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UNIT 4UNIT 4
A.5 CONTENT READING GUIDE: MIXTURES AND SOLUTIONS
In the table below, define the terms in your own words and provide examples for each.
Term
Mixture
Heterogeneous
mixture
Suspension
Tyndall effect
Colloid
Homogeneous
mixture
Solution
Solute
Solvent
Definition
Examples
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UNIT 4
A.5 MIXTURES AND SOLUTIONS
Types of Matter
Matter
Substances
Homogeneous
Chemical
formula can be
written for
components
Mixtures
Two or more
physically
separable
components in
varying ratios
Homogeneous
Element
One type
of atom
Compound
Two or more
atoms
chemically
bonded
Uniform
properties
throughout
Solutions
Often one
phase
Often, Ionic
sized
particles
Usually
pass light
Heterogeneous
Varying
properties
Colloids
Often,
suspended
microscopic
particles
Exhibits
Tyndall effect
Suspensions
Often, large
visual particles
that tend to
settle
Tend to block
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light
UNIT 4
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UNIT 4
A.6 MODELING MATTER: REPRESENTING MIXTURES
Throughout this course, you have represented the macroscopic world with drawings of matter at
the microscopic level. As you study aqueous mixtures, try to visualize what is in the mixture at
the molecular level. This activity will give you some practice drawing representations of
mixtures.
1. Draw a model of a container of a solution composed of a solvent that is a two-atom
compound of L and R, and a solute that is a compound composed of two atoms of element
D and one atom of element T.
2. Draw a model of the solution made when you dissolved sugar in Investigation 5 of
Investigating Matter A.3,
a. in cold water.
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b. in hot water.
c. Identify differences between the representations in 2a and 2b.
3. The element iodine (I) has a greater density in its solid state than in its gaseous state. Draw
models that depict and account for this difference at the particulate level. Iodine normally exists
as a two atom molecule.
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4. Draw a model you could use to help explain to a friend what happened when you shined the
flashlight through the solutions in Investigation 6 of Investigating Matter A.3.
5. Compare your drawings from Questions 1–4 with a classmate.
a. How are your drawings similar?
b. How are your drawings different?
c. Do the differences in your drawings indicate different ideas or simply different
representations? Explain.
6. Consider some limitations of the representations you have drawn.
a. What ideas and events are difficult to represent in these drawings?
b. What advantages would there be to using computer animations instead of static
drawings?
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UNIT 4
B.1 INVESTIGATING MATTER: WHAT SUBSTANCES DISSOLVE IN WATER?
The Riverwood News reported that Dr. Brooke believes a substance dissolved in the Snake
River is one likely cause of the fish kill. She based her judgment on her chemical knowledge
and experience. Dr. Brooke also has a general idea about which contaminating solutes to focus
on in the early stages of the investigation: those that dissolve easily in water and are harmful to
fish at elevated concentrations. Such background knowledge helps Dr. Brooke (and other
chemists) reduce the number of water tests required in the laboratory.
However, can Dr. Brooke and the EPA scientists completely rule out substances that appear not
to dissolve in water? What does it mean for a substance to be soluble versus insoluble? Is
anything truly insoluble in water? It is likely that at least a few molecules or ions of any
substance will dissolve in water. Thus, the term insoluble actually refers to substances that
dissolve only very, very slightly in water. For instance, lead(II) sulfide (PbS), which is considered
insoluble, actually dissolves very slightly in water—10–14 g (0.000 000 000 000 01 g) PbS per
liter of water at room temperature. See Figure 4.22 in the textbook. Could such low
concentrations of lead(II) ions lead to a fish kill? Such questions are answered by examining
toxicity data. (You will examine toxicity data later in this unit.)
Part I of this investigation will provide an opportunity to begin classifying various molecular and
ionic solutes as soluble or insoluble (very, very slightly soluble) in water. You will extend your
investigation in Part II.
Preparing to Investigate
Part I: What Solutes Dissolve in Water?
In Part I, you will design a procedure for testing whether several ionic and molecular substances
dissolve in water. (Your teacher will tell you which solutes to investigate. Write these in your
laboratory notebook.)
Solutes:
As a class, discuss how you could test whether substances like those listed in your laboratory
notebook dissolve in room-temperature water. With your partner, design a step-by-step
procedure to determine whether each solute is soluble (S) or insoluble (I) in room-temperature
water (see Figure 4.23 in the textbook).
The following questions will help you design your procedure.
