The student will:
- describe the molecular arrangement of solids and liquids.
- explain the incompressibility of solids and liquids.
- explain the low rate of diffusion in solids and liquids.
- explain the ability of solids to maintain their shape and volume.
- explain the ability of liquids to maintain their volume and inability to maintain their shape.
Liquids and solids differ from gases in that the particles (atoms, molecules, or ions) are much closer together, so the total volume of a liquid or solid is much closer to the sum of the volumes of the particles. The volume of a gas, as you may recall from the chapter on “The Behavior of Gases,” is related to the volume of the spaces between the particles and not to the volume of the particles.
At all temperatures above absolute zero, atoms and molecules are in constant random motion. The particles travel in a straight line unless they collide with another particle. In the absence of any attractive forces, this molecular motion would cause all substances to be in gaseous form. The fact that solid and liquid states exist tells us that there are forces that hold molecules and atoms together even when they are not chemically bonded. The forces of attraction that hold atoms and molecules together in solid and liquid phases are called intermolecular forces of attraction. These forces are different from chemical bonds, which are called intramolecular forces.
The phase (solid, liquid, or gas) of a substance is the result of a competition between the molecular motion that pushes the molecules apart and the attractive forces that pull them together. If the molecular motion is much greater than the attractive forces, the substance will be gaseous. If the molecular motion is nearly the same in strength as the attractive forces, the substance will be liquid, and if the molecular motion is much less than the attractive forces, the substance will be solid. When the molecular motion is increased or decreased by changing the temperature, the relationship between the molecular motion and the attractive forces changes, and the substance may change its phase.
The Structure of Solids and Liquids
The diagram below illustrates the molecular arrangements in the three phases of matter.
In the gaseous phase, the molecular motion dominates. The molecules of the substance are completely separated and move about independently of each other. The spaces between the molecules are very large compared to the size of the particles, so the measured volume of a gas is actually a measurement of the spaces between the molecules. In comparison, the molecular structure of the liquid phase has some spaces between the particles that allow the particles to move past one another, but the attraction between the particles is strong enough to prevent them from moving very far apart. In the solid phase, the forces of attraction have completely overcome molecular motion, and the movement of the particles has been reduced to vibrating in place. The particles cannot move past one another and are held in a tightly-packed pattern, so there is very little space between the particles.
For a brief animation showing the differences in molecular motion and relative position for gases, liquids, and solids (2d), see http://www.youtube.com/watch?v=s-KvoVzukHo (0:52).
For a video demonstration of a physical model of molecular motion and states of matter (2d), see http://www.youtube.com/watch?v=ynUso6rJ0rE (3:02).
Properties of Solids
The intermolecular forces of attraction in solids hold the particles so tightly in place that they cannot pull away from each other to expand their volume, nor can they flow past one another to change shape. Therefore, solids hold their own shape and volume regardless of their container. There is very little empty space in the solid structure, so solids are virtually incompressible. Since molecules cannot pass each other in the structure, diffusion or mixing is essentially non-existent beyond the surface layer.
Properties of Liquids
Attractive forces between molecules are a major factor in the behavior of the liquids. Since the particles in a liquid remain in touch with each other, liquids maintain their volume, but since the particles can flow past each other, liquids take the shape of their container. 100 mL of liquid will be 100 mL in any container, but because the liquid molecules are not held in a tightly-packed pattern like solids are, the molecules can move past one another, allowing the liquid to fit the shape of the container.
In gases, the distance between the molecules is so great that the size of the molecules themselves become inconsequential. A gas, then, is considered to be essentially empty space. If we consider the volumes of 6.02×1023 molecules of oxygen gas, O2, and 6.02×1023 molecules of Freon gas, CF4, at STP, we know that the volumes will be the same, 22.4 liters. This is because the sizes of the molecules themselves are negligible compared to the empty space in a gas. The fact that the Freon molecules are several times larger than the oxygen molecules makes no difference. In a gas, you are measuring the volume of the empty space. If we consider the volume of 6.02×1023 molecules of liquid oxygen and 6.02×1023 molecules of liquid Freon both at STP, we find the volume of the oxygen is about 28 mL and the volume of the Freon is about 55 mL. In the case of liquids, the volumes of the molecules themselves make a difference. In liquids, the volume of a group of molecules is related to the volume of the individual molecules, so equal moles of liquids do not occupy equal volumes under the same conditions. Since liquids have many more molecules in a smaller volume, they will have much greater densities than gases.
When a substance is compressed, it is not the molecules themselves that are compressed; it is the space between the molecules that is compressed. Gases have a great deal of empty space and are easily compressed. A pressure of 3.0 atm compresses a gas to one-third its volume at 1.0 atm. Liquids have very little space between molecules and do not compress easily – a pressure of 3.0 atm will have virtually no effect on the volume of a liquid. Liquids are used in hydraulic systems because of their ability to flow to fit their container and their incompressibility.
Diffusion in gases (mixing) is nearly instantaneous. If you release a colored gas in a container of non-colored gas, the color spreads evenly throughout the container in a second or two. In liquids, diffusion is a much slower process. The dye molecules require much more time to move from one side of a container to the other due to the smaller spaces between molecules and the almost constant collisions with other molecules.
- The molecular arrangement in solids is a highly organized, tightly-packed pattern with small spaces and molecular motion reduced to vibration in place.
- Molecules in a solid maintain both their own shape and their own volume.
- Solids are virtually incompressible and have little diffusion beyond the surface layer.
- Molecules in the liquid phase have some freedom of movement but their motion is much more restricted than that of gases.
- Liquids maintain their volume but take the shape of their container.
- Liquids are only slightly compressible.
- Diffusion in liquids occurs more slowly than in gases.
Further Reading / Supplemental Links
An interactive animated video showing the motion and arrangement of molecules in the three states of matter.
This video shows demonstrations that demonstrate the states of matter.
- The molar volumes of solid silicon and solid bromine under the same conditions are different and the molar volumes of liquid silicon and liquid bromine under the same conditions are different, but the molar volumes of gaseous silicon and gaseous bromine under the same conditions are exactly the same. Explain.