Mixtures and Compounds

We have already spent quite a bit of time discussing pure substances and mixtures. A pure substance is a substance that is made up of a single type of molecule. A mixture is a substance that is made up of two or more types of molecules. Another key distinction is that mixtures can be separated into two or more substances by physical or mechanical means, while pure substances cannot.

Air is an example of a gas that is a mixture. Air is approximately 78% nitrogen (N2), 21% oxygen (O2), and small amounts of other gases. If you could zoom in on a sample of air, you would find some N2 molecules mixed with some O2 molecules and some other molecules. And if you cooled the air, the O2 molecules would condense into a liquid first (at -182.95 °C), before the N2 molecules (at -195.79 °C). This means that you can separate the N2 molecules from the O2 molecules just by cooling the air to -190 °C and separating the liquid (O2) from the gas (N2).

nitrous oxide
nitrous oxide

On the other hand, nitrous oxide (N2O) gas (used by dentists and in aerosol spray cans) is not a mixture. It is made up of nitrogen and oxygen atoms, but those atoms are chemically bonded together into a single type of molecule. This makes nitrous oxide a compound and pure substance. If you could zoom in on a sample of nitrous oxide gas, you would only find N2O molecules. And if you cooled nitrous oxide gas, the N2O molecules would all condense into a liquid at -88.48 °C. You cannot separate the nitrogen atoms from the oxygen atoms without a chemical reaction that would transform the nitrous oxide into a completely different substance.

Everyday liquids

We have looked at a number of liquids in this unit, both mixtures and pure substances. Ethanol (C2H5OH), isopropyl alcohol (C3H8O), hydrogen peroxide (H2O2), ammonia (NH3), and acetic acid (CH3COOH) all exist as pure substances, but you are much more likely to encounter them as mixtures diluted in water. Water (H2O), of course, is a pure substance… as long as you ignore any dissolved gases or minerals that might be in it. Salt water, soda water, gasoline, and vinegar are all mixtures. It can be difficult to tell if a substance is a mixture or a pure substance just by looking at it. However, you often know a lot more about the substances around you than you realize.

What do you know about milk? I know that milk contains butterfat, vitamins (e.g., A and D), proteins, lactose (sugar), and calcium. I also know that the fats in milk tend to separate from the liquids over time. Because I know that fats, proteins, and sugars are all separate molecules, I know that milk is a mixture. Vitamin A is also a separate molecule, while vitamin D is the name for a collection of similar molecules. The calcium in milk is calcium phosphate [Ca3(PO4)2].

What do you know about orange juice? I know that orange juice contains fructose (sugar), vitamin C, and citric acid. I also know that orange juice can be made from concentrate by adding water. Those are all separate molecules, so orange juice is a mixture (you encountered the citric acid molecule in the reaction mechanism for respiration). And then there is the pulp. Pulp consists of vesicles… specialized cells that, in this case, contain juice, molecules called flavonoids, and dietary fiber.

What do you know about olive oil? I know that olive oil contains mostly triglycerides (fats), with some antioxidants and other flavoring agents. I also know that olive oil is made by extracting oil from olives. Because of how olive oil is made, it is extremely unlikely that all of the fat molecules in olive oil will be exactly the same. In fact, olive oil consists of triglycerides primarily made of oleic acid and palmitic acid chains. So, olive oil is a mixture.

oleic acid
oleic acid (C18H34O2)
palmitic acid
palmitic acid (C16H32O2)

What do you know about coca-cola? We have already discussed carbonated or soda water, so we know that coca-cola is a mixture of water and carbon dioxide molecules. Coca-cola also contains caffeine, and either cane sugar (sucrose) or corn syrup (fructose and glucose) as a sweetener, along with natural flavorings and caramel coloring (E150d).

caffeine (C8H10N4O2)

What do you know about honey? I know that honey is sugar dissolved in water and that it is made by bees using nectar from flowers. Honey is actually a supersaturated solution (you will learn more about solutions later in this unit). It is about 17% water, 38% fructose, and 31% glucose. Honey also contains other sugars (such as sucrose and maltose), and small amounts of pollen and other organic molecules. This makes honey a mixture.

What do you know about whipped cream? I know that whipped cream is cream whipped until it is light and fluffy. Cream itself is already a mixture since it is simply milk with a higher butterfat content. Whipping traps air bubbles in the cream, increasing its volume. Sometimes sugar (sucrose) is added to stabilize the whipped cream. Whipped cream in a can is often whipped with nitrous oxide instead of air. Nitrous oxide gas gives whipped cream more volume, but is much less stable.

