Matter commonly exists in four states: solid, liquid, gas, and plasma. Transitions between these states are physical changes, meaning the chemical identity of the substance remains the same. These transformations are governed by external forces that fundamentally alter the way the particles within the material interact.
The Underlying Mechanism: Particle Movement
Every substance is composed of tiny particles that are constantly in motion, a phenomenon described by the kinetic-molecular theory. The state of any given substance is determined by the balance between the attractive forces pulling its particles together and the energy driving them apart. In a solid, the particles are locked into relatively fixed positions by strong intermolecular forces, allowing them only to vibrate slightly.
As energy is added, the particles gain kinetic energy, causing them to move more vigorously. When kinetic energy is high enough to partially overcome the attractive forces, the ordered structure of the solid breaks down, resulting in the liquid state. Introducing more energy allows particles to completely break free from attractive forces, leading to the widely separated, chaotic movement characteristic of the gas state. The strength of the intermolecular forces varies between substances, which is why different materials require different amounts of energy to change states.
The Primary Driver: Thermal Energy and Temperature
Temperature is the most common external force driving state changes, as it is a direct measure of the average kinetic energy of a substance’s particles. Adding thermal energy is known as an endothermic process, and it causes the transitions from solid to liquid, liquid to gas, and solid directly to gas. The specific temperature at which a solid becomes a liquid is its melting point, while the temperature at which a liquid turns into a gas is its boiling point.
The reverse transitions—liquid to solid (freezing), gas to liquid (condensation), and gas directly to solid (deposition)—occur when thermal energy is removed in an exothermic process. For example, when liquid water is cooled, its particles lose kinetic energy until the intermolecular forces lock them into the solid structure of ice. The amount of energy absorbed or released during these transitions without a corresponding change in temperature is known as latent heat.
This “hidden” energy, such as the latent heat of fusion for melting, is used entirely to break the attractive bonds between particles rather than increasing their average speed. Consequently, a mixture of ice and water will remain at \(0^\circ\text{C}\) until all the ice has melted, even as heat is continuously supplied. Similarly, the latent heat of vaporization is the energy required to fully separate liquid particles into the gaseous state during boiling. The total energy required for a phase change depends on both the mass of the substance and the specific latent heat coefficient, which is a material property.
The Secondary Driver: Pressure
Pressure acts as an independent force capable of inducing a change in state, particularly for gases and liquids. Increasing external pressure on a gas forces its particles into closer proximity, which amplifies the influence of the attractive intermolecular forces. This compression can cause the gas to condense into a liquid, a process frequently used in industry to liquefy gases like propane or carbon dioxide.
Conversely, decreasing the pressure above a liquid makes it easier for the particles to escape into the gaseous phase. This is because the boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure pushing down on it. For instance, at high altitudes where atmospheric pressure is lower, water boils at a temperature below its standard \(100^\circ\text{C}\).
Increased pressure typically favors the denser state. For most substances, this means high pressure encourages the formation of a solid. The balance between temperature and pressure is mapped on a phase diagram, which shows the conditions under which a substance exists in a specific state.
Beyond Gas: Creating Plasma
The fourth state of matter, plasma, requires an energy input significantly greater than that needed to create a gas. Plasma is essentially an ionized gas, a highly energetic state where electrons have been stripped away from their atoms. This ionization creates a mixture of free-moving electrons and positively charged ions.
Achieving this state often involves temperatures exceeding \(10,000^\circ\text{C}\) or the application of a strong electromagnetic field. At these extreme energy levels, vigorous collisions between particles knock electrons out of their orbits. Naturally occurring examples include the material that makes up stars and the intense energy release of lightning. Plasma is artificially generated for technological applications such as neon signs and fluorescent lights.