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Gases, Liquids and Solids:

Gases, Liquids and Solids:. Swimmers (and all Canadians who have been rescued by the Coast Guard!) know that the human body is slightly less dense than water but rather more dense than air.

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Gases, Liquids and Solids:

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  1. Gases, Liquids and Solids: • Swimmers (and all Canadians who have been rescued by the Coast Guard!) know that the human body is slightly less dense than water but rather more dense than air. • Liquids and solids (high density) have small molar volumes. Gases have much larger molar volumes at “normal” temperatures and pressures. Gases are “easily” compressed!

  2. Table 1

  3. Table 2

  4. Condensed Phases and Gases at “Normal” Temperatures and Pressures • Condensed phases (solids and liquids) have relatively small molar volumes and “high” densities. • Gases have relatively high molar volumes and “low” densities • Simple Explanation – in condensed phases molecules are “touching” each other – no “empty” space.

  5. Gases at “Normal” Temperatures and Pressures • Gases are mostly empty space – and are thus easily compressed. This is not true at very high P and low T. (Demonstration with dry ice!) • Gases at low pressure can be condensed if subjected to a higher (external) pressure. Gases at high pressure will expand if the external pressure is reduced (propane barbecue). • There are many(!) pressure units.

  6. Pressure Units • By definition: Pressure = Force/Area • “Old” units for P: lb.in-2, mm Hg or torr • Modern or SI pressure units • P = Force/Area = N/m2 = kg.m s-2/m2 = Pascal • Standard atmospheric pressure = 101.325 kPa • 101.325 kPa = 1.01325 x 105 Pa (usual metric abbreviations) • We often measure atmospheric pressure using a barometer containing Hg or another liquid.

  7. The gaseous state of three halogens (group 17) Figure 6-1 General Chemistry: Chapter 6

  8. F W g x m g x V xd g x h x A xd = = = = = g x h xd P (Pa) = A A A A A Liquid Pressure liquid pressure is directly proportional to the liquid density and the height of the liquid column Figure 6-3 General Chemistry: Chapter 6

  9. Measurement of atmospheric pressure with a mercury barometer Standard Atmospheric Pressure 1.00 atm, 101.325 kPa, 1.01325 bar, 760 torr, ~760 mm Hg Figure 6-4 General Chemistry: Chapter 6

  10. Measurement of gas pressure with an open-end manometer Figure 6-5 General Chemistry: Chapter 6

  11. 1 Pa V 6-2 Simple Gas Laws PV = constant Relationship between gas volume and pressure – Boyle’s Law Figure 6-6 General Chemistry: Chapter 6

  12. Boyle’s Law: • The equation PV = constant is valid for a fixed amount of a particular gas at a fixed temperature. One could take two points on the previous graph say (V1,P1) and (V2,P2) and write • P1V1 = P2V2 = constant or just P1V1= P2V2 • This expression can be used to predict, for example, how the volume of a gas will change when the pressure is altered or….? We call this an initial state → final state problem.

  13. Class Example – Boyle’s Law: • At a particular temperature and a pressure of 242 kPa a sample of argon gas Ar(g) has a volume of 3.87 L. What will be the gas volume if the pressure is reduced to 88.6 kPa? (Mention the trichotomy axiom?)

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