What properties do gases have? | 16-18 years

In this activity, students explore key points about gases and their properties through a diagnostic task and a series of simple experiments.

Gases are a frequent source of misconceptions for students, even at post‑16 level.

Learning objectives

Students will know and understand that:

• Gases have mass.
• The space between gas particles is empty.
• Gases can be formed as products in chemical reactions.
• Gas particles can form bonds between them under certain conditions.

Sequence of activities

Introduction

1. Ask students to name gases they know.
2. Ask about the properties of these gases, for example, how do they behave?
3. Compile responses on a board, and keep for future reference.
4. Explain that the objective of the activities is to learn about the basic properties of gases.

Diagnostic task

Organise the class into groups of three or four. Give each student a copy of ‘What do you know about gases? Part 1’.

Circulate and support as students:

• Work through the diagnostic tasks on the sheet.
• Agree answers to the tasks.
• Select a spokesperson to feedback to the whole group.

Plenary 1

In a plenary:

• Enable each group spokesperson to give feedback.
• Invite students to explain why they think as they do.
• Review responses to the tasks.

Correct answers may be presented at the end.

Experiments

1. Introduce the experiment circus, during which the students investigate aspects of gas behaviour. Explain the procedure.
2. Give each student a copy of ‘What do you know about gases? Part 2’.
3. Supervise the students as they circulate around the six different experiments.
4. When they have all completed the six experiments, indicate that they should return to their groups to discuss their results.
5. Circulate and support as the groups:
   • Discuss answers to the questions.
   • Review responses to the diagnostic tasks.
   • Prepare to present answers to the class.

Plenary 2

In a plenary:

1. Allow each group to give its responses to the questions about the six experiments.
2. Review the responses to the diagnostic tasks.
3. Introduce induced dipole–dipole bonds between gas particles as responsible for some gas properties.
4. Collate new ideas about gas behaviour and compare these with the original list.
5. Invite students to summarise what they have learned about gases.

Feedback

On submitted work, give written feedback indicating the quality of thinking and any developmental points.

Commentary

The initial discussion is designed to open up what students know about gases and the basis for this.

The diagnostic tasks will reveal misconceptions. When carried out in groups, students need to listen to opposing points of view and be able to reason from scientific principles. This informal questioning helps students to gauge their understanding. Key questions with the experiments focus thinking. In the plenary, students give and receive feedback which will increase their confidence about their learning.

The written summary can reveal any lingering misconceptions. These can be addressed in the written feedback.

Practical notes

Equipment and chemicals

• Eye protection

Dissolving a tablet

• Beaker, 250 cm³
• Access to water supply
• Piece of effervescent tablet, eg Alka-Seltzer

Squashing air in a syringe

• Plastic syringe – size is not important, but larger syringes give more impressive effects

Perfume diffusion

• Air freshener or similar
• Stopwatch or other timing device
• Long tape measure to measure 10 m

Cooling air in a freezer

• Balloons
• Access to a freezer

Putting out a lighted candle

• Conical flask, 250 cm³
• Tea lights (small, metal-encased candles)
• Beaker, 1 dm³
• Matches
• Calcium carbonate chips, about 100 g
• Hydrochloric acid, 2 mol dm^-3 (IRRITANT), about 750 cm³

This is best set up in a draught-free area such as a fume cupboard.

Inflating a balloon

• Conical flask, 250 cm³
• Measuring cylinders, 50 cm³, x2
• Balloons to fit over the mouth of the conical flask
• Access to a balance weighing to 0.01 g
• Hydrochloric acid, 2 mol dm^-3 (IRRITANT), about 500 cm³
• Sodium carbonate solution, 2 mol dm^-3 (IRRITANT), about 500 cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• It is the responsibility of the teacher to carry out appropriate risk assessments.

Answers

What do you know about gases? Part 2

Dissolving a tablet

1. The gas did not exist – the atoms that make up the gas molecules were present in other molecules. The gas molecules were formed in the reaction.

Squashing the air in a syringe

1. The plunger springs back to release the air pressure built up inside the syringe.
2. The particles are moving fast and there are many in a small space.
3. The air would solidify.

Perfume diffusion

1. The particles of perfume are bombarded by air molecules and so are moved along in currents.
2. The perfume particles are large relative to the air molecules, so move slowly.
3. By using a fan to increase the movement; by heating the air.

Cooling air in the freezer

1. The air particles inside slow down as the temperature decreases, reducing the air pressure inside the balloon.
2. The heat of the room increases the kinetic energy of the air particles, creating more movement and hence more air pressure, and so expanding the balloon.
3. The air particles would stop moving completely.

Putting out a lighted candle

1. The gas produced in the reaction does not support combustion.
2. The gas was made in the reaction between the acid and marble chips.
3. The gas is denser than air. Other gases are the same density or less dense than air, so cannot be ‘poured’ in this way.

Inflating a balloon

1. The gas did not exist but was made in the reaction.
2. The mass reading stays the same. The gas is collected in the balloon and has mass, so when it is produced no mass change occurs.
3. The mass would decrease because the mass is no longer being recorded by the balance as the gas molecules are able to mix with the air.