Intermolecular Forces Intro

 11.1 States and State Changes

•        Most substances can be in three states: solid, liquid, and gas.

Solid Particles

•        The particles in a solid are very close together and have an orderly, fixed arrangement.

•        Solid particles can vibrate only in place and do not break away from their fixed positions.

•        Solids have fixed volumes and shapes.

Liquid

•        The particles in a liquid are very close together and have a random arrangement.

•        Liquid particles have enough energy to be able to move past each other readily, which allows liquids to flow.

•        Liquids have fixed volumes but can flow to take the shape of the lower part of a container.

•        Liquid particles can have cohesion, attraction for each other.

•       Liquid particles can also have adhesion, attraction for particles of solid surfaces.

•        The balance of cohesion and adhesion determines whether a liquid will wet a solid surface.

•        The forces of adhesion and cohesion will pull water up a narrow glass tube, called a capillary tube.

•        Below the surface of a liquid, the particles are pulled equally in all directions by cohesive forces.

•        The tendency of liquids to decrease surface area to the smallest size possible is surface tension.

Gases

•        The particles in a gas are very far apart and have a random arrangement.

•       The attractive forces between particles in a gas do not have a great effect, so the particles move almost independently of one another.

•       The shape, volume, and density of an amount of gas change depending on the size and shape of the container.

Changing States

•       Most substances can undergo six changes of state: freezing, melting, evaporation, condensation, sublimation, and deposition.

Temperature, Energy, and State

•        Generally, adding energy to a substance will increase the substance’s temperature.

Endothermic (energy in), exothermic (energy out)

•        But after a certain point, adding more energy will cause a substance to experience a change of state instead of a temperature increase.

Liquid Evaporates to Gas

•        Energy is required to separate liquid particles. They gain energy when they collide with each other.

•        If a particle gains a large amount of energy, it can leave the liquid’s surface and join gas particles.

•       Evaporation is the change of state from liquid to gas. Evaporation is an endothermic process.

•        Boiling point is the temperature and pressure at which a liquid and a gas are in equilibrium.

Gas Condenses to Liquid

•        When gas particles no longer have enough energy to overcome the attractive forces between them, they go into the liquid state.

•       Condensation is the change of state from a gas to a liquid. Condensation is an exothermic process.

•        Condensation can take place on a cool spring night, causing water vapor in the air to form dew on plants.

Solid Melts to Liquid

•        As a solid is heated, the particles vibrate faster and faster in their fixed positions.

•        At a certain temperature, some of the molecules have enough energy to break out of their fixed positions.

•       Melting is the change of state from solid to liquid. Melting is an endothermic process.

•       Melting point is the temperature and pressure at which a solid becomes a liquid.

Liquid Freezes to Solid

•        As a liquid is cooled, the movement of particles becomes slower and slower.

•        At a certain temperature, the particles are pulled together into the fixed positions of the solid state.

•       Freezing is the change of state from a liquid to a solid. Freezing is an exothermic process.

•       Freezing point is the temperature at which a substance freezes.

Solid Sublimes to Gas

•        The particles in a solid are constantly vibrating. Some particles have higher energy than others.

•        Particles with high enough energy can escape from the solid.

•        Sublimation is the change of state from solid to gas.

•        Sublimation is an endothermic process.

Gas Deposits to a Solid

•        Molecules in the gaseous state become part of the surface of a crystal.

•        When a substance changes state from a gas to a  solid, the change is often called deposition.

•        Deposition is an exothermic process.