# Kinetic Molecular Theory of Matter

## The Kinetic Molecular Theory of Matter

The kinetic molecular theory of matter explains how matter can change among the phases of solid, liquid, and gas.

### Learning Objectives

Describe the kinetic molecular theory of matter.

### Key Takeaways

#### Key Points

• All particles have energy, and the energy varies depending on the temperature the sample of matter is in, which determines if the substance is a solid, liquid, or gas. Solid particles have the least amount of energy, and gas particles have the greatest amount of energy.
• The temperature of a substance is a measure of the average kinetic energy of the particles. A change in phase may occur when the energy of the particles is changed.
• There are spaces between particles of matter. The average amount of empty space between molecules gets progressively larger as a sample of matter moves from the solid to the liquid and gas phases.

#### Key Terms

• kinetic molecular theory: Theory of treating samples of matter as a large number of small particles (atoms or molecules), all of which are in constant, random motion
• kinetic: Of or relating to motion.
• lattice: A regular spacing or arrangement of atoms/molecules within a crystal.
• phase: A component in a material system that is distinguished by chemical composition and/or physical state. Matter can exist in the solid, liquid, and gaseous phases.

### The Kinetic Theory: A Microscopic Description of Matter

The kinetic molecular theory of matter offers a description of the microscopic properties of atoms (or molecules) and their interactions, leading to observable macroscopic properties (such as pressure, volume, temperature). An application of the theory is that it helps to explain why matter exists in different phases (solid, liquid, and gas) and how matter can change from one phase to the next.

The three phases of matter: Notice that the spacing between atoms or molecules increases as we move from a description of the solid phase to the gaseous one.

The kinetic molecular theory of matter states that:

• Matter is made up of particles that are constantly moving.
• All particles have energy, but the energy varies depending on the temperature the sample of matter is in. This in turn determines whether the substance exists in the solid, liquid, or gaseous state. Molecules in the solid phase have the least amount of energy, while gas particles have the greatest amount of energy.
• The temperature of a substance is a measure of the average kinetic energy of the particles.
• A change in phase may occur when the energy of the particles is changed.
• There are spaces between particles of matter. The average amount of empty space between molecules gets progressively larger as a sample of matter moves from the solid to the liquid and gas phases.
• There are attractive forces between atoms/molecules, and these become stronger as the particles move closer together. These attractive forces are called intermolecular forces.

Interactive: Intermolecular Attractions and States of Matter: Explore how states of matter are related to the strength of intermolecular attractions.

### Example: Water

Let's take water as an example. We find that in its solid phase (ice), the water molecules have very little energy and cannot move away from each other. The molecules are held closely together in a regular pattern called a lattice. If the ice is heated, the energy of the molecules increases. This means that some of the water molecules are able to overcome the intermolecular forces that are holding them close together, and the molecules move further apart, forming liquid water. This is why liquid water is able to flow: the molecules have greater freedom to move than they had in the solid lattice. If the molecules are heated further, the liquid water will become water vapor, which is a gas. Gas particles have more energy and are on average at distances from each other which are much larger than the size of the atoms/molecules themselves. The attractive forces between the particles are very weak given the large distances between them.

Changes in phase: A change in phase may occur when the energy of the particles is changed.

### Diffusion

The kinetic theory of matter is also illustrated by the process of diffusion. Diffusion is the movement of particles from a high concentration to a low concentration. It can be seen as a spreading-out of particles resulting in their even distribution. Placing a drop of food coloring in water provides a visual representation of this process – the color slowly spreads out through the water. If matter were not made of particles, then we would simply see a clump of color, since there would be no smaller units that could move about and mix in with the water.

Interactive: Diffusion of a Drop: Click in the model to add a drop of dye. Watch how the molecules move through the water. Trace an individual molecule to see how it moves through the liquid.