Matter exists in different physical forms known as states. The state of a substance is determined by the collective behavior and energy level of its constituent particles. A phase change, or phase transition, is a physical process where matter moves from one state to another. This transformation is driven by the transfer of thermal energy, causing a shift in how its particles are organized.
The Three Fundamental States of Matter
The fundamental states of matter are solid, liquid, and gas, and each is defined by how its particles are arranged and interact. In a solid, particles are tightly packed together and held in fixed positions by strong intermolecular forces. This dense, fixed arrangement gives solids a definite shape and a definite volume, with particles only able to vibrate slightly around their mean position.
When a solid gains energy, it transitions into a liquid. In a liquid, the particles remain close but have enough energy to move and slide past one another. Liquids retain a definite volume but no fixed shape, conforming to the shape of the container they occupy. They are considered fluids due to the mobility of their particles.
A further increase in energy leads to the gaseous state, where particles are highly energetic and separated by large distances. Gases have neither a definite shape nor a definite volume. The forces of attraction between gas particles are negligible, allowing for free and rapid movement.
The Six Primary Phase Transitions
The six primary phase transitions describe the movement between these three states and occur in three opposing pairs.
The first pair involves the transition between solid and liquid. Melting is the process where a solid absorbs energy and changes into a liquid, such as an ice cube turning into water. The reverse process is Freezing, where a liquid loses energy and becomes a solid, exemplified by liquid water turning into ice.
The second pair describes changes between liquid and gas. Vaporization is the transformation from a liquid to a gas, which can occur through boiling or slower evaporation, like a pot of water turning to steam. The opposite is Condensation, where a gas releases energy and converts back into a liquid, which is how water vapor forms dew or fog.
The final two transitions bypass the liquid state entirely. Sublimation is the direct change from a solid to a gas, a phenomenon commonly seen when dry ice turns directly into a visible gas. The opposite is Deposition, where a gas changes directly into a solid, such as when water vapor forms frost on a window pane.
The Role of Energy in Causing Change
All phase changes are driven by the transfer of thermal energy, which affects the kinetic energy, or motion, of a substance’s particles. When thermal energy is added, the kinetic energy of the particles increases, causing them to move faster and further apart, leading to a transition to a more energetic state like a liquid or a gas. Conversely, removing thermal energy decreases particle movement, allowing intermolecular forces to pull them into a more ordered, less energetic state, like a solid.
The energy involved in these transformations is known as latent heat, which is absorbed or released without causing a change in temperature. For example, when ice melts, the added heat energy is not immediately increasing the water’s temperature. Instead, this latent heat of fusion is used solely to break the bonds holding the water molecules in their rigid solid structure.
Only once all the intermolecular bonds are broken and the entire substance has changed phase does the continued addition of heat begin to increase the temperature of the resulting liquid. This constant-temperature period demonstrates that phase changes require significant energy to overcome or establish the forces between molecules. This energy mechanism explains why processes like vaporization require a large energy input, known as the latent heat of vaporization, to fully separate the liquid molecules into a gas.