Read Sections 6.1 and 6.2 before viewing the slide show.

Unit 21 Gases, Liquids, and Solids and Their Transitions and Characteristics Identification of Molecular Level Differences (6.1) Terminology of Phase Transitions (6.1) Phase Diagrams (6.1) Supercritical Fluids (6.1) Comparison of Ionic and Molecular Compounds (6.2)

Identification of Molecular Level Differences (6.1) There are fundamental differences at the molecular level between gases, liquids and solids Gases have particles that are far apart and moving rapidly and randomly in their container Liquids have particles that are fairly close together but can still move past each other relatively slowly Solids have particles that are fairly fixed in position but can vibrate about those positions. To get an impression of these differences, work with the PhET simulation called States of Matter found at Either download it to your computer by hitting the blue download button or run it directly by hitting the green Run Now button at the site. To see the molecular level differences between states, hit the Solid, Liquid, and Gas buttons to the right side of the simulation. How does the appearance of the atoms compare to the descriptions of the three states above?

Simulation Questions (6.1) For the following, continue using the States of Matter simulation at: Place the state to Solid and increase the temperature by raising the slider at the bottom of the simulation. What do you observe about the atoms as the temperature increases? What do you observe about the atoms as the slider is moved down to cool the container? This is a molecular level representation of what happens during processes with which you are already familiar: Melting – conversion of solid to liquid (when you heat the solid) Vaporization – conversion of liquid to vapor (when you heat the liquid) Condensation – conversion of vapor to liquid (when you cool the vapor) Freezing – conversion of liquid to solid (when you cool the liquid) Sublimation – conversion of a solid directly to a vapor (cannot see this one with the demonstration – dry ice is a typical example of a substance that sublimes under normal conditions)

Transition Temperatures (6.1) If you were asked the boiling point of water your response would likely be 212 ºF (or 100 ºC ). Similarly for the freezing point the response would likely be 32 ºF (or 0 ºC). Those values, however, depend on the pressure at which they are measured. Consider the boiling point, for example. You already know that liquid water can be converted to vapor at room temperature – consider the fact that water evaporates in a glass at room temperature. What makes 100 ºC special? Consider the following: Some water molecules are always escaping into the vapor state and some are always condensing into the liquid state. When the rate of the escape equals the rate of the condensation, the system is said to be at equilibrium. The pressure of the molecules above the liquid at equilibrium is called the vapor pressure and its value depends upon temperature. The boiling point occurs when the vapor pressure equals the pressure being exerted on the system. The typically quoted boiling point for water of 100 ºC is the temperature at which the vapor pressure of water is 1 atmosphere. For any material, the normal boiling point is the temperature at which its vapor pressure equals 1 atmosphere.

Phase Diagrams (6.1) The relationship between pressure and temperature is often summed up in a figure called a phase diagram, such as that to the right for water. A few key observations: Three main regions are labeled – solid, liquid, and vapor. The lines represent equilibrium lines between the phases separated by the lines. For example, the BD line is the equilibrium line between liquid and vapor. Notice that water achieves a vapor pressure of 1 atmosphere (101.3 kPa) at 100 ºC – the normal boiling point. At lower pressures, water will boil at a lower temperature and at higher pressures a higher temperature. The triple point (point B) occurs at one temperature and pressure. At this point, all three phases are in equilibrium. Point D represents the critical point. Above the temperature of 374 K no distinct liquid phase can exist – only a dense vapor state exists. The temperature at point D is called the critical temperature and the pressure is called the critical pressure. Image from

Supercritical Fluids (6.1) A fluid that is confined at temperatures above its critical temperature and pressure is called a supercritical fluid. Some supercritical fluids actually have practical uses. CO 2 is one of the most important supercritical fluids. Examples of its use: Supercritical CO 2 is used to extract caffeine from coffee beans. It is environmentally safe, relatively cheap, and easily recycled for reuse. Supercritical CO 2 can be used in dry cleaning to replace some chlorinated solvents Supercritical CO 2 is used as a degreaser. Supercritical CO 2 can be used as an extracting solvent for a wide variety of chemicals. Notice as the temperature goes up the line separating liquid and gas phases disappears until it is one dense gas. Image from

Comparison of Ionic and Molecular Compounds (6.2) Recall that ionic compounds form by the transfer of one or more electrons from a metal to a nonmetal, such as in NaCl. In molecular compounds electrons are shared, such as in water. NaCl H 2 O Notice the sodium ions and chloride ions are not actually bonded to each other – they are strongly attracted by the “+” and “-” charges, but are not connected by bonds. Water, on the other hand, consists of separate molecules – H 2 O – with two hydrogen atoms actually bonded to each oxygen atom. The different nature of ionic and molecular compounds leads to several important property differences. Image from Image from

Comparison of Ionic and Molecular Compounds (6.2) A summary table of the differences in properties between ionic and molecular compounds is given below. Ionic CompoundsMolecular Compounds Almost all are solids at room temperature Many are solids at room temperature, but some are liquids or gases High boiling and melting pointsLower boiling and melting points High energy required to melt – about times that of molecular compounds Much lower energy required to melt Solutions of ionic compounds in water conduct electricity Solutions of molecular compounds do not conduct electricity Most are crystalline, hard, and brittle Solids are usually much softer than ionic compounds

End of Unit 21 Slide Show