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Liquids and solids are formed by many atoms, ions, or molecules held closely together. In liquids the particles (atoms, ions, or molecules) can move freely but remain adjacent to the other particles. In solids not only are the particles adjacent, but the motion of the particles is limited. In gases (Chapter 8) the particles move freely and independently; that is, without attaching to other particles.

The atoms, ions, or molecules are held together by intermolecular forces. All these forces are based on the attraction of a positive charge to a negative charge. The magnitude of the force (U) is expressed by Coulomb's law

U = k(Q₁Q₂ / d)       (Equation 9.2)

where Q is the charge, d is the distance between the charges, and k is a proportionality constant.

The strongest intermolecular force is ion-ion attraction. This is the intermolecular force in ionic compounds (salts). The force is stronger for ions with higher charges (greater Q) or with smaller distances between ions. The distance between ions depends on the size of the ions and the arrangement of ions, known as the lattice structure. Substances with ionic forces of attraction exist in the solid state at room temperature.

                              >> Explore: Lattice Energy Tutorial

Polar molecules have partial charges and are held to each other by dipole-dipole intermolecular forces. Because these are only partial charges, unlike the full charges of ions, these forces are much weaker. Thus substances with dipole-dipole forces of attraction tend to be gases (occasionally liquids) at room temperature.

When ionic compounds are mixed with polar compounds, they may dissolve. The many small dipoles of the solvent may be more attractive than the other ions of the salt, creating ion-dipole forces. Dissolution will only occur if the ion-ion attraction is sufficiently weak and the ion-dipole forces are sufficiently strong. Because there is both a full and a partial charge in ion-dipole forces, these forces are stronger than dipole-dipole forces, but not as strong as ion-ion forces.>

Another special case of dipole-dipole forces is hydrogen bonding. Hydrogen bonds occur between molecules that have a hydrogen covalently bonded with an oxygen, nitrogen, or fluorine atom. Oxygen, nitrogen, and fluorine are the most electronegative atoms, which therefore attract most of the electrons of the covalent bond. When the other end of that covalent bond is a hydrogen, the electronegative atom is pulling all electrons associated with hydrogen away from it. Hydrogen acts almost as a positive charge. This high charge makes hydrogen bonds a very strong type of dipole-dipole force. Substances which hydrogen bond are typically liquids at room temperature.

                              >> Explore: Hydrogen Bonding in Water Tutorial

Nonpolar molecules are attracted to each other by temporary dipoles, which are called London or dispersion forces. Deforming the electron clouds surrounding the molecules creates temporary dipoles. With more electrons the electron cloud is more easily and more greatly deformed; thus the dipoles are larger and the attraction greater. Molecules with easily deformed electron clouds are called polarizable. Molecules with London forces are normally gases at room temperature.

                              >> Explore: Intermolecular Forces Tutorial

Forces of attraction can be overcome by increasing temperature and thus the kinetic energy (movement) of the particles. Consequently, when the temperature is sufficient to overcome some of the attraction, solids will melt. That temperature is called the melting point. In the reverse direction, energy is removed so that a liquid becomes a solid. This temperature is called the freezing point. The value of the melting point and freezing point is the same. The stronger the force of attraction, the higher the temperature must be to reach the melting point.

Similarly, kinetic energy allows particles on the surface of a liquid to escape the attractive forces and become a vapor. In a closed system, particles both escape to form vapor (gaseous state) and are captured by the liquid. When the number of particles escaping the liquid and the number of particles being captured by the liquid are the same, the system is in equilibrium. The pressure of the gaseous particles in such a system is called the vapor pressure. The vapor pressure depends only on the identity of the substance (attractive forces) and temperature. Substances with stronger attractive forces allow fewer particles to escape and thus have lower vapor pressures. An increase in temperature allows more particles to escape and increases the vapor pressure. When the vapor pressure is the same as the atmospheric pressure, any molecule can escape the liquid and the liquid boils. This temperature is called the boiling point.

Forces of attraction determine the solubility of substances. The forces of attraction between the solute and solvent must be stronger than those between the solute and itself for a substance to be soluble. This often occurs when the forces of attraction are similar, so polar substances dissolve in polar substances and nonpolar substances dissolve in nonpolar substances. In the case of ionic compounds, the solute-solvent forces can overcome the ion-ion forces with numbers. Since there are usually many more solvent molecules than solute molecules, the weaker ion-dipole forces can sometimes overcome the ion-ion forces. When water is the solvent, six waters typically surround each ion. These water molecules are called the sphere of hydration. When two liquids dissolve in each other in all proportions, the solutions are called miscible.

As seen in Chapter 5, solutes will modify the melting and boiling points of a solvent. A solute will also modify the vapor pressure of a solvent according to Raoult's law:

P_solution = X_solventP_solvent       (Equation 9.3)

                              >> Explore: Raoult's Law Tutorial

where P represents the vapor pressure of the solution and pure solvent and X is the mole fraction of solvent. (Mole fraction is defined in Chapter 8.)

Other properties of liquids are surface tension, the resistance to an increase in surface area, and viscosity, the resistance to flow. Both of these are higher with stronger attractive forces and weaker at higher temperatures.

The attraction of the substance to the wall of a container is called the adhesive force, and the attraction of the substance to itself is called the cohesive force. When the adhesive forces are stronger than the cohesive forces, the substance creeps up the sides of the container, forming a curved surface called a meniscus. In capillaries, there is sufficient wall that the substance actually climbs the walls of the capillary until gravity overcomes the adhesive forces. The rising of a liquid up a capillary tube is called capillary action.

                              >> Explore: Capillary Action Tutorial

In addition to temperature, pressure can change the physical state of a substance. A sufficient increase in pressure can force a gas to become a liquid and a liquid to become a solid. The graph of physical states at various pressures and temperatures is called a phase diagram. The phase diagram is used to determine the physical state of a substance for specific conditions of pressure and temperature, and how the physical state changes with pressure and temperature.

                              >> Explore: Phase Diagrams Tutorial

A typical phase diagram has solids at high pressures and low temperatures, gases at low pressures and high temperatures, and liquids in the middle. These physical states are divided with lines. A point on the line dividing liquids and gases is a pressure and temperature where both liquid and gas exist, and the substance boils. The line between liquids and solids represents the conditions where substances melt (or freeze). At low pressures and temperatures, there is a line dividing solids and gases that are the conditions for sublimation (turning a solid to a gas without passing through the liquid phase). At the triple point all three lines meet and all three phases exist. Unlike the melting point line, the boiling point line actually comes to an end at what is known as the critical point. Above this pressure (critical pressure) and temperature (critical temperature), the substance exists as a supercritical fluid. A supercritical fluid has most of the properties of a gas but solvates like a liquid.