Unit #1

Activity 8
A Matter of Stating the State of Matter

Activity Overview

The compressibility of a substance indicates the space, or lack of it, between the atoms or molecules of various states of matter.

Part A: Students put air and then water in a sealed syringe. In each case they try to compress the substance, and then discuss why gas is compressible and liquid is not.

Part B: Students view a computer model of a solid, a liquid, and a gas, and then discuss how this model helps to explain their previous experiment.

Learning Objectives

Students will:

• Diagram the atomic level structure of a solid, liquid, and gas.
• Describe the state of motion of atoms in a solid, liquid, and gas.
• Predict if a substance is compressible or not.
• Compare the macroscopic characteristics with microscopic characteristics for the three phases.

Conceptual Prologue

Macro-Micro Connection

Until this point, everything students have been studying and modeling has been in the gas phase. The hot air balloon is not just hot air. Without the fabric of the balloon to contain the hot air it would not fly. Without being able to carry liquid fuel, a hot air balloon ride would be short lived. So now, students will extend their studies to include other states of matter.

Other macro connections:

• The compressibility of gasses in the experiment can be explained through an understanding of the underlying structure.
• Gasses are used in some shock absorbers to act as a spring while liquids are used in hydraulic equipment because they won’t change volume under pressure.

Science Concepts

If a gas is put inside a syringe and the end is sealed, one can significantly depress the plunger of the syringe, compressing the gas inside. If a liquid is put inside the syringe instead of a gas, the plunger won’t be able to be depressed. The liquid resists being compressed into a smaller volume.

The results of these experiments demonstrate the fact that there is space between the molecules of a gas and very little or no space between the molecules of a liquid.

The atoms or molecules of a solid often form a crystal pattern [geometrically symmetrical packing of particles]. The atoms or molecules are generally spaced as closely as possible and vibrate in place. Because of this close packing, solids can’t be significantly compressed either.

The atoms or molecules of a liquid are also generally spaced as closely as possible. However, the atoms or molecules tend not to stay in one place. They slide by each other allowing the liquid to conform to its container. Because of this close packing, liquids can’t be significantly compressed.

Gasses have, comparatively, a great deal of space between their atoms or molecules. Because there is so much empty space between gas molecules, the volume of gasses can be significantly compressed, moving the molecules closer together. If you compress a gas enough you can form a liquid. This is, in fact, how many "gasses" are stored, because a very large amount of gas can be compressed into a small volume. Gasses generally take up 1000 times the volume of that same substance in the liquid state.

Naive Conceptions

• Liquids have atoms or molecules that are much further apart than solids.

Atoms or molecules of a liquid are packed almost as closely as that same substance in the solid phase. The main difference between the two is that the atoms of a solid vibrate around in place while the atoms of a liquid can flow by each other.

• Gasses consist of a continuous substance.

Students will often maintain that a gas can be made of atoms but that there must be something between the atoms. The idea of total vacuum or "nothingness" between the atoms is often hard for them to believe.

Activity Design and Execution

Major Science Concepts:	• states of matter
Assumed Previous Knowledge:	•That substances are made of atoms which are in continual motion.
Time:	• Part A: approximately 20 minutes • Part B: approximately 30 minute
Materials:	For each group: • A syringe with no needle. • Water.
Advanced Preparation: (if any)	• None

Investigative Question: What is the difference between a solid, liquid, and gas?

Part A:

1. Have students fill a syringe with air and block the end of the syringe with their finger. Ask them to compress this gas as much as they can.
2. Have them do the same thing but put water in the syringe, making sure to remove any air before trying to compress the water.
3. Ask the students to explain why they think the gas was compressible and the liquid was not.

Part B:

1. Using Workbench software have students open the "States of Matter" activity.
2. The software will then prompt them to click on one of the substances to zoom down to the atomic level. This will give students a picture of what each state of matter looks like at the atomic level.
3. The software will then prompt them to highlight one atom and observe its movement.
4. After viewing all states of matter students will be asked to describe the difference in the motion of single atoms within each state.
5. Revisit the discussion of why the syringe behaved the way it did in the previous experiment.

Assessment

Have students write several things in their notebooks:

1. Pick one substance of your choice from each state of matter. Then draw a diagram and write a description of what this substance looks like at the atomic level. Predict if this substance should be compressible or not. Explain why by referring to your diagrams.
2. On a microscopic scale atoms look to be just about as crowded in a liquid as in a solid. What are some of the ways you can think of to decide whether a substance is solid or liquid?
3. If you wait long enough, then some materials that seems solid will flow very slowly like a liquid. How can you explain this?

Internal Notes:

• See computer lab D for mock up of part B.