Key Idea: There are differences in the spacing, motion, and interaction of atoms and molecules that make up solids, liquids, and gases.

Students are expected to know that:

  1. The particles of a gas (atoms or molecules) are much farther apart than the atoms or molecules of a liquid or a solid. Because atoms or molecules of a gas are so far apart they rarely come in contact with each other. In solids and liquids, the atoms or molecules are packed closely together.
  2. Although the motion of atoms or molecules of a solid is severely restricted, the atoms or molecules are constantly moving back and forth in all directions with respect to a fixed position. In liquids, the motion of the atoms or molecules is limited but the atoms or molecules can still move rapidly back and forth with respect to a fixed point, around a fixed point, and past each other from one fixed point to another. Atoms or molecules of a gas move freely and spread out throughout the container they occupy.
  3. Similar to the pushes and pulls between magnets, atoms or molecules also push and pull on each other.
    1. In solids, the atoms or molecules pull strongly on each other and are linked together in rigid structures.
    2. In liquids, the atoms or molecules pull less strongly on each other, are more loosely connected, and form less rigid structures.
    3. In gases, the pull between atoms or molecules is so weak that they do not form structures.
  4. Hardness, flow, and compressibility are macroscopic properties for which students are expected to know the molecular reason.
    1. Solids are hard because the atoms or molecules are linked together tightly in rigid structures, not because the individual atoms or molecules are hard.
    2. The atoms or molecules of liquids can easily move past each other from one fixed point to another but do not move apart from one another and that this is why liquids flow and take the shape of their container but solids do not.
    3. Gases can be compressed because the atoms or molecules are not as close together as they could be, not because the individual atoms or molecules are soft.

Boundaries:

  1. Students are not expected to know the nature of the bonds between atoms or between molecules.
  2. They are not expected to know that atoms or molecules of a solid can sometimes move past each other.
  3. Students are not expected to know the terms rotation, translation, and vibration.
  4. They are not expected to know that plasma is a distinct state of matter, and they are not expected to know the properties of a plasma.