A change of state is the transformation of matter from one physical form—solid, liquid, or gas—into another.
Defining Changes of State
According to the provided reference, a change of state is the process of turning from a solid, liquid, or gas into another one of those states. This fundamental physical process involves matter transitioning between its distinct phases. Unlike a chemical change where the substance itself is altered, a change of state is a physical change; the chemical composition of the material remains the same. For example, when ice melts into water, it is still H₂O, just in a different state.
How Energy Influences State Changes
The driving force behind a change of state is energy. As highlighted in the reference, materials will change state depending on how much energy they receive or lose.
- Receiving Energy: Adding energy (usually in the form of heat) can cause a substance to move from a lower energy state (like solid) to a higher energy state (liquid or gas).
- Losing Energy: Removing energy (cooling) can cause a substance to move from a higher energy state (like gas) to a lower energy state (liquid or solid).
The temperature at which these changes occur at a specific pressure are known as melting points, boiling points, and freezing points.
Common Examples of Changes of State
We encounter changes of state frequently in everyday life. Here are some familiar examples:
- Melting: Solid to Liquid (e.g., ice melting into water). Occurs when a solid gains enough energy.
- Freezing: Liquid to Solid (e.g., water freezing into ice). Occurs when a liquid loses enough energy.
- Boiling/Evaporation: Liquid to Gas (e.g., water boiling into steam, puddles drying up). Occurs when a liquid gains enough energy.
- Condensation: Gas to Liquid (e.g., water droplets forming on a cold glass, clouds forming). Occurs when a gas loses enough energy.
- Sublimation: Solid to Gas (e.g., dry ice turning directly into carbon dioxide gas). Occurs when a solid gains enough energy to bypass the liquid state.
- Deposition: Gas to Solid (e.g., frost forming on a cold surface). Occurs when a gas loses enough energy to bypass the liquid state.
Understanding the Process
Each change of state involves overcoming or succumbing to the intermolecular forces holding the substance together in a particular state. Adding energy allows molecules to move more freely, breaking the rigid structure of a solid or the close contact of a liquid. Removing energy causes molecules to slow down, allowing intermolecular forces to pull them closer together into a more ordered state.
