• States of matter: gas, plasma, liquid, solid and Bose-Einstein condensates.
• Solid subgroups: crystals, glasses, polymers
• Liquid crystals
• Changes of state induced by temperature or pressure changes

• The various type of bonding (discussed in our last lecture) hold pairs or groups of atoms together, and in turn, provide bulk properties to large groups of atoms. The groups of atoms become organized into distinct states of matter. These states include gas, plasma, liquids and solids and Bose-Einstein condensates.
• The state of matter adopted by a group of atoms depends only upon three things, their elemental chemistry, the pressure and the temperature.
• Plasma: At extreme temperatures like in the sun, the energy is so high that the electrons do not remain attached to the atoms, and so charged nuclei are in a sea of electrons. Behaves a little like a gas, but, because of the free electrons, they can be confined inside of a strong magnetic field. 99.9% of the mass of the universe is in this state.
• Gas: any collection of atoms that is able to expand to take the shape and fill the volume of its container. Made up of inert atoms or molecules that are neutral and do not bond with their neighbors. Individual atoms or molecules bounce off of their neighbors.
• Liquids: A collection of atoms that has no fixed shape, but is able to maintain its volume. Usually made of molecules like the gases, but with weak intermolecular forces (polar) that hold them together. These weak forces are the source of surface tension. Surface tension is the force that separates a liquid from the air.
• Solids: groups of atoms that maintain both shape and volume. Bonding is strong and directional. Subgroups are crystals, glasses, and polymers.
• Crystals: The atoms are arranged with translational periodicity. Unit cell. E.g. gemstones, sand, salt. Most natural rocks are formed of interlocking smaller crystals. Your bones are crystalline.
• Glasses: no long-range translational periodicity. You can predict nearest neighbor bonding but not out further. In the example below we see a cartoon of the atomic structure of crystalline silica (made of silicon and oxygen) on the left, and silica glass on the right. Note that the crystal is very symmetric and its atoms display translational periodicity while the atoms in the glass are arranged in a more random fashion.

• Polymers: made of long molecules, like chains. The molecules are held to each other by polar bonds. E.g. hair, cotton, spider webs, plastics.

• Bose-Einstein Condensates: A recently discoved new state of matter. A collection of atoms that are cooled and compressed to the point that every electron is considered part of the entire collection and not associated with an individual atom. This is somewhat like plasma. However, all the nuclei are also interdependent, undergoing thermal vibrations as a group, behaving like a single entity. Extra stuff.

• E.g. liquid crystals: These are an interesting material and we are seeing their uses all over the place. For instance your watch. The liquid crystals are what are used to display the numbers.
• Liquid crystals are made up of long straight polymers that, in principle, can be placed in any position or orientation. However, if many are packed together then they become aligned and form a spaghetti-in-the-box type of arrangement.
• Some of these molecules can be made so that they are polar. In which case, if they are subjected to an electromagnetic field, then they will become aligned with this field. Otherwise, they are randomly oriented.
• So watches are made with these crystals all packed into predetermined little cells. Each cell can be electrically energized. In such a case, the polymers become aligned.
• Now then, the glass cover of your watch is made of a Polaroid material. This means that light can only pass through if it has a certain orientation. We apply an electric field and the liquid crystals become aligned perpendicular to the polarization. The result is that the figures come out black.

• Another example of a liquid crystal is provided by cell membranes. These are called lipid bilayers. The lipids are the polymers. They are polar, with one end that does not like to be near water. This end is in the middle of the arrangement, between the layers. It pushes the water out, thus insuring that the collection of lipids behaves like a membrane.

• We are quite familiar with changes in state induced by temperature changes. For instance, put water in the freezer and it freezes to ice. Put water into a pot and heat it and it turns to a gas.
• Temperature causes this behavior by changing the vibrational states of the atoms.
• An ice cube can never exceed 0° C, even if the water around it is above 0° C.
• You can also change state by changing pressure.
• E.g. making diamonds.