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Table of contents (under construction)

Composition of air – Gases: Composition, Misconceptions

Special topics

How do we pull liquids up through a straw? “Suction” doesn’t exist -> really just a difference in air pressure.

Does hot air really rise? (The balloon rises because the air pushing on the bottom of the balloon has a greater force than the downward force of the air on top of the balloon plus the balloon’s own weight.)

The gas laws

Putting Gases to work (an intro to the idea of gas laws)

Kinetic Energy – the energy of particles in motion.

We start by looking at an ideal, monatomic gas.  What do these terms mean?

ideal gas:

monatomic gas:

The temperature of this gas =  average kinetic energy

What is kinetic energy?


Ideal gas assumptions

* Gas made of very small particles (atoms or molecules.)

* So small, that the total volume of the individual gas molecules, added up, is negligible compared to the volume of the container that they’re in.

* Average distance between particles is large compared to their size.

* Particles all have the same mass.

* In constant, random, and rapid motion.

* They constantly collide among themselves, and with the walls of the container.

* All collisions are elastic.

* Gas molecules are considered to be perfectly spherical

* When not colliding, the particles don’t exert any forces on each other.

The average kinetic energy of the gas particles depends only on the absolute temperature of the system. The kinetic theory has its own definition of temperature, not identical with the thermodynamic definition.

The time during collision of molecule with the container’s wall is negligible as compared to the time between successive collisions.

Because they have mass, the gas molecules will be affected by gravity.

Are real world gases ideal like this?

Not at all. These are simplifying assumptions; they make it much easier to derive mathematical rules for how gases work.

If real world gases are not like this, then why make simplified (“wrong”) assumptions? Why use math formulas that we know are not perfectly correct (“wrong”)?

Answer: These assumptions make the math a hundred times easier, yet the practical, real-world results are almost exactly the same as what we’d get using the more exact assumptions and math.

Under most conditions – including all the weather that you have ever experienced – real world gases behave like ideal gases.

When do these assumptions fail?  Very pressure, or very high temperature.
This Java applet is a simulation that demonstrates the kinetic theory of gases. The color of each molecule indicates the amount of kinetic energy it has

App: Gas molecule simulation falstad.com/gas


The triple point

The triple point: Solid, liquid and gas at the same time


Pressure and volume

Atmospheric pressure: How does gravity affect the distribution of gas molecules in the air?

Use this applet:  http://www.falstad.com/gas/  See how changing gravity affects the molecules.

Brownian motion: we see dust particles, or pollen, moving around on still water – why do they move?


The kinetic model of gases: we usually can model gases as an “ideal gas”

Dalton’s law of partial pressure

Boyle’s law: pressure and volume

PV = c

(borrow these GIFs)



Charles’s law: volume and temperature


Gay-Lussac’s law: pressure and temperature


The combined gas law: pressure, volume and temperature

The ideal gas law: PV = nRT

Graham’s law is about the velocity of particles in a gas / Diffusion



Not all gases have the same density!


Apps and animations

Physics: Mechanics, Fluid Mechanics and Dynamics, Electromagnetism, Quantum

Chemistry: Thermodynamics, States of Matter, Chemical bonds, Water and solution, Reactions

Biology, Biotechnology, Nanotechnology,

Molecular workbench apps

Learning standards

Massachusetts Science and Technology/Engineering Curriculum Framework

8.MS-PS1-4. Develop a model that describes and predicts changes in particle motion, relative spatial arrangement, temperature, and state of a pure substance when thermal energy is added or removed.

HS-PS2-8(MA). Use kinetic molecular theory to compare the strengths of electrostatic forces and the prevalence of interactions that occur between molecules in solids, liquids, and gases. Use the combined gas law to determine changes in pressure, volume, and temperature in gases.

Next Generation Science Standards

5-PS1-1. Develop a model to describe that matter is made of particles too small to be seen

Disciplinary Core Ideas: Matter of any type can be subdivided into particles that are too small to see, but even then the matter still exists and can be detected by other means. A model showing that gases are made from matter particles that are too small to see and are moving freely around in space can explain many observations, including the inflation and shape of a balloon and the effects of air on larger particles or objects. (5-PS1-1)

MS-PS1-4. Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.

College Board Standards

Objective C.1.5 States of Matter

C-PE.1.5.1 Translate among macroscopic (e.g., a beaker of water), symbolic [e.g., H2 O(s)], and atomic–molecular level representations of states. Describe, using representations, the relative arrangement of particles in solids, liquids and gases. Or conversely, identify the state of matter depicted in atomic–molecular level pictures or animations.

C-PE.1.5.2 Explain why gases expand to fill a container of any size, while liquids flow and spread out to fill the bottom of a container and solids hold their own shape. Justification includes a discussion of particle motion and the attractions between the particles.

C-PE.1.5.3 Investigate the behavior of gases. Investigation is performed in terms of volume (V ), pressure (P ), temperature (T ) and amount of gas (n) by using the ideal gas law both conceptually and mathematically.

C-PE.1.5.4 Explain natural phenomena (e.g., cold air escaping from a tire or low atmospheric pressure on rainy days) in terms of the kinetic–molecular theory of gases.

C-PE.1.5.5 Construct atomic–molecular level representations of changes that occur when thermal energy is added to a pure substance. Explain, using these representations, why the continuous addition of thermal energy to a pure substance will generally result in a change of state (not a chemical reaction).

C-PE.1.5.6 Explain, in terms of molecular motion, why liquid water expands when it freezes, whereas most substances expand when heated (e.g., mercury in a thermometer). Provide examples of instances where the expansion of water upon freezing is important (e.g., ice floating on water acts as an insulator in ponds to keep temperature of the rest of the water above freezing).

Common Core Math

Analyze proportional relationships and use them to solve real-world and mathematical problems.




Recognize and represent proportional relationships between quantities.


Decide whether two quantities are in a proportional relationship, e.g., by testing for equivalent ratios in a table or graphing on a coordinate plane and observing whether the graph is a straight line through the origin.


Identify the constant of proportionality (unit rate) in tables, graphs, equations, diagrams, and verbal descriptions of proportional relationships.


“Is the NGSS Going to Ruin High School Chemistry?” By Pete A’Hearn and Wanda Battaglia, California Classroom Science, 10/19/2015 http://www.classroomscience.org/is-the-ngss-going-to-ruin-high-school-chemistry



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