1. What particular observations will allow you to judge how well each solute dissolves in
water? That is, how will you decide whether to classify a given solute as soluble or
insoluble in water?
2. Which variables will you have to control? Why?
3. How should the solute and solvent be mixed—all at once or a little at a time? Why?
In designing your procedure, keep these concerns in mind:
• Do not let any solutes make contact with your skin.
• Follow your teacher’s directions for waste disposal.
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Construct a data table for recording observations and data during Part I of the investigation.
When you and your partner have agreed upon a written procedure and an accompanying data
table, ask your teacher to check and approve your written plans.
Procedure:
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Part II: What Factors Affect Whether a Solute Will Dissolve?
In Part II, you will broaden your investigation. Your teacher may assign your group to investigate
one of the questions listed below, or you may investigate a different question.
Here are some possible questions:
•
•
•
How does temperature affect whether a solid dissolves in water?
Do particular groups of ionic compounds (e.g., ionic compounds containing sodium ions,
carbonate ions, or chloride ions) exhibit similar dissolving behavior in water?
For substances that do not dissolve noticeably in water, are there other solvents in which
they will dissolve?
It is best to gather evidence for Part I before planning your procedure for Part II. However, keep
your Part II question in mind as you carry out your procedure for Part I.
Question for Part 2:
Gathering Evidence
Part I: What Solutes Dissolve in Water?
Before you begin, put on your goggles, and wear them properly throughout the investigation.
Follow your teacher-approved procedure to investigate whether each specified solute dissolves
in water. Record all data in your data table. Dispose of all materials as directed by your teacher.
Part II: What Factors Affect Whether a Solute Will Dissolve?
Once you know the question you will address in this investigation, design a procedure that will
allow you to collect the data necessary to answer your question. Can you use the same
procedure you designed for Part I? If not, what parts of that procedure should be revised? In
considering your Part II procedure, keep the questions listed in Part I in mind. Write down the
procedure and construct the data table or tables that you plan to use for this investigation.
Have your written procedure and data table or tables approved by your teacher. Then collect
and record data for your investigation. Dispose of all materials as directed by your teacher.
Wash your hands thoroughly before leaving the laboratory.
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Part II Procedure:
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Interpreting Evidence
1. Based on your observations, classify each solute investigated in Part I as either soluble
or insoluble in water.
2. State in a few sentences how you decided whether each solute in Part I was soluble or
insoluble.
3. Compare your classifications in Part I with those of the rest of the class. Are there any
differences? If so, how can you explain those differences?
Making Claims
4. State the question you were addressing in your Part II investigation.
a. Write two or three sentences that describe your conclusions (answering your
question).
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b. On what evidence are your conclusions based?
c. How confident are you in your conclusions?
d. What additional tests would you like to perform to be more confident in your claims?
Reflecting on the Investigation
5. You investigated the dissolving behavior of both ionic and molecular substances. Many
of the substances you studied in Unit 1 were ionic, while the majority of substances you
encountered in Units 2 and 3 were molecular.
a. At this point, what is your definition of an ionic compound?
b. What is your definition of a molecular compound?
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c. Based on your current definitions, classify each substance you encountered in Part I
and Part II (including water) as either ionic or molecular.
d. Based on your current classifications, is it possible to make the claim that all ionic
substances are soluble in water? Why or why not?
e. Based on your current classifications, is it possible to make the claim that all
molecular substances are soluble in water? Why or why not?
6. According to your data, which solutes tested are least likely to be dissolved in the Snake
River? Explain, and support your explanation with evidence from your completed
investigation.
You now know how some solids and liquids behave in water. In the next sections, you will learn
more about the process by which ionic and molecular substances dissolve in water. Applying
what you have learned about intermolecular forces will help you understand and predict why
some ionic and molecular substances dissolve in water and why others do not.
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B.1 INVESTIGATING MATTER: WHAT SUBSTANCES DISSOLVE IN WATER?
DATA TABLE
Part I: What Solutes Dissolve in Water?
Solute
Observations
Soluble (S) or Insoluble (I)
Part II: What Factors Affect Whether a Solute Will Dissolve?
Tested substance/property
Observations
Soluble (S) or Insoluble (I)
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UNIT 4UNIT 4
B.2 CONTENT READING GUIDE: DISSOLVING IONIC COMPOUNDS IN WATER
Based on your reading illustrate how the ionic compounds NaCl and CaCO3 might dissolve in
water.
