Prescriptive Activities: Facet Cluster 2.2: PF#51-­56 Teacher Page Prescriptive #51-56
50 The student does not understand the processes and energy changes involved in dissolving
51 The student believes ionic substances do not break up into ions when dissolved.
52 The student believes that when an ionic compound dissolves in water that the ions that
constitute the formula unit stay together, as salt molecules, rather than separating as charged
particles in the water.
53 The student believes that when an ionic compound containing a polyatomic ion dissolves the
polyatomic ion breaks up into its constituent atoms.
54 The student believes that when a salt separates into ions when dissolving, that the identity of
the substance changes.
55 The student believes molecular substances break up into ions when dissolved.
56 The student believes molecular substances break up into atoms when dissolved.
Materials:
Student handout
copper (II) sulfate (or any soluble salt)
copper (II) carbonate (or any insoluble salt)
sugar
water
filter paper
funnel
Erlenmeyer flask
50 mL beaker (3)
stir bar
conductivity tester
Internet access
Investigation:
1. In each of the small beakers add 20 mL of water.
2. Add about 1 gram (g) of sugar to the first beaker, 1 g of copper (II) sulfate to the second
beaker and 1 g of copper (II) carbonate to the third beaker.
3. Give each beaker a stir (if you re-use the same stir bar, be sure to clean it before switching
solutions.) After stirring, record your observations (What color do you see? Is it cloudy or
clear?).
4. Allow each beaker to sit for 3 min. Do you see any solids collecting on the bottom of the
beaker? What color is the liquid?
5. Use a conductivity tester to test each solution. Be sure to clean the conductivity tester before
switching solutions.
6. Filter each solution in a different filtration setup. Set up the filter paper and funnel over the
Erlenmeyer flask. Stir the solution and quickly dump the whole thing into the filter paper.
After it has filtered, where is the compound – on the paper or with the liquid? What evidence
do you have to support this?
Prescriptive Activities: Facet Cluster 2.2: PF#51-­56 Teacher Page Reflection Questions:
1. What is water made of? How does the water get through the filter paper? In other words,
what is happening at the small particle level?
2. Did the copper (II) sulfate pass through the filter paper? What does this tell you about what
happens to the copper (II) sulfate when mixed with water, or what size are the particles in the
solution?
3. Did the copper (II) carbonate pass through the filter paper? What does this tell you about
what happens to the copper (II) carbonate when mixed with water, or what size are the
particles?
4. If a compound dissolves in water we say it is soluble in water. One of the copper compounds
is soluble, one is not. Which copper compound is the soluble one? What data or evidence did
you use to make this decision?
Information: conductivity is the ability to have electricity or an electrical current travel
through a substance. Electricity or electrical current is basically caused by movement of
charged particles.
5. When an ionic compound is in the solid form, the ions are locked in place in what we call a
crystal lattice. Based on the information above, would this form of an ionic compound be
able to conduct electricity? Explain.
Prescriptive Activities: Facet Cluster 2.2: PF#51-­56 Teacher Page 6. When copper (II) sulfate dissolves in water it does conduct electricity. What must be
happening at the atomic level when copper (II) sulfate dissolves in water? How would this
allow it to conduct electricity?
7. Does the same thing happen to sugar when it is dissolved in water? Explain.
8. Create a diagram that could represent what is happening to the atoms in each of the solutions
you made. (Optional: have students copy these diagrams onto whiteboards or poster paper to
share and discuss as a whole class.)
Copper (II) Sulfate and water Copper (II) Carbonate and water Sugar and water Prescriptive Activities: Facet Cluster 2.2: PF#51-­56 Teacher Page Background:
When solids dissolve in water the way the solid breaks apart depends on the type of bonds
holding the ions or molecules together. When a substance dissolves in water it does so because
the interactions with the water molecules overcome the forces attracting the ions or molecules to
each other. To really understand solvation (the process of dissolving) we must differentiate
between ionic and covalent (molecular) compounds.
