Name:_______________________________________
Class Period _______
Date: ________________________________________
Student ID: ____________
UNIT 4
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.
Unit 4 | Water: Exploring Solutions
309
Name:_______________________________________
Class Period _______
Date: ________________________________________
Student ID: ____________
b. Write a caption 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.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
310
Chemistry in the Community, 6th Edition
© 2012 W. H. Freeman and Company
Name:_______________________________________
Class Period _______
Date: ________________________________________
Student ID: ____________
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.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Unit 4 | Water: Exploring Solutions
311
Name:_______________________________________
Class Period _______
Date: ________________________________________
Student ID: ____________
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.
312
Chemistry in the Community, 6th Edition
© 2012 W. H. Freeman and Company
Name:_______________________________________
Class Period _______
Date: ________________________________________
Student ID: ____________
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.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Unit 4 | Water: Exploring Solutions
313