First draw the molecular representation for all three
Water in a beaker
NaCl
CaCO3
Illustrate and describe the process for the dissolving of each:
NaCl
Description:
CaCO3
Description:
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B.4 MODELING MATTER: THE DISSOLVING PROCESS
So far in this section, you have investigated and learned about the process of dissolving both
ionic and molecular compounds in water. Figure 4.25 (page 421 in the textbook) uses a single
image to illustrate what happens when salt crystals dissolve in water. However, it is hard to
capture in one ―snapshot‖ the dynamic process of dissolving, so you might visualize a short
―movie‖ of the dissolving process in your mind. In this activity, you will represent these mental
movies as a series of drawings illustrating the dissolving process. You will also represent this
process using chemical language by writing chemical equations.
SAMPLE PROBLEM: Refer back to Figure 4.25 (page 421 in the textbook), which illustrates the
process of dissolving NaCl in one frame. Then, draw a five panel sequence illustrating NaCl
dissolving in water.
One way to visualize the sequence is described and illustrated below:
First panel: Sketch a beaker of water, with some water molecules drawn in. Beside the
beaker, draw a 2-D representation of the NaCl crystal Beside the beaker, draw a 2-D
representation of the NaCl crystal Beside the beaker, draw a 2-D representation of the
NaCl crystal (alternating small and large circles, with charges drawn in).
Second panel: The solid is placed in the water. Draw the crystal at the bottom of the
beaker.
Third panel: A few water molecules are interacting with a few ions, pulling the ions away
from the surface of the crystal.
Fourth panel: More water molecules have carried away ions.
Fifth panel: No solid remains. All ions are surrounded by water molecules, and the
crystal has dissolved.
1. Consider magnesium chloride, MgCl2, another soluble ionic compound.
a. Draw a five-panel sequence showing what happens when magnesium chloride is placed
in water.
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b. Write
UNITa 4caption for your sequence, making specific reference to the attractive forces
involved.
c. Compare your magnesium chloride ―dissolving movie‖ to the sodium chloride ―dissolving
movie‖ in the Sample Problem.
2. You determined whether chalk was soluble in water in Investigating Matter B.1, and Section
B.2 also used chalk as an example.
a. Draw a five-panel sequence showing what happens when chalk (calcium carbonate) is
placed in water.
b. Write a caption for your sequence, making specific reference to the attractive forces
involved.
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3. Another way to represent chemical and physical processes is by writing and interpreting
chemical equations. For example, the following equation represents dissolving table salt in
water:
a. What do the symbols (s) and (aq) mean?
b. Write a sentence that interprets this chemical equation in words.
c. Water is not shown as a reactant or a product in this equation. How do you know, based
upon this equation, that water is part of the process?
d. Review your results from Investigating Matter B.1 and write chemical equations for each
of the ionic compounds that dissolved in water.
e. You classified some ionic compounds in Investigating Matter B.1 as insoluble. How would
you write chemical equations for the interaction of these compounds with water? Use
chalk (CaCO3) as an example. Write a chemical equation and write one or two sentences
describing your reasoning.
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4. Now model the dissolving process for molecular compounds.
a. Choose one soluble molecular compound from Investigating Matter B.1.
b. Draw a five-panel sequence illustrating what happens when this compound dissolves in
water.
c. Write a caption for your sequence, making specific references to intermolecular forces.
d. Write a chemical equation that describes this process.
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5. In Section B.2, you learned about factors that partly determine whether a specific ionic
compound will dissolve in water. Suppose the following 2-D drawing represents ions in a
sodium bromide (NaBr) crystal:
a. Magnesium cations (Mg2+) are smaller than Na+ ions, and sulfide (S2–) ions are smaller
than Br– ions. Draw a 2-D representation of MgS that shows relative sizes and ion
charges, compared to the NaBr crystal depicted above.
b. Would you predict MgS to be more or less soluble in water than NaBr? Use your model
(and what you learned in Section B.2) to explain your prediction.
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6. Consider the Riverwood fish kill that opened this unit. None of the newspaper articles have
mentioned an oil spill (e.g., from a tanker truck) as a possible cause of the fish kill.
a. Why is an oil spill an unlikely cause of the Riverwood fish kill? What would be observed
at the macroscopic level? Draw a picture to illustrate your point.
b. Discuss how intermolecular forces play a role in this scenario. Your discussion should
include intermolecular forces within oil, within water, and between oil and water.
