1. Phase Changes and Behavior of Gases
Unit 3: Weather
Phase Changes and Behavior of Gases

2. Lesson 1: Weather or Not – Weather Science

3. ChemCatalyst
The table gives the current weather conditions in Miami, Florida (shown on the weather map in the following slide with a star). Predict the weather for later today. Indicate whether you think the current conditions will increase, decrease, or stay the same. Explain your reasoning.
Current Conditions at 1:30 P.M. in Miami, Florida
Temperature
82°F / 27.8°C
Pressure
29.95 in.Hg, falling
Fronts
Cold front to pass through today
Conditions
Mostly cloudy, wind gusts up to 30 mph NW
Humidity
71%

4. Key Question:
What causes the weather?

5. Objectives: explain the phenomenon of weather in general terms
list the variables meteorologists study or measure in order to predict the weather
describe the basic components of a weather map

6. Definition:
Weather – the state of the atmosphere in a region over a short period of time.
Weather is the result of interaction among Earth, the atmosphere, water, and the Sun.
It refers to clouds, winds, temperature, and rainfall or snowfall.

7. The activity:
You will be working in groups to examine several weather maps that all represent the same day
Look for patterns among the different variables on the weather maps

8. Factors affecting Weather:
Jet Stream:
High-level winds that are at least 57 mi/h (up to 190 mi/h) located in the upper atmosphere (4 miles up). These winds generally “steer” storms around the planet.

9. Temperature:
Bands of color are used to display variations in temperature. Typically reds represent hotter temperatures while blues represent colder temperatures. Temperature patterns are usually very defined – not random

10. Cloud Cover:
Weather maps use gray areas to designate the location of clouds

11. Fronts:
Depictions of warm and cold air moving into a region

12. Precipitation:
Raindrops and snowflakes are used to show where there is rain or snow

13. Air Pressure:
The H’s show the locations of large air masses with consistently high pressure. The L’s show where large air masses with consistently low air pressure are located.

14. Key Question Answered:
What causes the weather?
Weather consists of precipitation, clouds, winds, and temperature changes that result from interactions among the Earth, the atmosphere, water, and the Sun.

15. Check-In What do weather maps keep track of?
How do they help meteorologists?

16. Lesson 2: Raindrops Keep Falling – Measuring Liquids

17. ChemCatalyst
1. How is Rainfall usually measured? Describe the type of instrument you think is usually used.
Which of these containers would make the best rain gauge? Explain your reasoning.

18. Key Question
How do meteorologists keep track of the amount of rainfall?

19. Objectives: Identify a proportional relationship
Describe several methods for solving a problem involving proportional variables
Explain why rainfall is measured in terms of height rather than volume

20. The Activity:
Follow the procedure in Lesson 2 to experiment with different types of rain gauges

21. Explanation:
The data points on your graph show that volume increases in a steady and predictable way in relation to the height of the beaker.
Use the graph to estimate the volume of water in the beaker at 5.5 cm
Use the graph to estimate the volume of water in the beaker at 9.0 cm

22. Explanation (Cont.)
All of these methods are possible to predict because the height and volume of a container are proportional to each other for that container.
The data points for the beaker lie on a straight line going through the origin (0, 0). Whenever a graph shows this relationship, the two variables are proportional to each other.

23. Definition Proportional:
Two variables are directly proportional when you can multiply the value of one by a constant to obtain the value of the other.

24. Explanation (Cont.)
The data points for the Florence flask do not lie along a straight line.
Is this an example of a proportional relationship?

25. Summary
The volume of rainfall increases regularly in relation to the height of the rainfall.
The height of rain collected in a rain gauge does not depend on the diameter of the container.
A rainstorm that drops 1 inch of rain in one container will also drop 1 inch of rain in another container as long as the walls of the container are parallel.
The volume of rain collected does depend on the diameter of the container.
After a storm, a container with a larger diameter will have more water in it than one with a smaller diameter.

26. Examples:
Can you think of at least 3 more everyday examples of proportional relationships?

27. Volume = (area of base) • (height)
Summary (Cont.)
All proportional relationships can be expressed mathematically.
If a rain gauge must be a cylinder, the relationship between volume and height of rain can be expressed mathematically as:
Volume = (area of base) • (height)
The area of the base never changes.
An unchanging number in a proportional relationship is often referred to as the proportionality constant.

28. Definition Proportionality constant
The number that relates two variables that are proportional to each other. Often represented by: lowercase k.

29. Key Question Answered:
How do meteorologists keep track of the amount of rainfall?
Meteorologists measure the height of rain, because the volume of rain is directly proportional to the height.
Graphs of two variables that are proportional are always a straight line through the origin.
When one variable is proportional to the other, it is possible to make accurate predictions of other values when one data point is known.

