Matter, the physical substance of the universe, exists in different forms known as phases, and the distinction between these phases is fundamental to how we understand physics and chemistry. Whether a substance appears as a solid, liquid, or gas depends entirely on the behavior of its constituent particles. This behavior is directly controlled by the amount of energy contained within the substance. Adding energy causes a transformation, resulting in a shift from one phase to another.
Defining Kinetic Energy and Temperature
The energy responsible for these phase shifts is primarily kinetic energy, which is simply the energy an object possesses due to its motion. On a molecular scale, every atom and molecule is in constant motion, even in seemingly stationary objects. The faster a particle moves, the greater its kinetic energy. The movement of these microscopic particles is what we measure as temperature.
Specifically, temperature is the measure of the average kinetic energy of the particles within a substance. When a substance is heated, its particles absorb the energy, causing them to move faster, which raises the average kinetic energy and, consequently, the temperature. This direct relationship means that a higher temperature is always an indication of faster-moving particles and higher average kinetic energy.
Particle Motion in Solid, Liquid, and Gas
The three most familiar phases of matter—solid, liquid, and gas—demonstrate a clear progression of kinetic energy levels.
Solids
In a solid, particles are tightly packed and held in fixed positions by strong attractive forces. Their movement is restricted to vibration in place, giving a solid the lowest amount of kinetic energy among the common phases. Adding energy to a solid causes this vibrational motion to increase.
Liquids
When enough energy is absorbed, the particles gain sufficient kinetic energy to partly overcome the forces binding them together, resulting in a liquid. In the liquid phase, particles remain close but can now slide and tumble past one another, allowing the substance to flow and take the shape of its container. This freedom of movement means a liquid has a greater average kinetic energy than a solid.
Gases
The highest kinetic energy level among these three common phases is found in a gas. Particles in the gaseous state have absorbed so much energy that they have completely overcome the attractive forces between them. They move rapidly and randomly in straight lines, covering large distances between collisions. This free, high-speed motion allows gases to expand and fill any volume they occupy, making the gas phase the one with the highest average kinetic energy at typical temperatures.
Identifying and Explaining the Phase with the Most Kinetic Energy
While the gas phase exhibits the highest kinetic energy of the three common phases, there is a fourth state of matter that possesses a vastly greater energy level: plasma. Plasma is an ionized gas, meaning it is formed when a gas is subjected to such extreme temperatures or energy fields that its particles gain enough kinetic energy to fundamentally change their structure. This intense energy causes electrons to be stripped away from their atoms, a process called ionization.
The resulting plasma is not a neutral gas but a chaotic mixture of positively charged ions and free, negatively charged electrons. The energy required to break the electromagnetic bonds holding electrons to their atoms far exceeds the energy needed merely to move atoms faster, as happens in the transition from liquid to gas. This extreme energy input makes plasma the phase of matter with the most kinetic energy. Plasma is the most common state of matter in the visible universe, constituting over 99 percent of all ordinary matter. Naturally occurring examples include the sun and all other stars, along with phenomena like lightning on Earth.