# Gas

One **gas** it is one aeriform characterized from one critical temperature inferior to the ambient temperature; the aeriform ones for which that it does not happen find in the state of vapor.

In practical, a gas can also be defined like aeriform a not condensabile one to ambient temperature.

Moreover, by extension, all the aeriform ones that find to a advanced temperature that critic come sayings gas: an example is given from water vapor, characterized from one advanced critical temperature to that atmosphere (374 °C), but that it comes however defined like "water gas" when comes carried to exceed this temperature.

The gas, like all the aeriform ones, it represents state of the matter in which the interatomic and intermolecolari forces between single atoms or molecules of a substance are therefore small that these can ramble free in the space. For this a gas does not have a defined volume but it stretches to occupy all the space to its disposition, and it assumes the shape of the container that contains it, filling up it completely.

The word "gas" probably was coined from a chemical one fiammingo like transcription of its pronounces of the Greek word? (chaos).

In the running language one says however that one given substance "is a gas" when its temperature of boiling it is a lot under the ambient temperature, that is when it is found normally to the state of gas on the Earth. As an example he is normal to say that "methane is a gas while the iron it is not", although methane can very well be found to the liquid state (cooled under 161 °C) and the iron to the gaseous state (heated beyond the 2750 °C).

## The perfect gases

In physics and in thermodynamics the approximation is used generally dictates of i *perfect gases*: the gas that is comes considered constituted from punctiform atoms, that attraction forces of or repulsione between they moves free from and the walls of the container: this approximation leads to formulate the famous law like equation of state of perfect gases, that it describes, in conditions of thermodynamic equilibrium, the relation between pressure, volume and temperature of the gas:

<math>P \cdot V = n \cdot R \cdot T</math>

where P is pressure, V volume occupied from the gas, n the number of wharves of the gas and R universal constant of perfect gases. As an example, a perfect gas size occupies 22,4 liters to temperature of 0ºC and pressure of 1 atmosphere.

From this law from there two come down then others:

For one sure mass of gas to constant temperature, the product of the volume of gas V for its P pressure is constant.

<math> \left(P \cdot V \right)_T = K</math>

That is for one sure mass of gas to constant temperature, the pressures are inversely proporziona them to the volumes. The geometric figure that has for equation the expression is one equilateral hyperbola. The law of Boyle is a law limit is worth that is with good approximation but not in absolute way for all gases. A perfect gas or ideal gas that follows the law of Boyle perfectly does not exist. The shunting lines from the behavior of real gases are small much for a gas that finds to low pressure and one far temperature from that one of liquefaction. The transformation isoterma is therefore one variation of the volume and the pressure maintaining constant the temperature.

A perfect gas that to the temperature of 0°C occupies a V° volume and that it comes heated maintaining costs of it the pressure occupies to the temperature t a Vt volume expressed from the law

<math>V_P = V_0 \cdot \left(1 + \alpha_0 \cdot T \right)</math>

in which <math>V_0</math> it is the volume ocupato from the gas to 0°C and <math> \alpha_0</math> he is equal to 1/273,15. The temperature is expressed in degrees Celsius. The transformation isobar is one variation of the volume and the temperature to constant pressure. In a diagram pressure-volume parallel to the axis of the volumes is represented from a segment. Therefore the volume variation that endures a gas for the variation of temperature of every centigrade degree piles to 1/273 of the volume that the gas occupies to 0 centigrade ones.

The relation that elapses between pressure-volume and that one between temperature and volume, it allows to gain the relation between the pressure of a gas and the temperature when works to constant volume. A perfect gas that to the temperature of 0°C has a pressure p° and that it comes scaldato maintaining the volume constant finds,to the temperature t,to one expressed pressure pt from the law: pt=p°(1+at) the transformation isocora is a variation of the pressure and the temperature that happens maintaining the volume constant.

Beyond to the above-mentioned laws, for perfect gases it is worth also law of Avogadro: to equal conditions of temperature and pressure, if two gases occupy the same volume then they have the same molecule number.

## The real gases

The real gases are instead not made of punctiform molecules but every molecule occupies a determined volume, not null small but (therefore is not comprimibili indefinitely but they pass to the liquid or solid state if compressed beyond a sure limit) and without forces of interaction between they, and they are not expanded infinitely but they arrive to a point in which they cannot occupy more volume (this because between atoms much small is settled down a force, due to the chance variation of the electrostatic charges in single molecules, call force of Van der Waals). For this the law of perfect gases does not supply turns out to you takes care of to you in the real gas case, above all in conditions of low temperature or high pressure, while it becomes preciser in case than rarefied gases or to high temperature, when interatomic forces and molecular volume become negligible.

Therefore, in the case of real gases the equation of state of perfect gases must be modified: the volume to disposition of the gas will be (V - b), that is the volume occupied total less that one from its molecules, and the pressure instead will be corrected of a factor toV^{2} that it holds account of the forces of attraction between atoms.

Therefore the equation of state of real gases, it dictates also **equation of Van der Waals** it is:

<math> \left(P + ñ2 \cdot \frac{a}{V^2} \right) \cdot \left(V - n \cdot B \right) = n \cdot R \cdot T</math>

## Correlated voices

**Combustible gases**- Natural gas
- Biogas

- states of the matter
- solid
- liquid
- aeriform
- fluid
- vapor
- Kinetic theory of gases

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