When we physically mixed iron powder with sulfur powder, we were unable to get the iron and sulfur to mix on the atomic level. Even after grinding the powders into as fine a powder as possible, we still had large chunks, about 10 microns across, of pure iron and pure sulfur. This kind of mixture is called a heterogeneous mixture. If you sample a hundred atoms from one location in an iron and sulfur mixture, you might get one hundred iron atoms. Sample the same mixture in a different location and you might get one hundred sulfur atoms. The chemical composition of the mixture is not the same throughout the entire mixture.

A homogeneous mixture that is mixed on the atomic or molecular level is called a solution. The term “solution” is typically used to describe liquid mixtures, but it is possible to have gas and solid solutions as well. Air is an example of a gas solution. It has the same basic composition throughout (78% N2 and 21% O2), and the N2 and O2 molecules are mixed on the molecular level. Alloys are examples of solid solutions. Common brass is 37% zinc (Zn) and 63% copper (Cu). It is made by melting zinc and copper together, forming a liquid solution, and then allowing the solution to cool and set as a metal solid. The zinc and copper atoms in a piece of common brass alloy are mixed on the atomic level. You do not have chunks of pure zinc and chunks of pure copper like we did in our iron and sulfur mixture.

In table salt (NaCl), the sodium and chlorine atoms are also mixed on the atomic level, but table salt is not a solid solution. Remember, table salt is a pure substance and not a mixture at all. Table salt has a single melting point at 801 °C (sodium melts at 97.72 °C and chlorine melts at -101.5 °C). Meanwhile, the zinc in a common brass alloy will start to melt at 419.53 °C, and the zinc will not finish melting until 1084.62 °C. Solutions are mixtures of two or more pure substances that have a consistent chemical composition throughout and are mixed on the atomic or molecular level.

A solution is formed when a solute is dissolved in a solvent. In general, the primary substance in the solution is called the solvent, while the secondary substance is called the solute. Therefore, for common brass alloy, the copper would be the solvent and the zinc would be the solute simply because there is more copper than zinc. (Don’t ask me which would be the solvent if the alloy was 50% zinc and 50% copper… I have no idea!) Water is, by far, the most commonly used solvent.

Water can form solutions with solid, liquid, and gas solutes. Table salt (NaCl) is an example of a solid solute, ethanol (C2H5OH) is an example of a liquid solute, and dissolved oxygen (O2) is an example of a gas solute. In all three cases, the solute enters the liquid state when it is dissolved in the liquid water solvent.

Some solutes, such as ethanol, are completely miscible in water. This means that ethanol will dissolve in water no matter how much ethanol you add. On the other hand, oil is generally considered immiscible in water because none of the oil will dissolve in the water. Instead, an oil and water mixture will separate into separate oil and water layers.

In an ethanol and water mixture, the mixture has a consistent chemical composition throughout the entire mixture, and the ethanol and water molecules are mixed on a molecular level. This makes ethanol and water a solution. In an oil and water mixture, the top half of the mixture is pure oil and the bottom half of the mixture is pure water. The mixture does not have a consistent chemical composition throughout. Oil and water is a heterogeneous mixture.

Some solutes are neither completely miscible nor completely immiscible in water. n-Butanol is an alcohol that is partially soluble in water. This means that small amounts of n-butanol will dissolve in water, but large amounts will not.

A mixture of 9.5 mL of n-butanol and 100 mL of water will form a homogeneous solution. The n-butanol molecules will be fully mixed with the water molecules on a molecular level. However, if you increase the amount of n-butanol in the mixture to 100 mL, not all of the n-butanol molecules will able to dissolve in the water, and you will end up with a heterogeneous mixture with two distinct layers. In the bottom layer, 7.7 mL of n-butanol will still dissolve in 81.3 mL of water, forming a homogeneous solution. And in the top layer, 19.7 mL of water will dissolve in the remaining 92.3 mL of n-butanol, forming a second homogeneous solution. This is considered a heterogeneous mixture because the chemical composition of the top layer is different than the chemical composition of the bottom layer. The top layer consists of 49% n-butanol molecules and 51% water molecules. The bottom layer consists of 1.8% n-butanol molecules and 98.2% water molecules.

n-Butanol and water solutions can have many different compositions depending on the recipe used to prepare the solution. Mixing 3.17 mL of n-butanol and 100 mL of water makes a solution with one chemical composition. Mixing 6.34 mL of n-butanol and 100 mL of water makes a solution with a different chemical composition. The composition of a solution is typically given as a ratio… between either the amount of solute and the amount of solvent, or the amount of solute and the amount of solution.

The percentage of n-butanol in solution #1 can be expressed in three ways: in a volume fraction, mass fraction, or mole fraction. Because solution #1 has a mole fraction of 0.6%, you know that 0.6% of the molecules in the solution are n-butanol molecules and 99.4% are water molecules. Volume and mass fractions are useful when measuring out quantities to prepare a solution. It is much easier to measure out 3.17 mL or 2.57 g of n-butanol to be mixed in water than to measure out 0.035 moles. Mole fractions are useful when you are counting out molecules or preparing a solution for a chemical reaction.

The amount of n-butanol in solution #1 can also be expressed as a concentration. The concentration of a solution is the amount of solute per unit volume. If you were to mix up a liter of solution #1, it would contain 24.9 g (or 0.339 mol) of n-butanol. When a solution is more concentrated, it has a higher percentage of n-butanol.

You should notice that the volume of the solution (103.22 mL) is not the same as the volume of the solute + the volume of the solvent (3.17 mL + 100.00 mL = 103.17 mL). When you are trying to prepare a solution with a specific concentration, you have to make sure that you are basing the concentration of the solution on the volume of the total solution. To prepare 100 mL of solution #1, you should measure out 2.49 g of n-butanol in a graduated cylinder, and then slowly add water until the solution reaches a volume of 100 mL. As you have seen with other alcohol and water mixtures, you cannot assume that the volume of two substances will simply add together.

When an n-butanol solution reaches a mole fraction of 1.8% in water, you cannot add any more n-butanol to the solution. Any additional n-butanol will separate out into a separate layer. When no more solute can be added to a solution, we say that the solution is saturated… and that the solubility of the solute is equal to the maximum amount of solute that can be added to a unit volume of solvent. (Please note that solubility is based on the volume of the solvent while concentration is based on the volume of the solution.)

A miscible solute like ethanol basically has a solubility = ∞ g/L, while an immiscible solute like oil has a solubility = 0 g/L.