Covalent compounds dissolving in water:
Covalent compounds are made of two or more nonmetals covalently bonded together, such as
glucose (C6H12O6), which is a type of sugar.
Imagine what sugar water might taste like if when you dissolved the sugar it separated into
the atoms it is made of. In other words, imagine drinking a cup of water with carbon
floating around in it. What might that taste like? Think about the fact that pencil leads are
made of graphite which is pure carbon.
Shown below is a molecular representation of glucose molecules dissolved in water. White balls
represent hydrogen atoms, red balls represent oxygen atoms, and grey balls represent carbon
atoms. In the glucose molecule, hydrogen atoms attached to oxygen atoms are partially positive
and the oxygen atoms have a partial negative charge. The partially negative oxygen atoms within
the water molecules are attracted to the partially positive hydrogen atoms on the glucose
molecules. Conversely, the partially positive hydrogen atoms on the water molecules are attracted
to the partially negative oxygen atoms on the glucose molecules. Notice that the individual
glucose molecules remain intact!
δ+ H O molecule 2
δ-­‐ δ+ Glucose molecule Take a look at the website listed here to learn more about how molecular compounds dissolve.
http://www.inquiryinaction.org/chemistryreview/dissolving/
Click on the forward arrows to move through the slides and double click on the videos to watch
the animations.
Prescriptive Activities: Facet Cluster 2.2: PF#51-­56 Teacher Page Ionic compounds dissolving in water:
When ionic compounds dissolve in water or
other polar solvents the process is different
than for covalent compounds. When ionic
compounds dissolve in water, the ions
interact with the polar water molecules (the
small blue and black molecules shown here)
Cl-­‐ and this interaction overcomes the
electrostatic attraction holding the ions
Na+ together in the crystal. This process of
dissolving is called dissociation. The
positively charged ions (e.g. the purple Na+
shown here) will become surrounded by the
partially negative, oxygen side of the water
molecules and the negatively charged ions
(e.g. the green Cl- shown here) will become
surrounded by the partially positive hydrogen side of the water molecules. It is important to
recognize that the ions which make up the solid crystal maintain their individual charges even
when dissolved.
Polyatomic ions in solution
It is important to understand that when an ionic solid containing a polyatomic ion dissolves the
polyatomic ion remains intact and keeps its charge. For example, when solid sodium sulfate
(Na2SO4) is place into water it will dissociate in Na+ and SO42- ions.
To learn more about this process watch the video at the link provided here.
Dissociation of Salt (:53):
http://www.youtube.com/watch?v=EBfGcTAJF4o&feature=player_detailpage After watching the videos above answer the following questions.
Questions:
1. Describe the differences, at the atomic level, between ionic compounds and covalent
compounds dissolving in water.
H2O Prescriptive Activities: Facet Cluster 2.2: PF#51-­56 Teacher Page 2. Draw a representation at the atomic level of potassium iodide (KI), an ionic compound,
dissolved in water.
3. Draw a representation at the atomic level of calcium sulfate (CaSO4) dissolved in water.
Sulfate is a polyatomic ion.
δ-­‐ 4. Draw a representation at the atomic level of the covalent compound,
ammonia (NH3) dissolved in water. Show the water molecules and
the ammonia molecules (structure shown at right).
H N H H δ+ Ammonia (NH3) References
Dissolving animations from ACS with explanations of solute, solvent, etc…
http://www.inquiryinaction.org/chemistryreview/dissolving/
Animation of NaCl Dissolving (:53):
http://www.youtube.com/watch?v=EBfGcTAJF4o&feature=player_detailpage Image of glucose molecule taken from:
http://jchemed.chem.wisc.edu/JCEWWW/features/MonthlyMolecules/2007/index.html
Image of sodium chloride molecules dissolving taken from:
http://intro.chem.okstate.edu/1215F99/Lecture/Chapter7/Lec92799.html