30. Check-In
Suppose you find a cylindrical rain gauge contains a volume of 8 mL of rain for a height of 2 cm of rain. How can you calculate the volume of rain for a height of 10 cm of rain in this same container?

31. Lesson 3: Having a Meltdown – Density of Liquids and Solids

32. ChemCatalyst
Water resource engineers measure the depth of the snowpack in the mountains during the winter months to predict the amount of water that will fill the lakes and reservoirs the following spring.
Do you think 3 inches of snow is the same as
3 inches of rain? Explain your reasoning.
How could you figure out the volume of water that will be produced by a particular depth of snow?

33. Key Question
How much water is present in equal volumes of snow and rain?

34. Objectives
Make density calculations, converting volumes of liquids and solids
Explain how phase changes affect the density of a substance
Use density equations to calculate the volume of water in a sample of snow or ice

35. The Activity:
Part 1, Data collection, will be completed as a demonstration
Work with your partner to complete the questions in the activity packet

36. Key Points:
You can determine the density of water by measuring the mass of a certain volume of water
D = m/v

37. Key Points (Cont.):
The densities of snow and ice are less than the density of water
Ice has a density of 0.92 g/mL
Snow has a density of 0.50 g/mL

38. Key Points (Cont.):
The graph you created allows you to compare the densities of the different phases of water
The steeper the line, the greater the density of the substance

39. Key Points (Cont.):
The relationship D = m/v can also be written as m = DV
When Density is constant, the mass and volume are proportional.
The proportionality constant is Density

40. Key Points (Cont.):
For any proportional relationship, the graph is a straight line that passes through the origin, (0,0).

41. Example Problems:
Imagine that you have a box with Volume 14.5 mL. What mass of ice will just fill this box?
You have 12 g of snow with density of 0.50 g/mL. What volume does this snow occupy in milliliters?
If you have 100 mL of snow, what volume of water do you have?

42. Wrap Up:
Scientists measure snowpack in terms of depth – meters or feet – and then make conversions to obtain the volume of water.
This is another application of conservation of mass
When a substance changes phase, its density changes. The mass stays the same, and the volume changes.

43. Check-in:
Imagine that you have equal masses of snow and rain. Which has a greater volume? Explain your thinking?
What is the mass of 14 mL of rainwater?

44. Lesson 5: Absolute Zero – Kelvin Scale

45. ChemCatalyst
Researchers have recorded the temperature on Triton, a moon of Neptune, as -235 ˚C.
1. Do you think carbon dioxide, CO2 would be a solid, a liquid, or a gas at this temperature? Explain your thinking.
2. What do you think is the coldest temperature something can get to? What limits how cold something can get?

46. Key Question:
How cold can substances become?

47. Objectives:
Describe the relationship between the Celsius and Kelvin temperature scales
Explain the concept of absolute zero
Describe the motion of gas molecules according to the kinetic theory of gases.

48. The Activity:
Work with your partner to answer the questions in part 1 and 2
While working on part 1 of the activity, one partner should log onto a computer

49. Key Points:
On the Celsius scale, the temperature at which the volume of a gas is theoretically equal to zero is -273 ˚C.

50. Conversions: To convert from degrees Celsius to Kelvin:
K = ˚C + 273
To convert from Kelvin to degrees Celsius:
˚C = K - 273

51. Key Points (Cont.)
A temperature of 0 K is referred to as Absolute Zero.
This is considered to be the lowest temperature that could hypothetically be reached.
Although absolute zero has never been reached in a laboratory, in 2003 the lowest temperature reached in a lab was K

52. Celsius Scale Volume of Gas Versus Temperature
Graphs:
Celsius Scale Volume of Gas Versus Temperature

53. Volume of Gas Versus Kelvin Temperature
Graphs:
Volume of Gas Versus Kelvin Temperature

54. Key Points (Cont.)
The Model displayed in the simulation is the kinetic theory of gases.
The gas particles are constantly moving
The motion of gas particles is random
The gas particles move in straight lines
The speeds of the particles are not all the same
Gas particles have a lot of space to move around in. (They are tiny compared to the space they are found in.)
Gas particles change directions when they hit each other or the walls of the container

55. Key Points (Cont.)
The temperature of a gas is a measure of the average energy of motion of the gas.
Scientists hypothesize that if it were possible to reach absolute zero, the motions of atoms and particles would stop.

56. Check-in: Describe three features of the motions of gas particles.
Use the motions of gas particles to explain why gases expand when they are heated.

