1. The Nature of Gases
Gases have some interesting characteristics that have fascinated scientists for 300 years.
The first gas to be studied was air & it was a long time before it was discovered that air was actually a mixture of particles rather than a single gas.

2. The Nature of Gases
But this realization did not make the study of gas behavior more difficult.
Although air is a mixture of several different gases, it behaves much the same as any single gas.
Regardless of their chemical identity, gases tend to exhibit similar physical behaviors

3. The Nature of Gases
Gas particles can be monatomic (Ne), diatomic (N2), or polyatomic (CH4) – but they all have these characteristics in common:
1) Gases have mass.
2) Gases are compressible.
3) Gases fill their containers.
4) Gases diffuse
5) Gases exert pressure.
6) Pressure is dependent on Temp.

5. Kinetic Molecular Theory
There is a theory that modern day chemist’s use to explain the behaviors and characteristics of gases - the Kinetic Molecular Theory of Matter.
The word kinetic refers to motion.
The word molecular refers to molecules

6. KMT Assumption #1
The particles have an insignificant volume and are relatively far apart from one another.
There is empty space between particles.
No attractive or repulsive forces between particles.

7. KMT Assumption #2
The particles in a gas move in constant random motion.
Particles move in straight paths and are completely independent of each of other
Particles path is only changed by colliding with another particle or the sides of its container.

8. KMT Assumption #3
All collisions a gas particle undergoes are perfectly elastic.
No energy is lost from one particle to another, and the total kinetic energy remains constant.

9. We treat all gases as if they obey the KMT and call them “ideal gases”.
A hypothetical gas whose molecules occupy negligible space and have no interactions, and obeys the gas laws exactly.

10. Gases exert pressure
Gas particles exert pressure by colliding with objects in their path.
The sum of all of the collisions makes up the pressure the gas exerts.

11. The definition of pressure is a force per unit area.
So the total of all of the tiny collisions makes up the pressure exerted by the gas.

12. The gases push against the walls of their containers with a force.
The pressure of gases is what keeps our tires inflated, makes our basketballs bounce, makes hairspray come out of the can, etc.
Pressure = force per unit area

13. AIR PRESSURE

14. Earth’s Atmosphere About 10 km thick
Consists mostly of molecular nitrogen (N2) and oxygen (O2)

15. The air is made up of molecules.

16. Gravity pulls the air molecules toward the earth, giving them weight
Gravity pulls the air molecules toward the earth, giving them weight. The weight of the air molecules all around us is called the air pressure.
Your weight is the result of gravity pulling your mass down on the bathroom scales. Note that weight has units of a force, such as pounds.

17. High altitudes = lower pressure
Air pressure can be thought of as the column of air rising above us. As we go up in altitude, we get closer to the top of the column. Thus there are fewer molecules of air above us to be pulled down by gravity, so the air “weighs” less. Therefore, pressure always decreases as one goes up.
Low altitudes = higher pressure

18. Air pressure is equal in all directions.
Because air is a fluid, force applied in one direction is distributed equally in all directions. Thus the downward pull of gravity on air molecules produces air pressure in all directions.
Pressure = force per unit area

19. Barometric pressure goes down.
As elevation goes up
Barometric pressure goes down.
This is an inverse relationship.

20. Atmospheric Pressure Gas pressure and atmospheric pressure.
Adding air molecules increases the pressure in a balloon.
Heating the air also increases the pressure.

21. at 1 Atmosphere Pressure
at 10 Atmospheres Pressure
at 30 Atmospheres Pressure

22. A Barometer
is used
to measure air pressure.

23. In 1643, Evangelista Torricelli invented the barometer
Torricelli didn’t actually build a barometer, but he gave detailed instructions on how to build one, so he is given credit for the invention. He was actually trying to prove the existence of a vacuum. Many scientists in his day didn’t believe that a vacuum could exist, hence the phrase, “nature abhors a vacuum.”
In 1643, Evangelista Torricelli invented the barometer

24. Torricelli’s barometer used a glass column suspended in a bowl of mercury. The pressure of the air molecules pushed the mercury up into the glass tube.
The weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish.
The abbreviation “Hg” is the chemical symbol for mercury. Some kinds of pressure reading instruments, including some barometers, use the abbreviation “mmHg,” meaning “millimeters of mercury.” 760 mmHg is considered the standard “normal” atmospheric pressure at sea level. This unit is called a “torr,” after Torricelli.
To construct a mercury barometer, fill a tube with a liquid. Invert then tube in a dish of liquid holding your thumb over the top of the tube until the the tube is immersed in the bowl of liquid, the atmospheric pressure will keep the liquid in the tube from emptying such that the weight of the liquid in the tube equalize with the atmospheric pressures. (Do not do this with mercury because of its toxicity)
Mercury was used because it is a very heavy liquid, so the tube could be relatively short. The tube in a mercury barometer still has to be over a meter long. Students may want to try building a barometer using colored water. How high would the tube need to be? Merucy is about 11 times more dense than water. What if they used milk or some other liquid, would the height be the same?