Exploring our world: The energy of molecules

Why do different states of matter have different densities?

In the accompanying article, “Exploring our world: Why does ice float?,” we explored density and the relationship between mass and volume. But why do different states of matter have different densities?

Matter is made up of molecules. All molecules have energy and are in constant motion, they vibrate. This vibration is the energy of the matter and can be measured as temperature. Most matter expands when heated, and when cooled it contracts. The more energy something has, the higher its temperature and the more its molecules are vibrating. When molecules vibrate more, they tend to take up more space. The reverse is also true for most matter; as the matter cools it vibrates less and generally takes up less space. You can help youth explore the energy of molecules.

You’ll need a small box, such as a shoebox, and enough marbles to make a loosely packed single layer in the bottom of the box. Each marble represents a molecule. Slowly move the box side to side so the marbles move slightly. Next, slowly move your finger through the marbles, keeping them in one layer. This is a model of how molecules behave as a liquid.

Next, move the box more vigorously side to side. Some marbles should break free of the layer and move on top of the other marbles. By moving the box more vigorously, you added energy to the “liquid” and the molecules, or marbles, vibrated more. They vibrated with so much energy that some of the molecules broke free of the liquid layer much like molecules do when they change state to a gas. When you stop moving the box, the energy reduces and the “gas” molecules condense back to their liquid state as the marbles settled back into a single layer in our model. Models are a great way to help youth understand their world and are included in the new Michigan Science Standards.

Another way to explore density with youth is to take a group of youth and explain each of them represents a molecule. Ask them to stand in a group with their hands on their hips. The youth are to move around but must remain in contact with the other molecules, the other youth. Explain this represents a liquid as you gently push your way between the youth. We can easily move through or stir liquids. Next, have the youth move farther apart as they keep moving and occasionally touching; this is a gas. Finally, ask the youth to pack tightly together and then attempt to move between them; this is a solid. Tap a table or other solid and explain it is not possible to move through a solid because its density increased. Ask the youth “Are you still moving?” The answer is yes; they are fidgeting and breathing. Even though it appears that a solid is not moving, the molecules are still slowing vibrating. The colder a solid is the less the molecules are moving.

Throughout this explanation the youth remained the same – so their mass was constant, but their volume or the amount of space they took up as a group changed as their energy seen as movement changed. As a gas they took up the most space and had the most energy, and as a solid they took up the least space and had the least energy. Solids generally have a smaller volume, but greater density than gases or liquids.

Using models is one of the eight Science and Engineering Practices identified by the National Research Council in “A Framework for K-12 Science Education as the best practices for engaging youth in science. To learn more about helping youth use models, read “Helping youth succeed in science – Part 3: Developing and using models,” part of a Michigan State University Extension series about the eight Science and Engineering Practices.

You can help youth explore their world by helping them discover answers to questions. For more ways to share science with youth in your life, please explore the MSU Extension Science and Technology website. For more information about 4-H learning opportunities and other 4-H programs, contact your local MSU Extension office.

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