Weather Unit. Welcome Back! Begin Semester 2 1/20/2010. Seating Changes Final Exam Averages S1 Routine Changes for S2 Review Expectations.

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1 Welcome Back! Begin Semester 2 Weather Unit I-2 to I-6 Seating Changes Final Exam Averages S1 Routine Changes for S2 Review Expectations Beginning of Class (ChemCatalyst) Chemistry Binders and note-taking Class Performance Looking Ahead ChemCatalyst 1. How is rainfall usually measured? Describe the type of instrument you think is usually used. 2. Which of these containers would make the best rain gauge? Explain your reasoning. How do meteorologists keep track of the amount of rainfall? Rain Gauges If the two containers below are next to each other in a rain storm. 1) Which one will collect more water? 2) Which one will fill up to a higher level? The data points show that volume increases in a steady and predictable way in relation to the height of the beaker. Proportional: Two variables are directly proportional when you can multiply the value of one by a constant to obtain the other value Other proportional relationships: The number of steps in a staircase is proportional to the total height of the staircase. The relationship between centimeters and inches is proportional. The number of pages in a book is proportional to the thickness of the book. 1

2 Check-In All these proportional relationships can be expressed mathematically. In each case, one number in the equation does not change. Proportionality constant: The number that relates two variables that are proportional to each other. It is represented by a lowercase k. Suppose you find that a cylindrical rain gauge contains a volume of 8 ml of rain for a height of 2 cm of rain. Describe how you might figure out the volume of rain for a height of 10 cm of rain in this same container. Proportional: Two variables are directly proportional when you can multiply the value of one by a constant to obtain the other value 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. 1. Do you think 3 inches of snow is the same as 3 inches of rain? Explain your reasoning. 2. How could you figure out the volume of water that will be produced by a particular depth of snow? You can determine the density of water by measuring the mass of a certain volume of water. The density, D, is the mass, m, divided by the volume, V: D = m V The densities of snow and ice are less than the density of water. 2

3 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 0.50 g/ml. What volume does this snow occupy in milliliters? If you have 100mL of snow, what volume of water do you have? If 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? ChemCatalyst The weather forecast in Moscow, Russia calls for a 60% chance of precipitation with highs reaching 30 C, while in Washington DC the weather forecast calls for a 70% chance of precipitation with highs reaching 50 F. 1. Which city will be warmer? Explain your thinking. 2. Do you think it will rain or snow in either of the two cities? Explain your reasoning. The weather forecast in Moscow, Russia calls for a 60% chance of precipitation with highs reaching 30 C, while in Washington DC the weather forecast calls for a 70% chance of precipitation with highs reaching 50 F. How is temperature measured? 3

4 Many thermometers contain a liquid, such as alcohol or mercury. Gases expand and contract as the temp changes. How is temperature measured? How is temperature measured? 200 F 100 C Almost all substances expand upon heating and contract upon cooling. 150 F 50 C The change in volume of a liquid can be used to measure temperature change. 100 F 37 C 50 F 20 C The relationship between Fahrenheit scale and the Celsius scale is described by the formula: 0 In 1724, German physicist Daniel G. Fahrenheit invented the first modern thermometer the mercury thermometer. C In 1747, Anders Celsius, a Swedish astronomer, created a thermometer with a different scale. F = 9/5 ( C) + 32 To set a temperature scale, you need at least two measurements. Melting point or melting temperature: The temperature at which a substance melts or freezes. At this temperature, both solid and liquid phases of the substance are present. The temperature is 37 C in Spain in July. How does this compare with body temperature, which is 98.6 F? Will 37 C feel warm or cold? Explain your reasoning Boiling point or boiling temperature: The temperature at which a substance boils or condenses. At this temperature, both liquid and gas phases of the substance are present. 4

5 Researchers have recorded the temperature on Triton, a moon of Neptune, was 235 C. 1. Do you think carbon dioxide, CO 2, would be a solid, liquid, or gas at this temperature? Explain your reasoning. 2. What do you think is the coldest temperature that something can get to? What limits how cold something can get? How cold can substances become? The lowest recorded temperature in the solar system was on Triton, a moon of Neptune, and was recorded to be 235 C. What is temperature? Temperature is a measure of the average motion of gas molecules. Temperature Conversions To convert between Fahrenheit and Celsius: F = 9/5 ( C) ( F 32) = C 9 To convert between Celsius and Kelvin: K= ºC ºC = K

6 Temperature Conversions A temperature of 0 Kelvin is referred to as absolute zero. The model displayed in the simulation is the Kinetic theory of gases. The model shows that gas particles are constantly moving. The motion of gas particles is random. Gas particles move in straight lines. This is considered to be the lowest temperature that could hypothetically be reached. Absolute zero has never been attained in the laboratory. ChemCatalyst The model displayed in the simulation is the Kinetic theory of gases The speeds of the particles are not all the same. Gas particles have a lot of space to move around. (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. A lava lamp contains a waxy substance and water, which do not mix, and a lightbulb 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 this happens. How can you predict the volume of a gas sample? How can you predict the volume of a gas sample? Charles Law: You can use the proportionality constant (k) to calculate the volume at any temperature in degrees Kelvin. The proportionality constant (k) is V/T using Kelvin. k = V/T V 1 / T 1 = V 2 / T 2 Initial or starting condition New or ending condition 6

7 Charles s law states that volume, V, is proportional to the Kelvin temperature, T. The Kelvin scale assigns a value of 0 to the temperature of a gas with a volume of 0. 0 Kelvin is referred to absolute zero. Volume is proportional to temperature in degrees Kelvin. Volume is not proportional in Celsius so when you are solving problems comparing temperature (T) to volume (V) you need to convert to K ALWAYS!!! Imagine that you prepare a Graduation balloon. First thing in the morning you fill a balloon to a volume of 200 ml at 50 C. After several hours in the sun, the air inside the balloon has warmed to 85 C. Calculate the volume of the balloon at 85 C. ALWAYS ASK YOURSELF: What information is given to us in the problem, and what are we solving for? Imagine that you prepare a Graduation balloon. First thing in the morning you fill a balloon to a volume of 200 ml at 50 C. After several hours in the sun, the air inside the balloon has warmed to 85 C. Calculate the volume of the balloon at 85 C. Answer: 222 ml Imagine that you prepare a Graduation balloon. First thing in the morning you fill a balloon to a volume of 200 ml at 50 C. After several hours in the sun, the air inside the balloon has warmed to 85 C. Calculate the volume of the balloon at 85 C. Think about Predict whether the volume will increase or decrease. Convert temperatures to K. Find the V/T ratio (or k). Imagine that you prepare a Graduation balloon. First thing in the morning you fill a balloon to a volume of 180 ml at 50 C. After several hours in the sun, the air inside the balloon has warmed to 85 C. Calculate the volume of the balloon at 85 C. Solution: Find the proportionality constant (k): V/ T = 200 ml / 323 K = 0.62 ml /K Then solve: 0.62 ml / K = V / 358 K 222 ml = V 7

8 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? Check-in Answer: Prediction Since the temperature decreases, the volume gets smaller. T V Conversion Convert the temperatures to Kelvin by adding 273 to each. T 1 = = 313 K T 2 = = 263 K Find the Constant (k) Determine proportionality constant. V /T = 120 L / 313 K = 0.38 L / K Do the math! Now use the constant to determine the new volume at -10 C (263 K) L/K=? L / 263K = 100 L Does your answer make sense? Check that the volume is smaller. (It is.) 8