Thermal Energy and Particle Motion

The science background provides teachers with more in-depth information on the phenomena students explore in this unit on discovering matter. Below is an excerpt from the science background section.

Energy Changes Matter

Matter can only change when enough energy is present. This is why you need a pot of boiling water to cook pasta. Energy is the ability to do work. Work is any change in position, speed, or state of matter due to force (a push or pull that acts on an object, changing its speed, direction, or shape). Examples of work include heating an object or moving an object.

Energy can either be stored or in motion. Energy that is stored is called potential energy. The energy of motion is called kinetic energy.

Energy is never created or destroyed, but it can change from one form to another. For example, all matter has a form of chemical potential energy that is stored in the bonds holding together atoms and molecules. That chemical energy is what allows new molecules and compounds to form. We’ll explore this idea more a little later. That chemical energy can also be transferred. For example, whenever we eat food, we absorb some of the food’s chemical energy.

All matter also has a form of energy called thermal energy, which is the motion of atoms and molecules in a substance or object as its temperature increases. Temperature is a measure of heat and is measured in Celsius with a thermometer. Heat is not the same as thermal energy. Heat refers to the thermal energy transferred from one object to another. The faster that atoms and molecules move, the more thermal energy they have.

The amount of thermal energy present in a substance determines whether that substance is a solid, liquid, or gas. For example, ice cream is a solid because it keeps its own shape until something changes it. This is because solids have the least amount of thermal energy. As a result, the atoms in a solid are closely packed together. They are always moving, but because of how close they are, they can only vibrate in place. They cannot move past one another.

Changing State

When thermal energy is added to solids, the atoms or molecules begin to move more quickly. When enough energy is added, they will expand and become a liquid. This is what happens when you leave ice cream out on a hot day. The ice cream absorbs thermal energy from the environment, causing the ice cream to melt. The temperature at which a substance changes from a solid to a liquid is called its melting point.

Water and milk are both examples of liquids. The atoms in a liquid are less tightly packed. They are in constant contact with one another, but they have enough energy to slide past one another. Therefore, matter in a liquid state takes the shape of its container but has no shape of its own. When thermal energy is removed, the kinetic energy of the atoms and molecules decreases, causing them to slow down. When enough thermal energy is removed, the liquid will turn into a solid. This is called its freezing point. For example, water freezes when the temperature reaches 0 degrees Celsius (32 degrees Fahrenheit).

When enough thermal energy is added to a liquid, the atoms or molecules will move so quickly that the liquid will expand, becoming a gas. This is called a substance’s boiling point. The molecules in a gas have so much energy that they move far apart and bounce around randomly. As a result, matter in a gas state has no shape and spreads out into space. If a certain amount of gas is taken out of one container and placed into a container that is twice as large, the gas will expand to fill the larger container.

Different substances have different melting points depending on how much energy is needed to change them from a solid to a liquid. This in turn depends on the kind of atoms that make them up.

Energy Transfer

Cold water in a pot eventually heats up and starts to boil when enough thermal energy is added. This happens because energy can transfer into or out of systems or objects.

Heat is energy that has transferred whenever two substances are at different temperatures. Heat always flows from faster moving atoms (a higher temperature) to slower moving atoms (a cooler temperature) until both substances reach the same temperature. This point is called equilibrium.

Conduction

Conduction is heat transfer that occurs when molecules collide. Remember that pot of water on a stove? When the stove is turned on, it heats up the surface of the pot. The warmer molecules of the pot collide with the cooler water molecules. This causes energy to transfer from the pot molecules to the water molecules.

This transfer of energy causes the kinetic energy (motion) of the water molecules to increase. When enough thermal energy is transferred into the water, the water will begin to boil and eventually turn into water vapor, a gas. This happens when its atoms speed up to the point where they spread out, filling the room.