Video: Compounds and Mixtures

In this video, we will learn how to describe the ways that substances can combine. We’ll look at the similarities and differences between atoms, molecules, compounds, and mixtures, and discuss the possible types of mixture.


Video Transcript

In this video, we will learn how to describe the ways that substances can combine. We’ll look at the similarities and differences between atoms, molecules, compounds, and mixtures and discuss the possible types of mixture. There are currently 118 different elements, and they can be found on the periodic table of elements. Each element is a type of atom or ion, with a different number of protons in the nucleus.

It’s easy to describe atoms. For instance, this is an atom of hydrogen. It has one proton in its nucleus and one electron in the surrounding space. This particular atom of hydrogen does not have any neutrons at all. An atom is simply a nucleus made of protons and neutrons with enough bound electrons to make it neutral. If we bring two atoms of hydrogen together, they form a covalent bond sharing their electrons, holding them together. So what we produce is two atoms of hydrogen making up a molecule of hydrogen.

A molecule is simply two more atoms covalently bonded together in a discrete unit. Drawing molecules as nuclei surrounded by clouds of electrons gets quite complicated. So let’s draw the hydrogen molecule in a more simple way. We can use the element symbol for hydrogen connected by a single line to form a covalent bond between the two hydrogen atoms. Our next guest is a fluorine atom. It has nine protons in the nucleus and nine electrons in the surrounding space. This particular fluorine atom has 10 neutrons as well. We can simplify this by representing the whole atom with the element symbol for fluorine, the letter F. If we bring two atoms of fluorine together, they also form a covalent bond. We now have a fluorine molecule.

In the right conditions, if we bring these things together, the hydrogen molecule and the fluorine molecule break apart. And new stronger covalent bonds form between the hydrogen atoms and the fluorine atoms, producing a new substance called hydrogen fluoride. We started with molecules containing one element, either hydrogen or fluorine. And we produced a new type of molecule containing two different elements, hydrogen and fluorine. We call this type of substance a compound, a thing that is composed of two or more separate elements, with the added restriction that the elements must be in a fixed ratio. For instance, in hydrogen fluoride, we have one hydrogen atom for every fluorine atom. It’s important to recognize that fluorine, hydrogen, and hydrogen fluoride are all molecular substances. But out of the three, hydrogen fluoride is the only compound.

Atoms can gain or lose electrons and become ions. Ionic substances are always compounds. Ions come together in structures of alternating positive and negative ions. These structures can go on forever, so they’re not like molecules. But ionic substances are always composed of two or more elements. And they generally have a repeating structure, which means that we know the ratio between positive and negative ions. So they have a fixed composition. For example, sodium chloride is composed of positive sodium ions and negative chloride ions. For each sodium ion, there’s one chloride ion and the formula for sodium chloride is NaCl.

Generally speaking, nonmetal elements react with other nonmetal elements to produce molecular substances although some react to produce continuous lattices. Also, speaking generally, nonmetals react with metals to produce ionic substances. But what about metals and metals? You may have heard of alloys like brass. Brass is a combination of copper and zinc. However, most of the time when we melt metals together, they don’t form regular patterns. Some brasses contain only 10 percent zinc while others contain 65, with the rest being copper. So brass, as an alloy, is not a molecule. And because its composition can change freely, it’s not a compound. Instead, we say that alloys are mixtures.

In everyday language, a mixture can mean lots of different things. In chemistry, the word is more specific. A mixture is two or more substances mixed together, but not reacted together. In a chemical mixture, the substances retain their original chemical behavior. Let’s have a look at some examples. This is a box of oxygen atoms. It’s a pure substance because all the particles are the same. Of course, oxygen atoms will naturally stick together and form oxygen molecules. Again, we have a pure substance because all the particles are exactly the same. They will react in the same way.

This is what we’ll have if we pause things partway through, a mixture of oxygen atoms and oxygen molecules. All these substances are examples of the element oxygen. But in the middle, we have two different substances, two different forms of the element oxygen. This means that we have a mixture, a mixture of oxygen atoms and oxygen molecules. Oxygen atoms react differently to oxygen molecules, so we have two different substances.

