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Gases

 Frogs breathe differently than humans do ! Frogs breathe differently than humans do ! According to Boyle’s law when volume increases pressure decreases. A gas moves from high pressure region to low pressure region. Contraction of diaphragm in humans expands the lungs and hence the pressure in lungs becomes lower than atmospheric pressure, therefore the air rushes into the lungs. Interestingly frogs push the air into their lungs themselves. Frog takes a mouthful of air and then contract its mouth cavity. As the volume goes down, the pressure goes up (Boyle’s Law), so the air in the frog’s mouth is pushed into the lungs.

Learning Objectives

After completing the topic, the student will be able to:

  • Explain how gases differ in their macroscopic properties from liquids and solids.
  • Define the pressure of a gas, give and interconvert different units of pressure.
  • Explain the laws governing behaviour of ideal gases and apply those laws to real world situations.
  • Derive the ideal gas equation and use this equation to calculate the thermodynamic properties such as pressure, volume and temperature etc.
  • Calculate the partial pressure of each gas in a mixture using Dalton’s law.
  • Discuss the kinetic molecular theory of gases.
  • Define the effusion and diffusion phenomena and examine how their rates are related to molar mass.
  • Understand why the gases deviate from ideal behaviour and use the van der Waals equation to correct these deviations.
 Gas balloons Gas balloons Balloon carnivals which provide a great deal of entertainment are the result of inflation of balloons with the hot air. Balloonists control their crafts with the knowledge of properties of gases.
Gases

We breathe air. We feel a breeze blowing over our body. We feel the temperature of air. But with all this familiarity with air around us, we get very little idea about what is air, or why it behaves the way it does.

For solids, their behavior can be studied using laws of motion laid down by Newton. For liquids, theories could be developed to explain the static and dynamic situations on the basis of these laws. But for gases there was a problem. Newton’s laws could not be applied to thermodynamic variables like pressure, volume and temperature. Behavior of gases could be studied with thermodynamics.

Scientists later were convinced that Newton’s laws were applicable to all matter and even to gases. Although there was enough evidence to indicate that air consists of molecules with a lot of empty space between them, the problem was how to apply Newton’s laws to these molecules.

The difficulty in applying Newton’s laws of motion to the very large number of molecules was in arriving at the behavior of a system, when motion of each of the very large number of constituent particles is known. By averaging the values over the large number of molecules, relations connecting variables such as pressure and temperature of a gas to the speeds of its molecules could be obtained. Questions such as why a gas occupies the volume of its container or why an ideal gas obeys the gas law could be answered on the basis of the theory of gases.

The theory of gases, which tries to explain the gas laws and properties of gases on the basis of the assumption that molecules of gases are constantly in random motion, is called the kinetic theory of gases.

 Effect of temperature Effect of temperature When liquid nitrogen is poured on the balloon the gas in the balloon is cooled and its volume decreases. When a gas at a constant pressure is subjected to heat, its volume increases.
Behavior of gases

Gases behave differently when compared to liquids and solids. This may be due to their low weight which is due to large volume occupancy, less intermolecular forces etc., Let us discuss how a gas behaves when it is subjected to the external changes.

Relation with pressure: When a sample of gas is confined to a container which has a variable volume (such as balloon, piston) an external force applied will compress the gas resulting in lower volume. By removing the external pressure the container retains its original shape leading to the increase in volume of gas. This kind of variation of volume will be less appreciated in case of solids and liquids.

Relation with temperature: When a gas at a constant pressure is subjected to heat, its volume increases; when cooled, its volume decreases. This dependence on temperature is 100 times more pronounced in the case of gases when compared to the liquids or solids.

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