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.
