. The light or chemical energy being used to heat a substance will be converted into kinetic energy to increase the speed of the particles. The overall measure of all of the kinetic energy of all the particles gives us our measurement of temperature.

Now let’s go into the structure of an atom: in the center there is a nucleus with all the hadrons (the protons and neutrons) and moving randomly around the nucleus are the electrons. Heisenberg’s Uncertainty Principle tells us that it is physically impossible to know both the speed and position of an electron at a single moment in time. While this is an undisputed fact of the universe, it doesn’t prevent us from using probability to make a few predictions!

We can predict that at some point, all of the electrons will be on only one side of the atom leaving the entire other side exposed. A partial negative charge will exist on the populated side and, with no electrons cancelling it, the protons allow for a partial positive charge on the vacant side. This is called an “instantaneous dipole” and it can occur when there is a large majority of electrons on one side.

The instantaneous dipole will induce a dipole on neighboring atoms. The negative charge of one atom will attract the positive charge of the other atom. The attraction between these atoms is a spontaneous process, meaning it will happen on its own.

Let’s combine these ideas. Electrons are always moving around causing induced dipoles. Partially charged molecules will stick together automatically when they are nearby. A change in temperature is a change in the speed of the molecules’ movement. The speed of molecules directly relates to the amount of time spent nearby other molecules.

We’ll first look at a substance that has no attachment between its molecules. The molecules are moving very quickly in random directions. If those molecules slow down, they can get close enough to stick together briefly but the attraction isn’t strong enough to hold the fast molecules for long.

As we slow down our substance, the molecules begin to form temporary attachments. They latch on, let go, and then latch on to other nearby molecules. If the molecules slow down even more, the attachments become more permanent. The substance will align to a specific shape maintained by the interlocking nature of its molecules. The first scenario described a gas, the second described a liquid, and the last scenario described a solid.

This is why phases of matter exist! Matter will automatically attract itself due to induced dipoles caused by the random movements of electrons. The temperature of a substance determines whether its molecules have enough energy to break free of those automatic attractions. Phases can also be caused by a change in pressure. Higher pressure pushes molecules closer together to cause more chance of induced dipole attachment. Lower pressure allows molecules to diffuse away from their attractions in order to reach equilibrium. The low concentration of molecules in the low pressure environment pulls them away from the induced dipoles.

A fourth phase of matter exists that we call plasma; it is when a gas is given enough energy for the electrons to hop between atoms of the substance. The transferring of electrons also causes attractions between the molecules. Plasma, despite being superheated, takes on a more liquid-like state. Matter within stars, lightning and fluorescent light bulbs are in a plasma state.

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