# Chemistry

Learning physics and chemistry easily and freely - Science for elementary school, middle school and high school

Free online chemistry lesson for elementary school, middle school and high school.

## Pressure

1) What is pressure ?

A gas pushes upon all surfaces with which it is in contact: it is said it exerts a pressure.

2) What is atmospheric pressure ?

The atmospheric pressure corresponds to pressure exerted by air from the atmosphere.

A simple experiment demonstrates the existence of atmospheric pressure:
A glass is completely filled with water. A sheet of paper is pressed against the glass and then the glass is returned.
Water remains in the glass and doesn't flow.
Despite its weight the water is kept inside the glass because the outside air pressure is greater.

3) Pressure units

The legal unit of pressure is Pascal (Pa)
Is also used:
- The hectopascal (hPa) = 100 Pa 1 hPa
- The bar (bar) 1 bar = 100000 Pa

4) How to measure pressure ?

The atmospheric pressure is measured with a barometer.
At sea level atmospheric pressure is about 101300 Pa or 1013 hPa or about 1 bar but it may vary and leads to high pressures (anticyclone) corresponding to areas of good weather or it can leads to low pressures (depression) corresponding to areas of bad weather.

When a gas is enclosed its pressure is measured with a manometer.

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