Phases of Matter

As we discussed in the early lessons of this course, there are three phases of matter: solid, liquid, and gas. The general behavior of these phases should be familiar to you from day-to-day life. Every day you breathe air (gas phase), drink water or other drinks (liquid phase) and walk around on roads and sidewalks (solid phase). You don't need me to tell you much about the human-scale differences between the phases, or that sometimes substances can change phase: water melts and freezes, evaporates and condenses, etc.

However, we should talk a little bit about what the phase behavior of matter looks like at the scale of atoms and molecules. In a solid, liquid, or gas, what are the atoms and molecules doing? What is it that makes one substance (like silver) solid at room temperature while another (water) is a liquid and a third (oxygen) is a gas?

In solid matter, the atoms/molecules/ions making up the substance are packed together very close to each other. In addition, they generally have a very regular, repeating pattern to their structure.

Because they are so tightly and regularly packed together, atoms cannot move past or around each other. They do still move somewhat - by vibrating back and forth in place at high speeds over short distances. But relative motion is impossible. This is why solids hold their shape and size; the atoms are locked into position, thus they cannot flow or deform easily.

The atoms/molecules/ions in liquid matter are also fairly densely packed. This is reflected in the fact that ice is about as dense as water. However, the particles do not have as rigid and regular a structure as they do in solids.

This means particles, while they can't move apart from each other, can move around each other. This relative motion of particles allows liquids to flow and deform into whatever shape their container has. Note: some liquids do not flow as easily ... this is because molecules get "snagged" on each other, known as viscosity.

In gases, particles are not packed densely together, in fact they do not touch at all except for occasional collisions. Mostly, they are totally independent of each other, flying through space until they collide with some barrier, bounce off, and go flying in another direction.

As a result, gases have no fixed shape or size. A balloon filled with gas can be deformed, squished, or expanded. When you squish a balloon, the molecules inside keep flying through space and bouncing off the balloon walls, mostly unchanged.

What Controls Matter's Phase?

As you probably know, the way to get a substance to move from solid to liquid to gas phase is to heat it. This should track with what we've just discussed. Heating increases the movement of molecules, causing them to separate from each other and move more independently.

However, not all substances melt or vaporize at the same temperature. If I heat water to 100 °C it will boil; however, a block of salt heated to the same temperature stays happily solid and will remain so up to hundreds or thousands of degrees. What is different about water and salt that makes salt so much harder to melt?

Hopefully, you already have an inkling of the answer based on the previous lesson. As you have learned, salt (NaCl) is an ionic compound that adopts a network structure, with ions tightly bound to one another. However, water forms molecules. While internally the bonds in these molecules are very strong (H₂O doesn't easily fall apart into H and O atoms), it is quite easy to separate individual molecules from each other.

In this lesson, we will expand on this idea. Our goal is to develop an understanding of how the composition and bonding of substances determines their basic physical properties: their melting and boiling points, and how they mix with other substances. To get an idea of where we are going, consider this question: water (a liquid at room temperature) is a molecular substance, but so are table sugar (a solid at room temperature) and methane (a gas at room temperature). What differences between these compounds can explain this difference in properties?

Obviously, right now, you cannot answer this question. But by the end of the lesson you will be able to. In order to get there, we need to start by learning about the three-dimensional shapes of molecules.

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