Learning Set 2 • What Is in Air?
2.1 Understand the Question
What Is in Air?
Get Started
Observe
You will observe a video that shows two stoppered test tubes. One test tube
contains air. The other test tube contains pure oxygen. A glowing splint will be
put into the test tubes. In this video, you will be observing the properties of two
substances. Both substances are gases. The way a gas reacts with a glowing
splint is one property of a gas. Pay close attention to what happens to the
glowing splint in each of the test tubes. Record what happens to the splint.
Communicate
Share Your Observations
What happened to each splint in the video? For example, did the splint
go out, did the splint burst into flames, or did nothing happen? How do
your observations help you compare air and oxygen? Think about how the
demonstration in the video provides evidence about what makes up air. Share
your ideas about how you might describe the gas in the test tube with air.
As you listen to the ideas of others, decide if you agree with what is being
presented. If you do not agree with a group’s idea or supporting statement,
ask questions or offer advice.
Project-Based Inquiry Science
AQ 60
2.1 Understand the Question
Mixtures
You may have thought that air and oxygen were the same thing. The
results of the test tube demonstration show that the splint reacted
differently in oxygen alone than in air. Therefore, it seems that air must
be more than oxygen alone. Oxygen is an example of a pure substance.
A pure substance is made up of only one type of particle. Oxygen is made
up of only oxygen atoms. Water is a pure substance made up of only
water molecules.
Based on your observations, air appears to be a mixture rather than
a pure substance. A mixture is a combination of different substances.
In a mixture, the substances themselves do not change when they are
combined. Instead, they retain their individual properties, and they can
be separated from one another.
There are many types of mixtures in the world. You can sort them into
different categories based on the way the mixture is made and how it
might be separated. For example, your breakfast may contain two types
of mixtures. The milk you pour on your cereal is a mixture. The milk you
buy at the grocery store is a homogeneous mixture. A homogeneous
mixture has the same amounts of its different substances in every part
of it. In other words, it has the same composition throughout. The milk
on one side of your bowl is not different from the milk on the other side
of your bowl. All of milk is the same.
pure substance:
a type of matter
composed of a
single type of
particle.
mixture: two or
more substances
combined
such that each
substance may
retain its own
identity.
homogeneous:
any mixture that
has a uniform
composition of
substances.
Your breakfast cereal of strawberries, cornflakes, and
blueberries is a heterogeneous mixture. The milk you
put on your cereal is a homogeneous mixture.
AQ 61
AIR QUALITY
Learning Set 2 • What Is in Air?
heterogeneous: a
mixture that varies
in composition
from one part to
another.
Your cereal with strawberries, cornflakes, and blueberries is another
type of mixture, called a heterogeneous mixture. A heterogeneous
mixture does not have the same composition throughout. The number
of blueberries in each spoonful will likely vary from spoonful to spoonful.
You can usually tell by looking at a mixture whether it is heterogeneous
or homogeneous.
The three states of matter you observed in Learning Set 1—solid,
liquid, and gas—can combine to form mixtures. These mixtures can be
heterogeneous or homogeneous. For example, if you place a spoonful
of sand (a solid) in a glass of water (a liquid) and stir, the sand will sink
to the bottom. This is a heterogeneous mixture of a solid and a liquid.
The water at the bottom of the cup has more sand than the water
at the top. However, if you place table salt in water and stir, the salt
mixes with the water and forms a homogeneous mixture called salt
water. Every spoonful of the mixture contains the same amount of salt
and water.
Salt and water make a homogenous mixture. Sand and water make
a heterogeneous mixture.
Project-Based Inquiry Science
AQ 62
2.1 Understand the Question
Reflect
You observed two test tubes, one containing air and one containing pure
oxygen. You also read about different kinds of mixtures. With your group,
answer the following questions. Be prepared to share your answers with
the class.
1. List two homogeneous mixtures of liquids in liquids. Present your
reasoning to support why you think the mixtures are homogeneous.
2. List two heterogeneous mixtures of solids in solids. Present your
reasoning to support why you think the mixtures are heterogeneous.
3. Do you think clean air is a homogenous or heterogeneous mixture?
What evidence do you have for your answer? How do you think you
could find out if your answer is correct?
Predict
What substances are in air? Everyone
probably agrees that oxygen is in air.
It is important to think about what
other gases, liquids, or solids might also
be in the mixture called air. On your
Substances in Air page, make a list of
the substances you think are present in
air. Use your reading and the video you
observed to help you.
In the first circle on your Substances in
Air page, create a pie chart to show the
percent composition of each substance
you think is in air. Percent composition
relates the amount of a substance to the
amount of the entire substance. Sketch
and color what you think is the amount
of each substance in air. For example, if
you think half of the particles in air are
oxygen atoms, color one-half of the circle
with one color and label it “oxygen.” Add
each of your substances to your pie chart,
showing the percentage of each.
AQ 63
AIR QUALITY
Learning Set 2 • What Is in Air?
percent: out of
one hundred.
Percent
A percent describes a number out of 100. You use the symbol %
to represent a percent. If 50 out of 100 people answer a survey,
50 percent (50%) of the people answered the survey. If you spend
$20 out of $100, you spent 20 percent (20%) of your money.
10
10
9
9
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
1 2 3 4 5 6 7 8 9 10
50
= 50%
100
1 2 3 4 5 6 7 8 9 10
20
= 20%
100
Communicate
Share Your Ideas
Share with the class your ideas about what substances are in air and how
much of each substance you think is in air. Notice the similarities and
differences among your classmates’ ideas. The disagreements you have
about what is in air will be good questions for the Project Board.
Save your Substances in Air page to use in the next section.
Project-Based Inquiry Science
AQ 64
2.1 Understand the Question
Update the Project Board
You now know that air is a mixture of oxygen and some other substances, but
you do not know what those substances are. You also do not know yet whether
air is a homogeneous or a heterogeneous mixture. In the rest of this Learning
Set, you will be learning more about what is in air. But first, you will update
the Project Board. In the What do we think we know? column, list what you
think you know about what substances are in air. In the What do we need
to investigate? column, list questions you need to investigate in order to fully
understand what is in air. Your class probably had many disagreements about
what is in air and how much of each substance is in air. Together with your
class, formulate questions that you need to answer to come to agreement.
What’s the Point?
A pure substance is made up of only one type of matter. A mixture is a
combination of different substances. In a homogenous mixture, the substances
are mixed evenly throughout. In a heterogeneous mixture, the composition
varies throughout.
AQ 65
AIR QUALITY