Understanding Changes of State: Melting, Boiling, and Sublimation

Laws of Melting

When a pure substance melts, it exhibits specific behaviors:

1. It melts at a specific temperature, called the melting point.
2. While melting, the temperature remains constant, even with the coexistence of solid and liquid phases.
3. All pure liquids, when sufficiently cooled, solidify at the same temperature at which they melt.
4. During solidification, the temperature remains constant.

Melting Point and Solidification

The melting and solidification points of a pure substance are characteristic properties that can be used to identify it.

A substance whose temperature varies during a state change cannot be considered a pure substance.

Vaporization

Vaporization is the change of state from liquid to vapor (or gas). It can occur in two ways: boiling and evaporation.

Laws of Boiling

1. A liquid boils at a specific temperature, which depends on the pressure. For example, the boiling temperature of water is 100°C at 1013 hPa.
2. While boiling occurs throughout the entire mass of the liquid, the temperature remains constant.

Boiling

Boiling is the transition from liquid to vapor that occurs at a specific temperature (the boiling point) in a disorderly or noisy manner throughout the entire liquid. When the pressure is higher, the boiling temperature increases. If the pressure decreases, the boiling temperature is lower.

Latent Heat

Latent heat is the energy required to melt one kilogram of a substance that is already at its melting temperature. For example, the latent heat of fusion for ice is 334.4 J/g, and for water, it is 2060 J/g. The formula is Q = m * L, where Q is heat, m is mass, and L is latent heat.

Latent Heat of Boiling

The latent heat of boiling is the heat required to boil one kilogram of a substance that is already at its boiling temperature.

Evaporation

Evaporation is the slow transition from liquid to vapor that occurs when the temperature is lower than the boiling temperature. It occurs only on the free surface of the liquid, which is the surface in contact with the atmosphere.

Factors Affecting Evaporation Rate

1. Type of liquid: Liquids that evaporate very quickly are considered volatile.
2. Surface area: The larger the free surface area, the faster the liquid evaporates.
3. Temperature: The higher the temperature, the faster the evaporation.
4. Ventilation: The more wind there is, the faster the evaporation.

Kinetic Theory Interpretation of Evaporation

On the free surface, particles with a high enough speed can overcome the attractive forces of the other molecules and evaporate. As the temperature increases, the number of particles with sufficient speed increases, and evaporation is faster. When the free surface area is larger, there are more molecules in contact with the air. Wind carries away evaporated particles, preventing them from returning to the liquid, thus making evaporation faster. Evaporation absorbs energy because the faster molecules leave, and the liquid cools, absorbing energy from the environment.

Sublimation

Sublimation is the direct transition from solid to gas. The direct passage from gas to solid is called deposition or crystallization. Some substances that sublime include dry ice, camphor, and naphthalene.