Charles's Law

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Objective

SWBAT explain why the volume of a gas sample increases and the density decreases as the temperature increases.

Big Idea

Help your students connect what they know about the density of gases and heat transfer to understanding this gas law.

Getting Started

In this first lesson on gas laws, you are digging for student ideas and helping them make connections between the relationship of temperature, volume and pressure of gasses. 

You open with a thought experiment, move on to a teacher led demonstration and then have students complete a short investigation. 

Materials:

  • 2 1000 ml beakers
  • hot plate
  • water
  • 2 balloons
  • student handouts

Elicit

15 minutes

I open this lesson with a thought experiment that won't be easily answered here but pushes students to think about their assumptions about temperature, volume, pressure, altitude and density. It connects to gas laws and to atmosphere dynamics. 

Use it as a tool to hear students’ ideas and collect some data on their background knowledge and misconceptions. 

I set it up as follows:

Mark was wondering what would happen to a bunch of inflated helium balloons if he climbed with them several hundred feet up the side of a mountain where the temperature was 20° C cooler than at the base. 

 

1) What can you tell me about how the balloons might change after his climb up the mountain? (Think about temperature, pressure, volume.)

2) What will happen to Mark's balloons if he were to descend many feet below sea level into a cavern?

Record your answers and questions in your journal. Below is a sample student response:

 

Engage

15 minutes

After hearing student ideas, I perform a short demonstration for the class and then have them tell me what they observe going on. 

Before class starts, slightly inflate each balloon making one just smaller than the diameter of the 100 ml beaker. You will use this one the most. It should easily slide down just barely touching the sides. Blow the other balloon up larger than the diameter of the beaker and set it aside. 

Pour 500 mL of water into one of the 1000 mL beakers and place it on the hot plate with the heat turned to high. 

Fill the other beaker with 500mL of ice water and set it to the side. 

When you are ready, hold up the smaller of the two balloons and ask the class to predict what will happen to the balloon when you place it into the baker with the hot water. After taking their responses, go ahead and submerge the balloon and watch as the volume increases. Ask students to tell you what is happening to the gas molecules that makes this change. Connect it back to heat transfer and have them tell you by what means of heat transfer are the molecules being excited by.

Next, point to the beaker of ice water and ask the students what they think will happen to the balloon if you submerge it into the ice water after it expanded in the hot water. As before, take their predictions then carefully remove the balloon from the hot beaker and place it into the cold water. Ask students to describe what is happening -- making sure to make those connections to what is occurring on the molecular level -- as you did before (MS-PS1-4).

Help guide their responses by asking them questions that make the connection between change in temperature and change in volume. In other words as the temperature increases, the volume increases.

But what about pressure? Go back to the thought experiment responses and ask them how air pressure factors into this scenario. You may need to explain to them or ask them a series of guiding questions about the air pressure in the classroom. By whatever means you get there, you want to help them conclude that the air pressure is constant.

Here is a video of the demonstration. 

Explore

20 minutes

Next, students work as a table group or in pairs as they use the Charles's Law Student Handout to explore how the volume of the gas is related to its temperature.

They are calculating how changes in the volume of the gas relate to temperature changes. This lesson uses a unit of measure that some middle school students may not be familiar with. Assuming that the symbol for proportionality constant (K) is new, you may want to explicitly tell them what "K" means.

I like to work through the first three problems as a class, as even my brightest students have a number of questions about these relationships. This is a great opportunity to collaborate with your math teacher so they can support this work in their class.

In problem three, students make the connection to why we have to use Kelvin when calculating Charles's law. If they need some help making the connection, return to the lesson on absolute zero to make the case why we have to have a unit like Kelvin that allows us to reach zero volume in order achieve the proportionality constant.

Note: This lesson was adapted from Living by Chemistry W. H. Freeman and Company 2012.