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Chemistry Honors/AP

Chemistry 13: 1-2

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WebLecture

Solutions and Their Behavior

Outline

Key Equations

PurposeFormulaVariables
Concentration as molalityc = n/mc: concentration in molality
n: moles of solute
m: mass of solvent in kg
Concentration as molarityC = m/vC: concentration in molarity (molar)
n: moles of solute
v: liters of total soltion
Mole fraction of component AXA = nA / (nA + nB + nC...+nn)

Solutes, Solvents, and Solutions

Units of Concentration

We tend to think of solutions as the result of dissolving a dry powder like salt in a liquid like water, but the term actually applies to a wide range of chemical situations, solid, liquid, and even gas! Metal alloys like bronze are solid solutions: bronze is a case where tin is dissolved into copper to form a homogenous solution as liquids, then allowed to cool and solidify. Oxygen dissolves in water, which is a good thing, or fish would not be able to survive.

A solution is a homogeneous mixture composed of only one phase. This means that when we dissolve a solid or gas solute into a liquid solvent, the solute takes on the liquid phase of the solvent. Since a solution is a mixture, its components may be separated by physical, rather than chemical means. We can recover common salt (solute) from a saltwater solution by evaporating the water (solvent). However, interesting chemical reactions occur in solutions: the dissolution of ionic substances into their ions is one such reaction. As we shall see, ions in solution are the basis of electrical systems that depend on chemical batteries.

Chemists have different ways of expressing the concentration of a solute in a solution, depending on what they want to do.

The Solution Process

As with any process, we need to examine the energy flow into and out of the solution system when a solute dissolves in a solvent.

A common example is the dissociation of an ionic compound (a salt) in water. This is a two-step process.

  1. Dissociation occurs first as the bonds between the ions are broken. This usually involves energy input as work is done to separate the mutually attracted ions.
  2. Hydration occurs as the ions form bonds with polar water molecules.
The enthalpy of solution is the net change in overall energy of the system between the dry salt + water state and the dissolved salt in solution state. While both breaking ionic bonds and forming H-bonds with water molecules may involve large amounts of energy, the difference between the two steps is often very small, and slightly exothermic — meaning that most reactions where a salt dissociates will be spontaneous and product-favored.

Not every substance dissolves easily in a solvent. A good rule of thumb is that "like dissolves like", where "like" refers to the polarity of the substances involved. Water is a good solvent because as a polar covalent substance, it can readily dissolve other polar covalent or ionic (the extremity of polar covalence) substances. It cannot readily dissolve non-polar substances. We say that non-polar substances are immiscible in water.

Another common situation involves fats and acids. Oil and vinegar don't mix well, leading to interesting taste sensations in salad dressings, and the classification of salad dressing as a suspension of oil droplets in vinegar.

Practice with the Concepts

Molality is used to indicate final concentration when we prepare a solution.

What is the molality of the solution when we dissolve 5.844g table salt (NaCl) in 1.0 L water?

Optional Readings