4.1 Water in Zero Gravity - an Introduction to Intermolecular Forces If you go far enough out in space, for instance, onto the International Space Station, gravity becomes negligible, and the laws of physics act differently than here on Earth. Just how might water act in a place of zero gravity? 4.2 Intermolecular Forces There are three types of small particles: nonpolar particles, polar particles, and ions. These three types of particles can be attracted to one another in six ways: nonpolar/nonpolar, polar/nonpolar, ion/nonpolar, polar/polar, ion/polar, and ion/ion. The first five types of interactions are considered attractive intermolecular forces (IMFs). The sixth type of interaction (ion/ion) is considered a bond. The classification as a bond or an IMF depends mainly on the strength of the interaction. 4.3 Application of IMFs: Evaporation, Vapor Pressure, and Boiling Points There are many ways in which IMFs manifest themselves in the macroscopic properties of materials. Three closely related physical properties describe the strength by which particles in the liquid phase are attracted to each other: evaporation, vapor pressure, and boiling point. For a set of substances at a given temperature, the stronger the IMFs, the lower the vapor pressure. For a set of substances at a given pressure, the stronger the IMFs, the higher the boiling point. 4.4: Intermolecular Forces in Action: Surface Tension, Viscosity, and Capillary Action Surface tension, capillary action, and viscosity are unique properties of liquids that depend on the nature of intermolecular interactions. Surface tension is the energy required to increase the surface area of a liquid. Surfactants are molecules that reduce the surface tension of polar liquids like water. Capillary action is the phenomenon in which liquids rise up into a narrow tube called a capillary. The viscosity of a liquid is its resistance to flow. 4.5: Solids, Liquids, and Gases: A Molecular Comparison The state of a substance depends on the balance between the kinetic energy of the individual particles (molecules or atoms) and the intermolecular forces. The kinetic energy keeps the molecules apart and moving around, and is a function of the temperature of the substance and the intermolecular forces try to draw the particles together. 4.6 Phase Changes Almost all materials will change phase if the temperature and/or pressure are changed. These phase changes involve a change in potential energy and a change in entropy, a measure of the possible allowed arrangements of the particles of the substance. In this course we will discuss phase changes for solids, liquids, and gases. Keep in mind that it is also possible for substances to have more than one form of crystalline solid (polymorphs), and that a fourth phase, supercrtical fluid, also exists. 4.7: Pressure: The Result of Particle Collisions Gases exert pressure, which is force per unit area. The pressure of a gas may be expressed in the SI unit of pascal or kilopascal, as well as in many other units including torr, atmosphere, and bar. Atmospheric pressure is measured using a barometer; other gas pressures can be measured using one of several types of manometers. 4.8: Gases Because the particles are so far apart in the gas phase, a sample of gas can be described with an approximation that incorporates the temperature, pressure, volume and number of particles of gas in an equation known as the ideal gas law. The simple form of the ideal gas law allows for quick estimations of how gas samples will act under specific conditions, like the use of air to pump water from underground. The key to understanding how such a pump works is realizing that suction is not a force. 4.9 Solutions - What Mixes Together and Why One of the most important applications of intermolecular forces is to use them to predict if substances will form a solution. An often cited adage is "Like dissolves like", which means that nonpolar substances mix with nonpolar substances, and polar substances mix with polar substances. But what about ions? And what about water, which is a very polar substance? 4.10 Factors Affecting Solubility Both temperature and pressure can affect the solubility of a solute in a solvent. An increase in solvent temperature may increase or decrease the solubility of a liquid or a solid solute, depending on whether the solvation process is endothermic or exothermic. An increase in solvent temperature almost always decreases the solubility of a gaseous solute. Pressure changes only affect the solubility of gaseous solutes, and the pressure change must involve the solute gas itself, not any other gas. 4.11 Suspensions and Colloids 4.12 Practice Problems