You are now familiar with why and how certain substances dissolve in water. You also know
that solutions can contain different amounts of solute—think of putting a pinch of salt into a pot
of boiling water versus adding a cup. Later in this unit, you will research and report
concentrations of oxygen dissolved in water that are necessary for fish to survive. To interpret
these data, we next explore how to describe and measure concentration.
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UNIT 4
B.6, B.9, B.10 EXTRA PRACTICE: DESCRIBING SOLUTION CONCENTRATION,
SOLUBILITY AND SOLUBILITY CURVES, AND FACTORS THAT AFFECT
SOLUBILITY
Using the solubility curves found in figures 4.31 on page 436 and 4.32 on page 438 in your
textbook, solve the following problems.
1. What is the solubility of potassium nitrate (KNO3) in 100.0 g of water at 80.0 °C?
2. What is the solubility of potassium chloride (KCl) in 100.0 g of water at 50.0 °C?
3. What is the solubility of sodium chloride (NaCl) in 100.0 g of water at 90.0 °C?
4. What is the minimum temperature needed to dissolve 180.0 g of KNO 3 in 100.0 g of
water?
5. What is the minimum temperature needed to dissolve 35.0 g of KCl in 100.0 g of water?
6. At what temperature do KCl and KNO3 have the same solubility?
7. How much more KCl will dissolve at 90.0 °C than at 20.0 °C?
8. If 50.0 g of NaCl is mixed with 100.0 g of water at 80.0 °C, how much will not dissolve?
9. If 15.0 g of KCl is added to 100.0 g of water at 30.0 °C, how much more must be added
to saturate the solution?
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dissolved?
11. If a saturated solution of KCl at 90.0 °C is cooled to 30.0 °C, how much of the solid will
precipitate?
12. How much NaCl will dissolve in 350.0 g of water at 70.0 °C?
13. How much KCl will dissolve in 50.0 g of water at 50.0 °C?
14. Classify as saturated or unsaturated a solution that contains 90.0 g of KNO3 in 100.0 g of
water at 60.0 °C.
15. Classify as saturated or unsaturated a solution that contains 50.0 g of KCl in 100.0 g of
water at 70.0 °C.
16. What temperature is needed to dissolve twice as much KNO3 as can be dissolved at
30.0 °C in 100.0 g of water?
17. What is the solubility of O2 in 1000.0 g of water at 24.0 °C?
18. How much more soluble is O2 in water at 16.0 °C than at 40.0 °C?
19. At what temperature is the solubility of O2 10.0 mg in 1000.0 g of water?
20. What mass of O
2
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can be dissolved in
2000.0
at 30.0
10.gIfofa water
saturated
solution of KNO3 at 20.0 °C is heated to 80.0 °C, how much more could be
°C?
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Show work and units in all problems.
1. A solution of seawater contains 3.0 g salt in 97.0 g of water. What is the concentration of
this solution expressed as percent salt by mass?
2. What is the concentration of a sugar water solution that has 10.0 g of sugar in 40.0 g of
water expressed as percent sugar by mass?
3. An acidic solution is made by dissolving 4.0 g of acetic acid in 35.0 g of water. What is the
concentration of the solution expressed as percent acetic acid by mass?
4. A saturated solution of KNO3 at 80 °C has 160.0 g of KNO3 in 100.0 g of water. What is the
concentration of this solution expressed as percent potassium nitrate by mass?
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5. At 26 °C, water is saturated with oxygen when 0.008 g of oxygen gas is dissolved in 1000.0
g of water. What is the concentration of the solution expressed as percent oxygen by mass?
6. A solution that has 0.0045 g of lead (II) ions (Pb2+) dissolved in 1000.0 g of water is
considered dangerous to human health. Calculate the concentration of the solution
expressed as percent lead (II) ions by mass.
7. A concentrated commercial solution of hydrochloric acid (HCl) is 37% by mass. If you had a
solution of 550 g of the acid, how many grams of HCl would be in the solution?
8. Arsenic ions are a high risk to aquatic life at a concentration of 0.44 ppm. If a 500.0 g
sample of water with this concentration is measured, how many grams of arsenic ions are
present in the solution?
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5. It is sometimes said that ―time is money.‖
a. If you spent twice as much time purifying your sample, would that extra time
investment pay off in higher water quality?
b. If you spent about 10 times as much time, would that extra investment pay off?
Explain your reasoning.
6. Municipal water-treatment plants do not use distillation to purify water.
a. Why?
b. What advantages would distillation offer?
7. Are there types of contaminants that the procedures you used would not be capable of
removing? Give examples and describe possible ways of removing them.