Matter: Changes of State

Understanding Changes of State

Changes of state are physical transformations that occur when a substance transitions from one state of matter to another. These include melting, freezing, evaporation, condensation, sublimation, and deposition.
These changes are caused by the transfer of energy into or out of the substance, altering the internal energy of the system.
During a change of state, the substance maintains its own chemical composition, meaning that the substance itself does not change, only the state in which it exists.

An increase in the internal energy of a substance typically comes from heat energy being transferred to the substance, causing a change of state
- Melting refers to the transition from solid to liquid
- Evaporation and boiling refer to the transition from liquid to gas
When the internal energy of a substance decreases, heat energy is transferred from the substance, leading to a different change of state
- Freezing refers to the transition from liquid to solid
- Condensation refers to the transition from gas to liquid
In certain conditions, it’s possible for substances to change directly from solid to gas (sublimation) or from gas to solid (deposition) without passing through the liquid phase.

The temperature of a substance plays a critical role in changes of state. It represents the average kinetic energy of the particles, which relates to how fast they’re moving.
The boiling point and melting point of a substance are specific temperatures at which these changes of state occur.
For most substances, increasing the temperature will result in a change of state from solid to liquid to gas, while decreasing the temperature will result in a change from gas to liquid to solid.
However, during the exact moment of a change of state, the temperature of the substance remains constant despite the continuous input or removal of heat energy. This is because this energy is used to weaken or strengthen the forces between particles, not to change their speed.

Pressure also affects the state of matter and can cause a substance to change state.
For example, applying pressure to a gas can force the particles closer together, causing it to become a liquid.
Conversely, reducing the pressure on a substance can cause it to change from a liquid to a gas.
A substance’s boiling point will lower if the external pressure is decreased, and will rise if the pressure is increased.

The particle model of matter can help explain these changes of state, as it describes how particles move and interact.
Particles in a solid are closely packed together and vibrate in fixed positions, but don’t have enough energy to break free from their positions.
When energy is added (such as by heating), the particles vibrate more, may break free from their positions, becoming particles in a liquid. They are still close together but can move past each other, causing the substance to flow.
If more energy is added, the particles gain enough energy to break away from each other, becoming particles in a gas. These particles are spaced far apart and move rapidly in all directions.