Solutions are homogeneous mixtures: they are everywhere the same down to the atomic/molecular level
Solutions are stable and do not spontaneously separate
Suspensions are heterogeneous mixtures: on the level of atoms and molecules they are different from one location to another
Suspensions, given time, will separate themselves into the distinct components of the mixture: think muddy water allowed to stand in a bucket for a day or so
The most important difference between solutions and suspensions is the size of the particles; suspensions consist of large particles that do not dissolve and solutions consist of particles which are mixed thoroughly on the smallest scale
Chemists speak of two components in a solution: the solvent and the solute
The solvent is the substance present in the largest amount: the solvent is 50% or more of the solution
Water is the most common solvent we think of; other solvents include tetrachloroethylene (C2Cl4) which is used in dry cleaning and tetrahydrofuran (C4H4O) which is used in organic chemistry
The solute is the substance present in the smallest amount: it is 50% or less of the solution
This distinction may not seem important with a solute like solid NaCl; it is particularly important for solutions of liquids
Solubility between two substances depends in large part on the polarity of the substances in question
Polar substances mix well with other polar substances and ionic compounds will typically dissolve in them as well
Non-polar substances (such as oils) mix well with other non-polar substances and usually do not dissolve ionic compounds
Solubility
When one substance does not dissolve in a given solvent it is said to be insoluble
Often, ‘insoluble’ substances do in fact dissolve to some extent; they simply do so in such small amounts that we ignore the fact in order to simplify things
In regard to liquids solubility is called miscibility; if two liquids mix readily they are miscible; if not they are immiscible
The maximum amount of a solute that can dissolve in a given solvent defines the limit of solubility
This maximum solubility is dependent on temperature; some solutes such as sodium acetate dissolve in much greater amounts at high temperature while others, such as gases, dissolve in much smaller amounts at high temperature
Maximum solubility is also dependent on pressure, especially as concerns gases; this is why soda and beer is fizzy and the label warns you that ‘contents are under pressure’
Solubility graphs typically show the maximum concentration of solute in a given solvent at a range of temperatures
Concentration
Describing maximum solubility requires units of concentration
Concentration describes how much solute is present in a solution either by comparing masses of solute and solvent or numbers of moles per liter or kilogram
Some units of concentration are: mass percent (% by mass, grams solute per 100 grams solution); parts per million (ppm, grams solute per million grams solution); parts per billion (ppb, grams solute per billion grams solution); molality (m, moles of solute per kilogram solvent); and, most importantly for our purposes, molarity (M or mol/L, moles of solute per liter of solution)
Notice that many of these units of concentration refer to the solution as a whole for the ‘per ______’, molality by contrast refers to 1 kilogram of solvent
Molarity (abbr. molar, M, and mol/L) depends on the total volume of the solution meaning the volume of the solute plus the volume of the solvent
Molarity can be used in stoichiometry problems as a way to find the number of moles of a reactant or product given the number of liters of solution at a certain concentration
To make a solution of a given molarity one must add the desired number of moles to a 1 L volumetric flask add some solvent, mix well and only then add the solvent to the fill line
Predicting Solubility
There is a reason why some substances dissolve and some do not
The reason is that substances have to have a similar overall polarity to form a homogeneous mixture
Predicting the solubility of two substances depends heavily upon differences in electronegativity between atoms
The general rule is like dissolves like
If all of the bonds in a molecule are nonpolar covalent then the molecule is soluble in nonpolar solvents and insoluble in polar solvents
If the polar covalent bonds in a molecule lead to a separation of charge so that there is a positive end and a negative end then the molecule is soluble in polar solvents and insoluble in nonpolar solvents
If the bonds in a substance are ionic then that substance is soluble in polar solvents and insoluble in nonpolar solvents
Use the following steps to determine solubility
Determine the nature of the bonds in each substance. Are they ionic, polar covalent, or nonpolar covalent? If the difference in electronegativity is between 2.2 and 3.3 then the bond is ionic. If the difference in electronegativity is between 0.4 and 2.1 then the bond is polar covalent. If the difference in electronegativity is between 0 and 0.4 then the bond is nonpolar covalent.
If the molecule has only nonpolar covalent bonds then it will only dissolve in nonpolar solvents.
If the molecule contains polar bonds then you need to find out the geometry of the molecule. If the molecule has separation of charge that is equal in opposite directions (like CCl4), then it is soluble in nonpolar solvents. If the molecule has a separation of charge that is unequal overall then it is soluble in polar solvents (like H2O).
Emulsifiers
Emulsifiers are substances that allow immiscible liquids to mix homogeneously
They do this because they have both a polar part and a nonpolar part so that they dissolve in both of the immiscible liquids
Soap is a common emulsifier; it works because one end of the molecule is charged and polar and the other end (and we’re talking a long molecule here) is completely nonpolar
The polar end dissolves in the water you wash the dishes with; the nonpolar end dissolves in the grease and dirt and allows the water to wash it away
Bit of trivia: the reason raw eggs are used in the original recipe for Cæsar salad dressing is that they act as an emulsifier and produces an emulsion of the oil and vinegar