57. Lesson 6: Sorry Charlie – Charles’ Law

58. ChemCatalyst
A lava lamp contains a waxy substance and water, which do not mix, and a light bulb at the base. As the bulb heats the waxy substance, it rises. Near the top of the lamp, the waxy substance cools and falls. Explain why you think this happens.

59. Key Question:
How can you predict the volume of a gas sample?

60. Objectives:
Explain Charles’ law and use it to solve simple gas law problems involving volume and temperature
Explain two methods for determining the volume of a gas if its temperature is known

61. The Activity:
After completing the lab activity, you should have a good idea of Charles’ law.
Work with your partner to complete the activity packet.

62. Key Points:
Charles’ Law: For a given sample of gas at a certain pressure, the volume of gas is directly proportional to its Kelvin temperature

63. The proportionality constant, k, indicates how much the volume of a gas changes per Kelvin
Because of this relationship, we can do calculations for the same sample of gas very simply by comparing initial and final conditions

64. Steps for Solving Charles’ Law Problems
Convert all temperature values to Kelvin
Solve problem using this equation for Charles’ Law
Charles’ Law Equation:

65. Example Problem:
The first thing in the morning, you fill a balloon with air to a volume of 180 mL at 50˚C. After several hours out in the Sun, the air inside the balloon has warmed to 85˚C. Calculate the new volume of the balloon.

66. Key Points:
Because the volume is proportional to temperature, the graph of volume versus temperature for a gas sample is a straight line that goes through the origin, (0,0).

67. Check for Understanding:
Why do you think that the lines of the graph have a different steepness?
The slope of the line, k, is different for different quantities of gas. The balloons must be filled with different amounts of gas

68. Key Points: Hot air rises because it is less dense than cooler air.
When the temperature of a mass of air increases, the molecules move faster and the volume increases. Because D = m/V, the density of the air decreases when the Volume gets larger.
Less dense substances float on denser substances, so a larger warm air mass floats above a larger cooler air mass.

69. Check-in:
A sample of gas has a volume of 120 L at a temperature of 40˚C . The temperature drops to –10˚C. If nothing else changes, what is the new volume of the gas?

70. Lesson 7: Front and Center – Density, Temperature, and Fronts

71. Chem Catalyst:
Large air masses form over different regions of land and ocean. These air masses have a consistent temperature and moisture content.
What patterns do you notice in the temperatures and moisture content of the air masses shown on the map?
Why do you think clouds form when the Continental Polar air mass collides with the Maritime Tropical Air mass?
Use the concept of density to explain why warm air in the maritime Tropical air mass rises, while the cold air in the Continental Polar air mass descends.

72. Key Question:
How do weather fronts affect the weather

73. Objectives:
Explain the roles of temperature and density in the movement of cold and warm air masses
Describe the weather patterns associated with warm fronts and cold fronts

74. Review:
Weather fronts represent regions of cold and warm air moving into a region
Cold Front
Warm Front

75. The Activity:
Working in groups, use the weather transparencies and your activity packets to answer the questions

76. Key Points:
Fronts occur between the boundaries of warm and cold air masses.
In general, warm (tropical) air masses move up across the North American continent from the south
Cold (polar) air masses move down across the continent from the north

77. Key Points: Warm and cold air masses have different densities
Warm air masses rise above cold air masses because they are less dense.
When this happens, clouds form.
The cold temperatures cause water in the warm air mass to change phase from a vapor to a liquid.
This is why fronts are associated with storms and rain.

78. Key Points: The weather associated with cold and warm fronts differs.
Clouds associated with cold fronts, tend to be thicker, puffy clouds similar to those seen with thunder storms.
Clouds associated with warm fronts, tend to be thinner and not as puffy. In advance of warm fronts, you may have days of clouds before the rain actually arrives.
Precipitation tends to occur at or just behind cold fronts, and in advance of warm fronts.

79. Graphics: Cold Front: Cold air overtakes warm air
Warm Front: Warm air overtakes cold air

80. Wrap up:
Interactions among temperature, volume, and density of air masses contribute significantly to the formation of weather.
Weather Generalizations:
Fronts occur at the boundaries between warm and cold air masses.
Warm air, which is less dense, layers over the denser cold air.
Clouds and steady light rain form ahead of a warm front. Clouds and heavy showers form at and behind a cold front.
On weather maps, Ls are closely associated with fronts, while Hs appear away from the fronts.
Hs are associated with clear skies.
Ls are associated with storms and cloudy skies.

81. Check-in:
A warm front is approaching your hometown, and it is only one day away. What would you expect to observe in the way of weather?