The Handy Chemistry Answer Book. Justin P. Lomont

equal. After some math, one can find a relationship between the number of components of a system, the number of free variables (such as temperature, pressure, or the fraction of a given component present in a mixture), and the number of phases that can be in coexistence. This relationship is:

      F = C – P + 2

      where F is the number of degrees of freedom, C is the number of independent components, and P is the number of phases.

      What is the difference between a homogeneous and heterogeneous mixture?

      A homogeneous mixture is one that is uniformly mixed and has the same proportions of components throughout the mixture. An example is a transparent solution of sugar dissolved in water (specifically one in which there is no undissolved sugar floating around). A heterogeneous mixture is one that is not consistent or uniform throughout, such as a glass of sugar water with some chunks of undissolved sugar also floating around.

      Can multiple liquid phases exist for a mixture?

      Yes. One familiar example is a mixture of oil and water. The immiscible oil and water phases are two different phases of liquid matter.

      Can there be more than one solid phase for a given substance?

      Yes, solids can adopt different types of microscopic arrangements. If there is a repeating pattern to the atoms in a solid, it is called a crystal. If the ordered structure exists for the entire material, then the phase is known as a single crystal (think of a diamond). If a sample is a bunch of individual crystals, then we refer to the material as polycrystalline. Lots of solids also just have no pattern to the arrangement of their atoms, and this class is known as amorphous.

      The arangement of atoms in a solid can take several form, depending on the arrangement of the atoms: A) crystalline, B) polycrystalline, or C) amorphous.

      What is the density of a material?

      Density is the mass (or weight) of a material divided by a unit of volume. Water, for example, has a density of 1.0 g/cm³ (1.0 gram per cubic centimeter).

      What determines the density of a substance?

      At the most basic level, density is determined by how close the atoms or molecules in a substance are packed, as well as the mass of those atoms. While it’s not quite as simple as assuming that the heaviest elements on the periodic table have the highest densities, having a high mass does help: heavy metals like iridium and osmium are the densest metals known to date. Remember that the density of a material does not depend on how much material you have; the density of 1 gram of lead is the same as the density of 1 kilogram of lead. Density is an intensive property, so changing the amount of a material you have does not affect its density.

      Why does ice float?

      Ice floats in water because it is less dense than water, though this is actually a very unusual case in terms of comparing the densities of the solid and liquid phases for a given substance. Most substances increase in density when moving from the liquid to the solid phase of matter, but H₂O does the opposite. When water freezes, it forms a network of hydrogen bonds between H₂O molecules, and because of the spacing of the molecules in this lattice, ice is less dense and floats in water.

      What is temperature?

      Temperature is a measure of the average kinetic energy of particles in a substance. What does that mean? “Average kinetic energy” is a precise way of saying how fast something is moving, in this case on a molecular level. The faster molecules vibrate, the hotter they feel because heat is being transferred from the object to your hands.

      How are the Fahrenheit, Celsius, and Kelvin temperature scales related?

      The Celsius and Kelvin scales use the same size degree (“incremental scaling” is the technical term), but set their zero values at different absolute numbers. Let’s explain that sentence a bit more: If you go up by one degree Celsius or one degree Kelvin, you’ve raised the temperature the same amount, but 0 °C (the temperature at which water freezes) is 273.15 K. Thus the two scales are offset from one another by 273.15.

      Fahrenheit is completely different though. Water freezes at 32 °F, and a change of one degree on the Fahrenheit scale is equal to a change of 0.55 °C.

      A comparison of the Fahrenheit, Celsius, and Kelvin scales.

      What makes metals feel colder than air to the touch?

      Metals feel colder than air when you touch them because they are good conductors of heat. The cold metal is able to conduct heat away from your hand through the entire object quickly, making it feel colder than the air around you.

      What is a boiling point?

      The technical definition of boiling point is the temperature at which the vapor pressure of the liquid phase equals the pressure of the surrounding gas (atmospheric pressure usually). This is a precise way of saying the temperature at which a liquid turns to vapor.

      What molecular properties lead to higher boiling points?

      There are a few factors that play important roles in boiling points of substances. The first is molecular weight: in general, heavier molecules have higher boiling points, which is pretty straightforward considering how we defined boiling point above (heavier molecules take more energy to move from the liquid to the gas phase).

      All other properties that affect boiling point deal with intermolecular forces, or interactions between molecules. Think of this like the affinity of one molecule of a substance to be attracted to, or stick to, another molecule. Noncovalent bonds, like ionic or hydrogen bonds, significantly increase boiling point. Why? Because to move to the vapor phase the molecules typically have to break these interactions. Dipole interactions and Van der Waals forces have similar effects (see “Atoms and Molecules”), but these interactions are weaker, so the effect they have on boiling points is smaller. Finally, branching of the carbon backbone of a molecule is also frequently touted as a factor leading to lower boiling points; while this is true, it is really the weakening of Van der Waals forces at work here too.

      What is a melting point?

      The melting point is the temperature at which a substance changes from the solid state to the liquid state. At this exact temperature, the two phases are in equilibrium, so fractions of the sample are constantly moving between the two phases of matter. In practice, it is pretty difficult to observe the exact melting point of a substance.

      What molecular properties lead to lower melting points?

      Most of the trends we talked about for raising boiling points hold true for melting points for most of the same reasons. There is one major exception, though. The more branched, and therefore compact, a molecule is, the higher its melting point, because, in general, compact molecules will pack better in a crystal lattice. The better packed a lattice is, the more stable it is, and the more heat (energy) it takes to break up that lattice and melt the solid.

      How do the boiling point and melting point of a substance change as a solute/impurity is added?

      The addition of a solute typically raises the boiling point and lowers the melting point of a substance. These effects are appropriately named “boiling point elevation” and “melting point depression.”

      Boiling points are raised when a nonvolatile solute (like NaCl) is added to a solution because the solute lowers the vapor pressure of the solution.