What do you know about jello? I know that jello is flavored gelatin and that it can be made by dissolving powdered gelatin and sugar in hot water, and then chilled and allowed to set. Gelatin is actually a protein. When it is dissolved in water, it can form a gel… a three-dimensional network of interconnected proteins that traps in water and sugar. Gels are mostly liquid, but the network of proteins prevents the sugar water from flowing. Jello is a mixture.

Everyday solids

We have also looked at a number of solids in this unit. Metals, such as aluminum (Al), iron (Fe), gold (Au), and copper (Cu) are pure substances. Diamond (C), graphite (C), table salt (NaCl), and cane sugar (C6H12O6) are pure substances. Baking soda (NaHCO3) is a pure substance, but baking powder is a mixture of baking soda and an acid salt. (Baking soda and baking powder are both leavening agents. Baking soda reacts with the acids in a batter to produce carbon dioxide, which causes the batter to rise. Baking powder, on the other hand, contains its own acid component.)

What do you know about wood? I know that wood comes from trees and that trees are living organisms made of plant cells. By the time those plant cells turn into wood, they have lost their cytoplasm and are functionally dead. However, cells in wood still contain water and cellulose in their cell walls (even oven-dried wood still contains water), other carbohydrates (such as lignins), and fatty acids. Wood is a mixture.

What do you know about plastic? I know that plastics are polymers. Polymers are long chains of repeating molecular units called monomers. Polyethylene is one of the most common plastics. It is used in plastic bags and other packaging materials. Its monomer is ethene (C2H4). The polymers in polyethylene consist of ethene molecules chemically bonded together, but the number of ethene molecules in each individual polymer can vary. Creating polymers that all have the same length is very difficult. This is why most plastics are mixtures.

VHydrocarbons and Polymers

What do you know about rocks? I know that there are three basic types of rocks (igneous, sedimentary, and metamorphic) and that rocks are made of minerals. While some rocks may be made of a single type of mineral, it is far more common for a rock to be a collection of particles and crystals made of different minerals. For example, limestone is a sedimentary rock. It is primarily made of calcium carbonate (CaCO3) from the skeletal remains of small marine organisms collecting on the bottom of the ocean. But there is also almost always going to be some sand, or quartz (SiO2), mixed in. Granite is igneous rock formed from cooling magma within the crust. Granite is a mixture of the two most abundant minerals in the Earth’s continental crust: quartz and feldspar (AlSi3O8 and Al2Si2O8).

What do you know about concrete? I know that concrete is a composite material consisting of crushed rocks (such as limestone or granite) mixed with sand, water, and cement. When the cement is mixed with water, it acts as a binder, holding the other materials together. Because limestone, granite, and sand are different substances, concrete is mixture. The most common type of cement, Portland cement, is also a mixture of calcium silicates (Ca3SiO5 and Ca2SiO4) and other molecules.

What do you know about alloys? I know that steel, brass, and bronze are alloys. Steel is a mixture of iron and carbon; brass is a mixture of copper and zinc; and bronze is a mixture of copper and tin. Combining a metal with a non-metal or another metal can enhance the properties of the primary metal. For example, steel is stronger than pure iron. So are alloys mixtures or pure substances? It is hard to tell just by zooming in and looking at the individual “molecules.” Remember, solids with metallic bonds do not have discrete molecules… metallic solids consist of atoms chemically bound in a crystal structure. To figure it out, we are going to have to go to the second part of our definition. Alloys are mixtures because they do not have a single melting point. If you heat a piece of brass, the zinc will start melting at 420 °C, and the copper will not finish melting until 1084 °C. This means that the mixture of copper and zinc has not formed a pure substance and can be separated.

Iron and sulfur mixtures and compounds

There is a common chemical reaction used to demonstrate the difference between a mixture and a compound. If you mix iron (Fe) powder and sulfur (S) powder together, and then heat the mixture over a bunsen burner, the iron will react with the sulfur to form a compound, iron sulfide (FeS).

