GASES

A GAS

- Uniformly fills any container.- Mixes completely with any

other gas- Exerts pressure on its

surroundings.- Is easily compressed.

PRESSURE

- is equal to force/unit area- SI units = Newton/meter2 = 1

Pascal (Pa)

4

VARIABLES AFFECTING GAS PRESSURE

There are three variables that affect gas pressure:

1) The volume of the container.

2) The temperature of the gas.

3) The number of molecules of gas in the container.

5

GAS PRESSURE

Gas pressure is the result of constantly moving gas molecules striking the inside surface of their container. The more often the molecules collide with the sides of

the container, the higher the pressure.

+ The higher the temperature, the faster gas molecules move.

BOYLE’S LAW

Robert Boyle -- Irish scientist (1627-1691)

Boyle’s Law states that at a constant temperature, the volume of a gas is inversely proportional to the pressure.

(The temperature and amount of gas must remain unchanged)

7

VOLUME AND PRESSURE

When the volume decreases, the gas molecules collide with the container more often and the pressure increases.

When the volume increases, the gas molecules collide with the container less often and the pressure decreases.

8

BOYLE’S LAW

• Inversely proportional means two variables have a reciprocal relationship.

(if one goes up, the other goes down)

(Notice Boyle’s Law graph shows a curved line)

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Pext

Pext

Volume is decreased

Volume of a gas decreases as pressure increases at constant temperature

FIGURE 12.6: ILLUSTRATION OF BOYLE’S LAW

11

CHARLES’ LAW

In 1783, Jacques Charles discovered the relationship between the volume of a gas and temperature

Charles’ Law states that with a constant amount of gas and a constant pressure, the volume of the gas is directly proportional to the temperature.

12

TEMPERATURE AND PRESSURE

When the temperature decreases, the gas molecules move slower and collide with the container less often and the pressure decreases.

When the temperature increases, the gas molecules move faster and collide with the container more often and the pressure increases.

CHARLES’S LAW

Directly proportional means two variables have a direct relationship.

(as one goes up, the other goes up,

as one goes down, the other goes down)

(Notice that Charles’ Law gives a straight line graph)

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Pext

Pext

Energy (heat) added

Volume of a gas increases as heat is addedwhen pressure is held constant.

Chapter 11 15

ILLUSTRATION OF CHARLES’ LAW

Below is an illustration of Charles’ Law.

As a balloon is cooled from room temperature with liquid nitrogen (–196C), the volume decreases.

WHY I NEED TO PAY ATTENTION IN 8TH GRADE SCIENCE! Crushed Tanker  We had some workers who didn't understand Physics. We take a Tank car without a negative pressure relief valve. We give the crew steam cleaning equipment and tell them to steam

clean our Tank Car. They Did a Great Job!!! To be perfectly honest, it’s a mistake most of us could have easily

made. Keep your brain in gear, at all times. The interior of the tank car was washed out & cleaned with steam.

Then all the outlet valves were shut and the tank car was sealed. All the workers went home for the evening and when they returned, this is what they found. Apparently as the tank car cooled, it collapsed. The shell on these tank cars is 7/16th's inch thick steel. It is hard to imagine the forces it took to do this much damage, to such a large steel object. Looking at the images, it appears to have been crushed as if it was a toy, or an empty pop can in your hand.

The tank car is smashed, crushed, or collapsed by an unseen force.

20

MOLECULES AND PRESSURE

When the number of molecules decreases, there are fewer gas molecule collisions with the side of the container and the pressure decreases.

When the number of molecules increases, there are more gas molecule collisions with the side of the container and the pressure increases.

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Increase volume to return to originalpressure

Gas cylinder

Moles of gasincreases

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Pext

At constant temperature and pressure, increasing the moles of a gas increases its volume.