Physical Behavior of Matter Review

Matter is classified as a substance or a mixture of substances.

The three phases of matter, i.e., solids, liquids, and gases, have different properties. Drawing and interpreting particle diagrams are favorite activities here.

A substance (element or compound) has a constant composition and constant properties throughout a given sample, and from sample to sample. Distinguishing mixtures from pure substances is a common question.

Elements are substances that are composed of atoms that have the same atomic number. Elements cannot be broken down by chemical change.

A mixture is not a substance because it is made up of two or more different elements and/or compounds. The proportions of components in a mixture can be varied. Each component in a mixture retains its original properties.

Differences in physical properties such as mass, particles size, molecular polarity, boiling point, and solubility permit physical separation of the components of the mixture. Sometimes they ask about separating mixtures. Separations exploit differences in physical properties: solubility (filtration or chromatography), boiling point (distillation)

Energy can exist in different forms, e.g., chemical, light, heat, nuclear. Energy conversion questions are common: chemical potential energy to kinetic energy (exothermic rxns); chemical potential energy to electrical (voltaic cells); electrical to chemical potential energy (electrolytic cells) Nuclear energy to kinetic energy (fission and fusion)

Heat is a transfer of energy (usually thermal energy) from a body of higher temperature to a body of lower temperature. Thermal energy is the energy associated with the random motion of atoms and molecules. Thermal energy = kinetic energy.

Temperature is the measure of the average kinetic energy of the particles in a sample of material. Temperature is not a form of energy. The bolded sentence above is tested somewhere on every single exam!!

Kinetic molecular theory (KMT) for an ideal gas states: All particles are in random, constant, straight-line motion. Gas molecules are separated by great distances relative to their size; the volume of the gas molecules is considered negligible. The molecules have no attractive forces between them. Collisions between gas molecules may result in the transfer of energy between gas particles, but the total energy of the system remains constant.

A consequence of statement b) is that equal volumes of different gases at the same temperature and pressure have the same number of molecules. Real gases have significant attractive forces between them and/or are not separated by great distances relative to their size. High pressures and low temperatures will create these conditions for gases. Under conditions of life on earth, H 2 O behaves as a real gas, while N 2 and O 2 behave ideally.

KMT Questions

Particles are in constant motion except at absolute zero (zero Kelvin). Kinetic molecular theory describes the relationships of pressure, volume, temperature, velocity, and frequency and force of collisions.

Ideal Gas Equation You need to be able to use the ideal gas equation (Table T). Any variable that is a constant can be dropped from the equation. Free response questions usually only require setting up the equation, not solving for it. Remember, Kelvin temperature must be used for all gas law calculations!!! The formula for Celsius – Kelvin conversion is also on Table T.

Ideal Gas Law Questions

The concepts of kinetic and potential energy can be used to explain physical properties that include: fusion (melting), solidification (freezing), vaporization (boiling, evaporation), condensation, sublimation, and deposition.

Sublimation questions are common, deposition is rare.

Heating and Cooling Curves Most questions are variations on heating and cooling curves. The horizontal lines are the phase changes. Be sure you know which one (melting- freezing or boiling- condensing) is which on the diagram.

At the melting point, solid and liquid phases are in equilibrium with each other. At the normal boiling point, liquid and gas phases are in equilibrium.

A physical change results in the rearrangement of existing particles in a substance. A chemical change results in the formation of different particles with changed properties.

Particle diagrams are commonly used on the regents to demonstrate phase changes.

Chemical and physical changes can be exothermic or endothermic. Table I shows heats of reaction (ΔH) for a number of combustion reactions, a bunch of synthesis reactions (often called the heat of formation), and some solubility changes (a physical change). There is usually one question using this table on every test. Often they ask for which reaction is endo/exothermic. Three have to have the opposite sign to the correct answer.

Which Reference Table Do I Use?

Phase Changes of Water Physical changes of water are given special attention. Information can be found on Table B. You should know when and how to use the “heat” equations on Table T. For phase changes, q=mH f and mH v. For liquid water temperature increases and decreases, q=mcΔT is used.

The structure and arrangement of particles and their interactions determine the physical state of a substance at a given temperature and pressure.

Intermolecular Forces Intermolecular forces created by the unequal distribution of electrons result in varying degrees of attraction between molecules. Hydrogen bonding is an example of a strong intermolecular force.

Small molecules that have equal or symmetrical charge distributions are non-polar, and are gases to very low temperatures and pressures (that is, they behave ideally under most conditions). Larger molecules, even those with symmetrical charge distributions, have larger attractive forces and higher boiling points and melting points.

The boiling point (distillation) is used to separate crude oil (a mixture of hydrocarbons) into fractions based on molecular size. Unequal distribution of charge is a euphemism for a polar molecule. These molecules have dipoles [partial (+) and (–) parts of the molecule]. For similar molecules, the larger the dipole, the higher the melting point and boiling point are.

Hydrogen Bonding Hydrogen “bonding” is the strongest dipole force. Only N, O, and F form hydrogen bonds. Although it may seem obvious, hydrogen bonds are only between these elements and hydrogen: N – H, O – H, and H – F. Remember, hydrogen bonding is not a true “bond”; it has 5% of the strength of a covalent bond.