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A chemistry independent study of the gas laws

General Chemistry/Gas Laws

Friday, June 6th, 1999 Gas Laws Since the days of Aristotle, all substances have been classified into one of three physical states. A substance having a fixed volume and shape is a solid. A substance, which has a fixed volume but not a fixed shape, is a liquid; liquids assume the shape of their container but do not necessarily fill it. A substance having neither a fixed shape nor a fixed volume is a gas; gases assume both the shape and the volume of their container.

The structures of gases, and their behavior, are simpler than the structures and behavior of the two condensed phases, the solids and the liquids Pressure and the Law of Boyle Quantitative measurements on gases were first made in a rational manner by the English chemist Robert Boyle 1627 - 1691. The instruments used by Boyle to measure pressure were two: A manometer is simply a bent piece of tubing, preferably glass with one end closed.

  • A substance, which has a fixed volume but not a fixed shape, is a liquid; liquids assume the shape of their container but do not necessarily fill it;
  • The barometer is a device for measuring the total pressure of the atmosphere.

When the liquid level in both arms is the same, the pressure of the sample of gas inside the closed end must equal the pressure of the external atmosphere since the downward force on the two columns of liquid is then equal. When the liquid levels are unequal, the pressures must differ. The difference in pressure can be measured in units of length of the vertical column of liquid.

A chemistry independent study of the gas laws

The mm Hg, or its modern version the torr, originated in this use of the manometer. Mercury is particularly convenient for use in manometers and barometers because at room temperature it has low vapor pressure, does not wet glass, and has a high density. Other liquids such as linseed oil or water have also been used in manometers. The barometer is a device for measuring the total pressure of the atmosphere.

Combined gas law

A primitive barometer can easily be constructed by taking a glass tube about a meter long, sealing one end, filling the tube completely with mercury, placing your thumb firmly over the open end, and carefully inverting the tube into an open dish filled with mercury.

The mercury will fall to a height independent of the diameter of the tube and a vacuum will be created above it. The height of the mercury column will be the height which the atmospheric pressure can support. The standard atmospheric pressure, one atmosphere atmis 760 mm Hg but the actual atmospheric pressure varies depending upon altitude and local weather conditions. For this reason barometers can be used to help predict the weather.

A falling barometer indicates the arrival of a low-pressure air system, which often means stormy weather.

  • In calculations for a gas above a liquid, the vapor pressure of the liquid must be considered;
  • A rising barometer indicates the arrival of a high pressure air system, and that often means clear weather;
  • A eudiometer is a device that measures the downward displacement of a gas;
  • When the liquid level in both arms is the same, the pressure of the sample of gas inside the closed end must equal the pressure of the external atmosphere since the downward force on the two columns of liquid is then equal.

A rising barometer indicates the arrival of a high pressure air system, and that often means clear weather. While mercury is again the most convenient liquid for use in barometers it is by no means the only liquid which can be used.

A chemistry independent study of the gas laws

Preparation of a water barometer and many of the early barometers did use water. With the manometer and barometer used together, the actual pressure of a sample of gas can be measured. Combining the barometer reading of atmospheric pressure with the manometer reading of pressure difference gives the actual pressure.

McQuarrie and Rock, Page 161 Units of Pressure Units of pressure were originally all based on the length of the column of liquid, usually mercury, supported in a manometer or barometer.

  • While mercury is again the most convenient liquid for use in barometers it is by no means the only liquid which can be used;
  • A rising barometer indicates the arrival of a high pressure air system, and that often means clear weather;
  • Ap chemistry a allan chapter 5 - gases 52 the gas laws of boyle, charles is independent of the type of gas molecule;
  • When the liquid level in both arms is the same, the pressure of the sample of gas inside the closed end must equal the pressure of the external atmosphere since the downward force on the two columns of liquid is then equal;
  • We begin by examining Boyle's Law in more detail;
  • The height of the mercury column will be the height which the atmospheric pressure can support.

By far the most common of these units was the mm Hg, although inches of mercury were also used in English-speaking countries. However, the modern SI unit of pressure is derived from the fundamental units of the SI.