Optional: Behaviour of Gases

Unit Overview

There are a variety of investigations which can contribute to students' understanding why gases behave as they do - examining some of the properties of gases, collecting data and analyzing the results, searching for trends and relationships. Students can recognize that macroscopic properties of gases can be explained by events that might be taking place at a molecular level.

From relationships deduced from observations about the properties of gases, students can develop an understanding of how the mathematical expression of various gas laws are developed. This unit should not be treated only as an opportunity to do mathematical manipulations. The basis of such calculations should be established through experimentation.

Students should become acquainted with the new definition of standard state pressure, referred to as SATP. They should recognize both STP and SATP while the transition to use of SATP is being made. An article on this topic is "The New Pressure Standard in Chemistry" by Geoff Rayner-Canham in CHEM13 NEWS, #192, February 1990, pages 1-2.

Factors of scientific literacy which should be emphasized

Foundational Objectives for Chemistry and the Common Essential Learnings

Apply kinetic molecular theory to understand the properties of gases.

Compare the behaviour of solids, liquids, and gases.
Investigate the behaviour of several gases while varying the temperature, pressure, volume or number of moles.
Use the kinetic molecular theory to make sense of observations about the behaviour of matter.

Recognize how knowledge about gas behaviour is used in science and in technology.

Consider the importance of Avogadro's hypothesis.
Identify industrial use and application of gases and their behaviours.
Recognize why particular conditions have been defined as standard temperature and pressure (SATP and STP).
Discuss the concept of molar volumes of gases.
Identify the SI units used to measure temperature, pressure, and volume.

Describe gas behaviour with mathematical equations.

Sketch graphs which express the relationships among temperature, pressure, volume and number of moles of gas.
Examine the link between the graph and the equation for a relationship.
Discuss the concept of an ideal gas.
Solve problems relating to the gas laws.

Strengthen understanding of gas behaviour by using knowledge of numbers and their interrelationships. (NUM)

Read and interpret graphs to deduce the relationships between the variables.
Develop and use their understanding of quantitative information through graphical analysis.
Understand the uncertainties inherent in measurement and in inductive logic.

Understand and use the vocabulary, structures and forms of expression which characterize chemistry (COM)

Incorporate the vocabulary of chemistry into talking and writing.
Use graphs and equations to explain to others about gases and gas behaviour.

Develop an understanding of how knowledge is created, evaluated, refined and changed within chemistry. (CCT)

Control variables, record experimental data, and analyze those data for patterns and relationships.
Understand the relationship between observations and measurements and their abstract expression as mathematical formulas.
Recognize the importance of mathematical expression to the ability to generalize and predict.

Suggested activities and ideas for research projects

Set a tube create from three clear plastic straws on a horizontal, dark background. Put a drop of concentrated HCl_(aq) on one Q-tip and a drop of concentrated NH₄OH_(aq) on the other.
Insert the Q-tips simultaneously into opposite ends of the tube. Record the time it takes for the vapours to diffuse and meet in the tube. Watch closely so that you will be able to tell when they meet. What is the formula of the white product formed? How far did each gas travel before they met? Is there a relationship between the mass of the gas molecules and the distance they travel. Can you express this relationship mathematically.
Note to teachers: The diffusion of ammonium hydroxide and hydrogen chloride through a glass tube forming a visible ammonium chloride front where they meet, most often done as a demonstration, can be done as a microscale activity. This makes it easy for each lab group to do the activity.
Clear plastic drinking straws substitute for a glass tube. Two or three straws spliced together give a long enough tube. Q-tip cotton swabs can be used to introduce the reactants into the tubes. One drop of concentrated reactant on each swab is enough.
A Q-tip, pulled through the tube with some thread or dental floss, will clean out the tube well enough for it to be used for second, third, and fourth trials. (This activity was adapted from CHEM13 NEWS, #203, April 1991, page 10, based on an idea contributed by Larry Benton Collins of Arlington, TX)
Get a short piece of plastic pipe which can be used to join two balloons together. Blow one balloon to about two thirds of its capacity and the other to about one third capacity. While holding the stems to prevent air loss, attach each of the balloons to the pipe. Predict what will happen when the stems of the balloons are released. Create an explanation of this phenomenon.

Sample ideas for evaluation and for encouraging thinking

Why is there a particular volume which one mole of any gas measured at some stated temperature and pressure occupies? Is there a volume which one mole of any metal measured at a stated temperature and pressure occupies?
V=kT is sometimes referred to as Charles's Law and sometimes as Gay Lussac's Law. PV=k is sometimes referred to as Mariotte's Law and sometimes as Boyle's Law. Why are there double names for these realtionships?
In the relationship V=kT, what are the units of the constant k? Why do constants appear in all gas law expressions, or indeed in any mathematical expressions which express the relationships of physical or chemical measurements?
Scuba and deep-sea divers take precautions when surfacing to avoid problems caused by dissolved gases in their blood and the gases in the alveoli of their lungs? What precautions do they take? What makes these precautions effective? What properties of gases make caution when diving essential?
Suppose a grade six student came to you and asked you to explain why balloons get larger when they are heated. Construct an explanation for the student. Describe any models, analogies or diagrams you would use during the explanation.
Suppose a grade six student came to you and asked you why hot air balloons rise when they are heated. Construct an explanation for the student. Describe any models, analogies or diagrams you would use during the explanation.
A motorist checking the tires of his car before starting out on a trip from Biggar to Kyle noticed that one tire was a bit low. Its pressure was 180 kPa, but he decided to go anyway. When he got to Kyle his sister pointed out that the tire was low. He measured the pressure again and discovered it was 205 kPa. What could have caused the change in tire pressure?
Is 180 kPa a low pressure for tires?