Now let’s consider what happens if we react two different elements together. On the left, we can see carbon atoms and oxygen molecules. It’s a mixture. Carbon atoms and oxygen molecules are two different substances. If we react them together, we produce molecules of carbon dioxide. If the amount of carbon atoms and oxygen atoms is just right, we’ll produce a pure substance, only carbon dioxide. In between, we have a mixture of carbon atoms, oxygen molecules, and carbon dioxide molecules. This time, we’ve turned a mixture into a pure substance.

Now let’s have a look at different types of mixture. When we form mixtures, we generally mix together solids, liquids, or gases. Mixtures involving solids and liquids can be very different. We could have large pieces or small pieces or atomic-scale pieces. When a mixture is this fine, we tend to call it a solution. The tiny particles are called the solute, and the stuff they’re mixed into is called the solvent.

Let’s have a look at some examples. Many rocks are example of solid–solid mixtures. Different minerals can be combined together in the same lump, and we call these materials conglomerates. Colored glasses can be made from various mixtures of silica and other minerals. For instance, blue glass can be made using cobalt oxide. And at the solution scale, we have alloys like brass. It’s perfectly acceptable to call alloys solid solutions.

Now what if, instead, we have two liquids? What happens with liquids depends on if they naturally mix or not. If you combine oil and water, they won’t mix on their own. But if you shake them up, you can get a mixture of the two, where there are small droplets of one in the other. If a mixture separates over time, we call it unstable. Milk is a stable liquid–liquid mixture of water, proteins, and fats. There are other chemicals in the milk that stop the fat droplets sticking together. However, it’s pretty much pure fat inside those droplets, so it’s not a solution.

But a good example of a liquid–liquid solution is white vinegar, which is a mixture of water and ethanoic acid, otherwise known as acetic acid. The acetic acid molecules are dissolved in the water, so acetic acid is the solute and water is the solvent. When water is the solvent in the solution, we say that we have an aqueous solution and use the letters aq to indicate that state.

Gas–gas mixtures are a little bit more uniform. Gases, by definition, are made of separated particles, so we can only get gas–gas solutions, like air, which is a mixture of nitrogen, oxygen, and a few other gases. So what about the other combinations? Soil is a mixture of solids, of clay and dirt, with a liquid, water. Meanwhile, blood is a suspension of many different solids, like red blood cells, although it also contains dissolved gases like oxygen and carbon dioxide. And sugar dissolves directly in water to form a solid–liquid solution. Of course, as well as having solids in a liquid, we can have liquids in a solid, like water in a wet sponge, tiny droplets of water in gelatin, as in jello. And we can even dissolve liquids in solids as when mercury dissolves into solid gold.

Now, let’s have a look at gases and liquids. A good example of a liquid–gas mixture is a fine spray like from a window cleaner bottle. If the droplets are small though, you can form stable mixtures like mist, where the droplets of water are so fine that they can float in the air. We come across the alternative gas–liquid mixtures quite often. The bubbles in a bubble bath are pockets of air surrounded by thin films of soapy water. If the bubbles are smaller, the foam will last longer, like shaving foam or whipped egg whites. But perhaps the most popular example is carbonated drinks. Carbonated water is a mixture of carbon dioxide and water. If you open the bottle, the carbon dioxide escapes. But if you leave the cap on, you have a stable mixture of water and carbon dioxide.

Finally, we have mixtures of solids and gases although these are perhaps less common. Dust in the air is a solid–gas mixture that’s unstable. But finer particles, like those in smoke, tend to stay suspended for longer. However, you can’t have a solid–gas solution because if you broke up solids until they were on the atomic scale, they wouldn’t be solids anymore.

A good example of a gas–solid mixture is a dry sponge, where we have pockets of air surrounded by solid. You can also get solid foams like polystyrene foam, which can be made using air or small hydrocarbons or halocarbons. You can also have gas–solid solutions. Palladium is a rare metal that will absorb large volumes of hydrogen gas. Whether we classify a mixture as unstable or stable depends on your point of view. I’ve marked off a few that seem reasonable, but there will be exceptions. And some of the other examples could be considered unstable depending on the time period.

