Phase transitions

Phase transitions

A phase transition is the change of a substance from one state of matter to another.

Chemistry

Keywords

change of state, melting, freezing, boiling, evaporation, condensation, sublimation, crystallisation, liquid, solid, gas, state of matter, physical property, temperature, pressure, transformation, exothermic, endothermic, diffusion, thermodynamics, chemistry, physical, _javasolt

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States of matter

  • solid
  • liquid
  • gas

Most substances can occur in nature in more than one state of matter. The three fundamental states of matter are the solid, liquid and gas phases. Scientists have discovered other states of matter as well which occur under extreme conditions.

Certain substances, such as water, can be easily observed in all three fundamental states of matter, while other substances, such as helium, are known in one state of matter under regular conditions, but change to another state under special conditions, such as very low temperatures or very high pressures.

The most important difference between the various states of matter is that, because of the different temperature or pressure, particles are attracted to one another to a different extent.

Solid

Particles move in solid substances as well, but usually slower than in substances of other states of matter. The force of attraction between the particles is very strong, therefore bonds form between them and they are held in a fixed position in which they vibrate.

Solids have a definite volume and shape.

There are two types of solids: crystalline or amorphous. In crystalline solids, particles form a regular repeating pattern. Examples include ice, diamond and graphite. Amorphous solids lack a regular arrangement of particles. Examples of amorphous solids are wax, bitumen, glass and most plastics.

Crystalline solids have an exact melting point, while amorphous solids do not, they melt gradually.

There is no sharp distinction between crystalline and amorphous solids: crystalline blocks may occur in amorphous materials. Most crystalline solids are not monocrystalline (consisting of a single crystal) but polycrystalline, that is, a large number of single crystals held together by thin layers of amorphous solid.

Liquid

In liquids, the particles move faster than in solids, but slower than in gases.
The attraction between the particles is weaker than in solids, so they are not in a fixed position, they can freely move within the substance but remain close to one another.

Liquids have a definite volume but no definite shape, so they take the shape of the container. Some liquids change their shape easily while others resist deformation a bit more. This property is called viscosity. Simply put, viscosity refers to the friction between the particles of the liquid. Although we commonly say that a more viscous liquid is thicker than another liquid, viscosity has nothing to do with density. For example, although olive oil is more viscous than water, it has a lower density.

There is no sharp difference between liquids of very high viscosity and amorphous solids: glass, for example, can be considered both an amorphous solid and a liquid of extremely high viscosity.

Gas

In gaseous substances, the particles move so fast and are so far apart that the influence of the attractive forces between them is much less effective.
They practically fly free and collide with each other and the wall of the container. The net force of the particles hitting the container wall is detected as gas pressure. That is, gas pressure is not only due to the mass, but also to the rapid movement of the particles.

Gases do not have a definite volume or shape, they always fill the available space and take the shape of the container.

Vapour is a substance in the gas phase, that is, above the boiling point but at a temperature lower than its critical temperature. Vapour can still be condensed to a liquid, but gases with a temperature above the critical temperature cannot.

Steam is water in the gas phase, formed when water boils or evaporates. Steam is invisible. In everyday language, however, the word 'steam' often refers to the white mist forming above boiling water, which is, in fact, the cloud of visible water droplets that form when steam condenses.

Phase transitions

  • solid
  • liquid
  • gas
  • melting
  • freezing
  • boiling / evaporation
  • condensation
  • deposition
  • sublimation

During phase transitions, chemical changes do not occur, that is, the particles do not change, only their location and speed do.

The state of matter of a particular substance is determined by the temperature and the pressure.
A phase transition can be induced by changing the temperature or by changing the pressure. For example, ice melts when pressure increases, while water boils when pressure is decreased.

The temperature at which melting takes place is called the melting point, and the temperature at which boiling occurs is called the boiling point. Both heavily depend on pressure.

For most substances, both the melting point and the boiling point can be precisely defined - in relation to pressure of course -, but for some amorphous solids the transition between two states of matter is continuous, that is, they do not have a well-defined melting point.

A direct transition can occur between the solid and gas state in the case of almost all substances, that is, solid substances may become gaseous without going through the liquid phase, and vice versa.

The change during which a solid becomes gaseous directly is called sublimation. An example of this process is the fuming of dry ice. The reverse process is deposition, an example for it is the formation of frost crystals from water vapour on the windows in winter.

There are two types of transition from liquid to gaseous state: evaporation and boiling. The difference between the two processes is that evaporation occurs only on the surface of the liquid and it occurs at any temperature. However, during boiling, vapour bubbles form within the liquid and rise to the surface, the occurrence of which is only possible at a certain temperature - the boiling point, when the vapour pressure in the bubbles is high enough to compensate for atmospheric pressure.

Processes

  • melting
  • freezing
  • boiling / evaporation
  • condensation
  • deposition
  • sublimation
  • heat transfer
  • heat reduction

Narration

Most substances can occur in nature in more than one state of matter. The three fundamental states of matter are the solid, liquid and gas phases. Scientists have discovered other states of matter as well which occur under extreme conditions.

The most important difference between the various states of matter is that, because of the different temperature or pressure, particles are attracted to one another to a different extent.

Particles move in solid substances as well, but usually slower than in substances of other states of matter. The force of attraction between the particles is very strong, therefore bonds form between them and they are held in a fixed position in which they vibrate. Solids have a definite volume and shape.

In liquids, the particles move faster than in solids, but slower than in gases.
The attraction between the particles is weaker than in solids, so they are not in a fixed position, they can freely move within the substance but remain close to one another. Liquids have a definite volume but no definite shape, so they take the shape of the container.

In gaseous substances, the particles move so fast and are so far apart that the influence of the attractive forces between them is much less effective.
They practically fly free and collide with each other and the wall of the container. The net force of the particles hitting the container wall is detected as gas pressure. That is, gas pressure is not only due to the mass, but also to the rapid movement of the particles. Gases do not have a definite volume or shape, they always fill the available space and take the shape of the container.

During phase transitions, chemical changes do not occur, that is, the particles do not change, only their location and speed do. The state of matter of a particular substance is determined by the temperature and the pressure.
A phase transition can be induced by changing the temperature or by changing the pressure.

A direct transition can occur between the solid and gas state in the case of almost all substances, that is, solid substances may become gaseous without going through the liquid phase, and vice versa. The change during which a solid becomes gaseous directly is called sublimation. An example of this process is the fuming of dry ice. The reverse process is deposition, an example for it is the formation of frost crystals from water vapour on the windows in winter.

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