# Gases Properties of Gases Gas Pressure The Nature of Gases Gases are compressible Why can you put more air in a tire, but can’t add more water to a glass.

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Gases Properties of Gases Gas Pressure

The Nature of Gases Gases are compressible Why can you put more air in a tire, but can’t add more water to a glass full of water? Gases have low densities Dsolid or liquid = 2 g/mL Dgas 2 g/L

Nature of Gases 1. Why does a round balloon become spherical when filled with air? 2. Suppose we filled this room halfway with water. Where would pressure be exerted?

Nature of Gases Gases fill a container completely and uniformly Gases exert a uniform pressure on all inner surfaces of their containers

Kinetic Theory of Gases The particles in gases Are very far apart Move very fast in straight lines until they collide Have no attraction (or repulsion) Move faster at higher temperatures

Barometers 760 mmHg atm pressure

Pressure vs Height

Parts of a Torricelli Barometer

Understanding Gases A.The downward pressure of the Hg in a barometer is _____ than (as) the weight of the atmosphere. 1) greater2) less 3) the same B.A water barometer has to be 13.6 times taller than Hg barometer (D Hg = 13.6 g/mL) because 1) H 2 O is less dense 2) Hg is heavier 3) air is more dense than H 2 O

Solution A.The downward pressure of the Hg in a barometer is 3) the same (as) the weight of the atmosphere. B.A water barometer has to be 13.6 times taller than Hg barometer (D Hg = 13.6 g/mL) because 1) H 2 O is less dense

Unit of Pressure One atmosphere (1 atm)  Is the average pressure of the atmosphere at sea level  Is the standard of pressure  P = Force Area 1.00 atm = 760 mm Hg = 760 torr

Unit of Pressure P = Force Area F = m x g m = mass and g = acceleration due to gravity m = d x V F = d x V x g

Unit of Pressure F = d x V x g V = L x W x H A = L x W P = d x g x A x H A P = d x g x H

Understanding Pressure When you drink through a straw you reduce the pressure in the straw. Why does the liquid go up the straw? 1) the weight of the atmosphere pushes it 2) the liquid is at a lower level 3) there is empty space in the straw Could you drink a soda this way on the moon? 1) yes 2) no 3) maybe Why or why not?

Solution When you drink through a straw you reduce the pressure in the straw. Why does the liquid go up the straw? 1) the weight of the atmosphere pushes it 3) there is empty space in the straw Could you drink a soda this way on the moon? 2) no Why or why not? Low atmospheric pressure

Types of Pressure Units PressureUsed in 760 mm Hg or 760 torrChemistry 14.7 lb/in. 2 U.S. pressure gauges 29.9 in. HgU.S. weather reports 101.3 kPa (kilopascals)Weather in all countries except U.S. 1.013 barsPhysics and astronomy

Understanding Units of Pressure A. What is 475 mm Hg expressed in atm? 1) 475 atm 2) 0.625 atm 3) 3.61 x 10 5 atm B. The pressure of a tire is measured as 29.4 psi. What is this pressure in mm Hg? 1) 2.00 mm Hg 2) 1520 mm Hg 3)22,300 mm Hg

Solution A. What is 475 mm Hg expressed in atm? 485 mm Hg x 1 atm = 0.625 atm (B) 760 mm Hg B. The pressure of a tire is measured as 29.4 psi. What is this pressure in mm Hg? 29.4 psi x 1.00 atm x 760 mmHg = 1.52 x 10 3 mmHg 14.7 psi 1.00 atm (B)

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