Unit 3: Weather, Exams of Law

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UNIT 3: WEATHER

Part 1 – Physically Changing Matter Part 2 – Pressing Matter Part 3 – Concentrating Matter

UNIT 3 PART 3:

CONCENTRATING MATTER

CHEMCATALYST 3 - 15 Earth’s Atmosphere Compare the atmosphere at sea level and at 34,000 ft, the altitude at which airplanes fly.

1. Describe at least three differences.
2. Explain why it is difficult to breathe at 34,000 ft.

NUMBER DENSITY

 The gas pressure increases as the number of gas

molecules per unit of volume increases.

P = k ( n / V )

 Number density: The number of gas particles per

unit volume. Number density = n / V

 The height of the water levels in a U-tube

indicates differences in pressure.

KEY POINTS  The composition of Earth’s atmosphere is not uniform. The density of the gas molecules in the air decreases with increasing altitude. This causes the pressure of the atmosphere to decrease with increasing altitude.  The number density of a gas is the number of gas molecules per unit of volume, n/V.  The pressure of a gas is directly proportional to the number of gas molecules per unit of volume. This relationship is written as P = k·(n/V).

REFLECTION!

 A balloon is filled with helium, tied off, and then

released. As it climbs into the air, its volume

slowly increases.

1. Explain what is going on with the helium atoms

inside the balloon and the air molecules

outside the balloon in terms of number density

and pressure.

CHEMCATALYST 3 - 16  There are two balloons. One is filled with helium, He, and the other with carbon dioxide, CO 2

1. Describe what happens when the balloons are released.
2. For the two balloons, state whether these properties are the same or different, and explain why: - pressure, P • number density, n/V - temperature, T • number, n - volume, V • density, m/V - mass, m

THE MOLE  Chemists use a unit called a mole to describe the number of gas particles in a sample.  Mole: A unit invented by chemists to count large numbers of gas particles. There are 602,000,000,000,000,000,000,000 particles in  1 mole = 602,000,000,000,000,000,000, = 6.02 × 10 23  (602 sextillion particles)

REFLECTION!

 One balloon contains 22.4 L of Ar, argon gas,

and another balloon contains 22.4 L of Ne, neon

gas. Both balloons are at 273 K and 1 atm.

1. Do the balloons contain the same number of

atoms? Why or why not?

2. Will the balloons have the same mass? Why or

why not?

IDEAL GAS LAW

EQ: How can you calculate the number of

moles of a gas if you know P , V , and T?

IDEAL GAS LAW  The ideal gas law allows scientists to relate gas pressure, volume, moles of particles, and temperature.  Ideal Gas Law: The ideal gas law states that PV = nRT , where R, the universal gas constant, is equivalent to the proportionality constant, k , for this equation. 𝑅 =

L ∙ atm mol ∙ K

IDEAL GAS LAW  Note that R is the same for all gases but the value of R does change depending on if the units change.  The number of moles can be converted to the total number of gas molecules by multiplying by 602 sextillion.  The ideal gas law can be used to solve for other variables besides n (like P , V , or T ).

HUMIDITY AND

CONDENSATION

EQ: What is humidity and how is it

measured?

CHEMCATALYST 3 - 18

1. Is there water vapor in the air right now?

What evidence do you have to support

your answer?

2. What do you think humidity means? How

does humidity depend on temperature?