Before we go on to some worked examples, we need to look at one more way that we classify mixtures. Stable and unstable talk about how a mixture will develop in future. We need to look at how mixtures are classified now. Let’s think about a drop of ink about to splash into water. We can consider both substances to be pure, although ink probably contains a few different substances. After we’ve added the ink and stirred everything together, we’ll have a mixture of ink and water that is even throughout. So the composition of the mixture is consistent. In between, the ink hasn’t mixed thoroughly with the water, so we have areas where there’s more ink and areas where there’s less. So we can say that the composition of the mixture is uneven.

We call a mixture with a consistent composition a homogeneous mixture, which literally means same kind. When the composition of a mixture is uneven, we call it a heterogenous mixture, meaning literally different kinds. It’s important to realize that homogenous and heterogenous describe the mixture as it is at the moment while stable and unstable describe how the mixture is going to develop. After all that, it’s about time we had some practice.

Which of the following best describes a compound? (A) Atoms chemically bonded together in a discrete unit. (B) A substance composed of two or more elements chemically bonded together in a fixed ratio. (C) A single atom of an element. (D) A mixture of different elements.

If you don’t remember what a compound is, it might help to think about the role of the word in other contexts, for instance, like compound interest, which is interest on interest. Or you could think about a compound fracture, which is a fracture of the bone, which also breaks the skin. These have associations of togetherness or combination or somehow layering together. Water is a good example of a compound. A molecule of water, H2O, is composed of one oxygen atom and two hydrogen atoms. So water is composed of two different elements. Sodium chloride is another example of a compound, but the chemical structure of it is completely different to water. Sodium chloride is made up of positive sodium ions and negative chloride ions, bound together in a lattice that, theoretically, could extend in all directions for infinite distance.

What these two compounds have in common is that they consist of two or more elements chemically bonded together. In the case of water, there are covalent bonds, and in sodium chloride, there are ionic bonds. The last thing we need to figure out is what fixed ratio means. For a compound containing two elements, it means that there’s a fixed ratio between atoms or ions of that element and the other element in the compound. So we have a one-to-one ratio in sodium chloride between sodium ions and chloride ions. The formula for sodium chloride is NaCl. Since it’s a one-to-one ratio, we don’t normally write the subscripts. But before we select this as our final answer, let’s have a look at the others.

The first statement suggests that compounds are atoms chemically bonded together in a discrete unit. An example of this would be two oxygen atoms bonded together in an oxygen molecule, which is not a compound because it contains only one element. So statement one actually refers to a definition of molecule. A single atom of an element is simply an atom. A single atom, for instance, an atom of argon, is definitely not a compound because it doesn’t contain two or more elements. A good example of a mixture of different elements is a combination of nitrogen and oxygen gases, which makes up the majority of the air we breathe. These molecules would not be chemically bound to each other, so we would not consider this mixture a compound. Therefore, of the four descriptions we’ve been given, the one that best describes a compound is a substance composed of two or more elements chemically bonded together in a fixed ratio.

Now let’s have a look at a question on mixtures.

Which of the following is true for mixtures? (A) Mixtures can only be made by mixing pure substances together. (B) A mixture is always formed when two substances react together. (C) Pure elements cannot form mixtures. (D) Mixtures cannot be separated into their components. Or (E) mixtures consists of particles of different substances mixed but not reacted together.

We can see in statement A that we can’t make a mixture by mixing pure substances together, like nitrogen gas and oxygen gas. But there’s nothing stopping us mixing another gas with that mixture to produce a new mixture. The next statement suggests that a mixture is always formed when two substances react together. However, if, for instance, we react carbon and oxygen gas together in a ratio of one carbon atom to one oxygen molecule, we’ll produce carbon dioxide in its pure form. So we won’t generate a mixture there. We know from mixing nitrogen and oxygen together that pure elements can form mixtures, so the next statement is false.

The next statement says that mixtures cannot be separated into their components. But if we have a sample of iron filings mixed with sulfur, we can use a magnet to pull out the iron filings, separating the mixture, which just leaves us with a definition of mixtures. Mixtures consist of particles of different substances mixed but not reacted together.

All that remains is to review the key points. Compounds are substances composed of two or more elements in a fixed ratio, and they’re generally covalent or ionic. Meanwhile, mixtures are two or more substances mixed together, but not reacted together. An unstable mixture is one that separates over time. A homogeneous mixture has an even composition throughout. And a heterogenous mixture has an uneven composition. Mixtures can involve solids, liquids, and gases, and they can even be aqueous solutions.

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