what is diff. between gas and vapour

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valentin68's profile pic

valentin68 | College Teacher | (Level 3) Associate Educator

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There is a fundamental difference between the state of gas and the state of vapor: substances as vapors can be liquefied, whereas substances as gases can not be liquefied. Explanation follows.

A real gas at high temperatures and low pressures behaves exactly like an ideal gas. It means that the equation `PV =constant` is respected no matter how high the pressure is, given a sufficient high temperature. When the temperature is lowered, initially at low pressures the equation `PV=constant` will hold, but by further increasing pressure, above low will fail to describe the behaviour of the substance. Liquid drops will begin to form in the substance until all the gas will be transformed to liquid by increasing the pressure.

In this state it is said that the liquid is in equilibrium with its vapors. Thus the name of vapor state.

In a diagram P-T (not P-V) of the states of matter there will be four different phases: solid, liquid, vapor (which will meet all in a point named the triple point of the substance) and gas.The gas and vapour phase will meet also in the so called critical point. See the figure attached.

Now you have the answer: if the substance is at a temperature higher than the critical temperature it will be gaseous (can not be liquefied by increasing pressure at whatsoever value), and will be vapor below the critical temperature (when by increasing the pressure enough it will become all liquid).

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amneet's profile pic

amneet | Student, Grade 10 | (Level 1) Honors

Posted on

The substance which is solid under ordinary condition,but on heating becomes in form of gaseous state is called vapour.example-water etc but the subtance which is in gas state under ordinary condition is called gas.example-carbon,hydrogen,oxygen etc.

imaankasana's profile pic

imaankasana | (Level 1) Honors

Posted on

A gas is a single well-defined thermodynamic phase, wheras a vapor is a mixture of two phases (generally gas and liquid). A cloud is made of water vapor, the clear sky contains gaseous water. Steam is vaporized water.

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