Fe + S → FeS

Iron is a silver-gray metal with a density of 7.874 g/cm3 and a melting point of 1538 °C. Iron conducts electricity and is ferromagnetic, which means that it is attracted to magnets. Sulfur is a soft, bright yellow crystalline solid with a density of 2.07 g/cm3, a melting point of 115.21 °C, and a boiling point of 444.6 °C. Under standard conditions, the iron atoms exist in a body-centered cubic crystal structure and sulfur forms a cyclo-S8 molecule.

iron atoms in crystal structure
cyclo-S8 molecule

Mixing iron powder and sulfur powder together gives you a silver-gray powder with specks of yellow. If you could zoom in on a sample of this powder, you would see particles of pure iron metal mixed with particles of pure sulfur molecules. Even grinding the mixture to a very fine powder in a mortar and pestle would still leave you with particles about 10 microns (1 × 10-5 m) across. And those pure iron particles would each contain over 10,000,000,000,000 iron atoms. There is simply no way to mechanically grind the iron and sulfur powders together so that they are mixed on the atomic level.

You can demonstrate that the silver-gray powder with specks of yellow is only a mixture of iron and sulfur, and not a pure substance, by heating it, pouring it into water, or running a magnet through it. When you heat the powder, the solid sulfur particles will melt at 115.21 °C and then the liquid sulfur will boil and turn into a gas at 444.6 °C; meanwhile the solid iron particles will remain solid. When you pour the powder into water, the solid iron particles will sink to the bottom and the solid sulfur particles will float on the surface. This is because the solid iron particles are denser than the solid sulfur particles. (The solid sulfur particles are also denser than water, but because of the small particle size, the surface tension of the water is enough to keep the solid sulfur particles from sinking.) And when you run a magnet through the powder, the solid iron particles will be attracted to the magnet and the solid sulfur particles will not be. (The demonstration is a little neater if you run a magnet beneath a sheet of paper with the powder on top of it.) Basically, we know that this is an iron and sulfur mixture because it does not behave like a pure substance and we can physically separate the iron from the sulfur.

You can transform the mixture of iron and sulfur powder into a pure substance by heating it until the iron atoms react with the sulfur atoms to form an iron sulfide (FeS) compound. Place a small amount of the mixed iron and sulfur powder in the bottom of a test tube. Plug the mouth of the test tube with a piece of mineral wool so that, when the sulfur transitions into a gas state, sulfur atoms cannot escape the test tube. Heat the powder mixture in the bottom of the test tube over a bunsen burner until the iron and sulfur reacts.

First, the sulfur particles in the mixture will melt, and then the sulfur will turn into a yellow gas. You will know that the iron and sulfur have started to react when the solids in the bottom of the test tube begin to glow a bright orange. Wait until the glow has spread throughout all of the solids, and then let the test tube cool down. The glow comes from energy released by the chemical reaction between iron and sulfur; the reaction is exothermic.

Once the reaction has completed, you should find a porous black substance, with no trace of yellow, in the bottom of the test tube. And this black substance will have completely different physical and chemical properties than the iron and sulfur mixture you started with. It will have a density of 4.84 g/cm3 and a single melting point at 1194 °C. It will not conduct electricity and it will not be magnetic. (Actually, I have conducted this demonstration half a dozen times, and each time, my black substance has been slightly magnetic. An explanation for why this might happen is below.) If you grind the black substance into a powder, the powder might be pyrophoric (it might ignite spontaneously in air) and it will not separate in water. This black substance is iron sulfide.

If you could zoom in on a sample of the iron sulfide, you would see iron and sulfur atoms chemically bonded together in a repeating crystal structure. The iron and sulfur are mixed on the atomic level. But more importantly, iron sulfide behaves as a pure substance with its own physical and chemical properties. This is the key difference between a pure substance that is a compound of iron and sulfur, and a simple mixture of iron and sulfur.

When iron and sulfur react cleanly, you get iron sulfide (FeS), which is non-magnetic. In order to react “cleanly,” each iron atom has to chemically bond with a sulfur atom. This almost never happens. Some iron atoms are going to re-bond with other iron atoms before they can encounter and bond with a sulfur atom. And some sulfur atoms are going to re-bond with other sulfur atoms before they can encounter and bond with an iron atom. This means that, instead of getting pure iron sulfide, you are likely to get iron sulfide (FeS) with small amounts of pure iron (Fe) and sulfur (S) mixed in. The presence of pure iron particles mixed in the iron sulfide is why the black substance in the bottom of test tube after the iron and sulfur chemical reaction is often slightly magnetic.

VConservation of Matter and Side Reactions

Iron sulfide is produced in hard-boiled eggs (it gives the surface of a yellow egg yolk its greenish color), and in bodies of water where organic matter decays under low-oxygen conditions (it gives swamp sludge its dark brown or black color).