The student will:
- describe the differences in molecular arrangement among solids, liquids, and gases.
- describe the basic characteristic differences among solids, liquids, and gases.
The kinetic molecular theory allows us to explain the existence of the three phases of matter. In addition, it helps explain the physical characteristics of each phase and why phases change from one to another. The kinetic molecular theory is essential for the explanations of gas pressure, compressibility, diffusion, and mixing. Our explanations for reaction rates and equilibrium in later chapters also rest on the concepts of the kinetic molecular theory.
The Assumptions of the Kinetic Molecular Theory
According to the kinetic molecular theory, all matter is composed of tiny particles that are in constant, random, straight-line motion. This motion is constantly interrupted by collisions between between the particles and surfaces, as well as collisions between the particles themselves. The rate of motion for the particles is related to the temperature. The velocity of the particles is greater at higher temperatures and lower at lower temperatures.
In our discussions of gases, we will be referring to what are called ideal gases. In an ideal gas, we assume that the molecules are points that do not take up any space. We also assume that there are no attractions between molecules. In real gases, however, the gas molecules do take up a small amount of space, and there can be slight attractions among the molecules. Later in this chapter, real and ideal gases will be discussed in more detail.
Phases of Matter
Earlier in the chapter “Matter and Energy,” we had discussed how matter can be categorized as mixtures or pure substances. We did not, however, discuss how substances can be found in different phases. Depending on the temperature and pressure, matter can exist in different forms or states known as phases. You are most likely familiar with the phases of dihydrogen oxide (H2O), known commonly as water. Besides existing as a liquid that we can drink, water can also exist as ice, which is a solid. When we boil water, we are producing water vapor, which is a form of gas.
In a solid, the molecules are held in a tightly packed pattern, as seen in the figure above. As a result, the molecules hold a set position in spite of random motion. Molecular motion is reduced to vibrating in place. In comparison, the molecules in a liquid touch each other but are not held in a pattern. The liquid structure has holes in it, which allow molecules to pass each other and change positions in the structure. In a gaseous substance, the molecules are completely separated from each other and move around independently. Most of the volume of a gas is empty space. Scientists also recognize one more phase of matter called plasma. A plasma is a type of ionized gas with unique properties that distinguishes it as a fourth phase of matter. In this text, however, we will be primarily concerned with the solid, liquid, and gas phases.
Characteristics of Solids
The molecular arrangements in the three phases account for the various characteristics that differentiate the phases. For example, the mixing of particles is almost non-existent in solids. This is because the molecules cannot pass by one another in the tightly packed pattern. Solids are essentially incompressible because when a substance is compressed, it is the spaces between molecules that are compressed, not the molecules themselves. Since solids have almost no empty space in their structure, they do not compress. Solids maintain their shape and volume, as seen in the figure below. A 25 mL rectangular piece of copper has the same shape and volume when it is resting on the table top as it does inside a beaker.
Characteristics of Liquids
In liquids, mixing occurs more readily because there are spaces between the molecules that allow the molecules to pass each other. The spaces between the molecules in liquids are small, so liquids have very little compressibility. Liquids maintain their own volume, but they take the shape of their containers, as seen in the illustration below.
A 25 mL sample of liquid in a graduated cylinder has a volume of 25 mL and the shape of a cylinder. If the 25 mL sample is placed in a beaker, the liquid still has a volume of 25 mL, but now it has the shape of the beaker. The structure of the liquid keeps the particles in touch with each other so that the volume does not change, but because the particles can slide by each other, the particles can flow to fill the shape of the container.
Characteristics of Gases
Mixing in gases is almost instantaneous because there are no inhibitions for particles to pass one another. The volume of a gas is nearly all empty space, so particles are able to move freely. Gases are highly compressible because of the great amount of empty space, which allows the particles to be pushed closer together. Gases do not have either their own volume or their own shape. They take both volume and shape from their container.
- In the solid phase, the molecules are held in a highly organized, tightly packed pattern.
- Due to the tightly packed pattern of molecules in a solid, solids maintain their own shape and volume and do not mix readily.
- In the liquid phase, molecules are in touch with each other but they are loosely packed and may move past each other easily.
- Due to the loosely packed structure of a liquid, liquids maintain their own volume but take the shape of their container. They are also able to mix readily.
- In the gaseous phase, molecules are completely separate from each other.
- The volume of a gas is mostly empty space.
- Due to the structure of gases, they take both the volume and the shape of their container, and they mix almost instantaneously.
Further Reading / Supplemental Links
The learner.org website allows users to view the Annenberg series of chemistry videos. You are required to register before you can watch the videos, but there is no charge to register. The video called “A Matter of State” examines matter in its three principle states - gases, liquids, and solids - and relates the visible world to the submicroscopic.
This website provides more details about the kinetic molecular theory.
- Automobile brakes have a hose full of liquid connecting your brake pedal on one end to the brake pads on the other end. When you press on the brake pedal, the force is transferred through the liquid and presses the brake pads against the wheels to slow or stop them. Brakes that use liquid in this fashion are called hydraulic brakes and the liquid is called hydraulic brake fluid. Why don't they use air in the brake lines instead of liquid?
- Why would it cause a problem if some air got into your liquid-filled brake lines?
- If you had a 250 mL container full of helium gas, and you transferred all of the gas into an empty 1.00 liter container, would the 1.00 liter container be full or only 1/4 full?