Init 4/2/2015 by Daniel R. Barnes

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HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms. (Sodium, from the left (group 1A), with one valence e-, loses 1 e- to become Na+. Chlorine, from the right (group 7A), with seven valence e-, gains 1 e- to become Cl-.)

HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles. (Sodium chloride is a robust solid at room temperature, with a strong crystal structure and a high melting point due to the powerful ionic bonding between its cations & anions. However, the polarity of the water molecule allows water molecules to dissolve salt, forming an aqueous solution of solvated ions.)

HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties. (Sodium and chlorine, being a metal and a nonmetal, react to form sodium chloride. The sodium loses electrons, which chlorine gains. The positive sodium ions are attracted by ionic bonds to the negative chloride ions.)

NGSS Justification of LessonNGSS in boldface (Barnes’ justifications in italics)

HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs. (Salt will dissolve in water faster if the water is hot. Higher temperatures mean faster-moving particles, so ions are more likely to break free of their crystal lattice AND faster-wandering water molecules will arrive at the surface of salt crystals at a higher frequency and carry away solvated ions faster. Salt dissolving in water may not be a chemical reaction per se, but the principle of temperature speeding it up is very much akin to the principle that higher temperatures speed up chemical reactions.)

HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials. (The connection between this lesson and this standard is disturbingly similar to the connection to HS-PS1-3 . . . )

HS-PS3-2: Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative positions of particles (objects). (Higher temperature salt water means faster-moving water molecules and more furiously-jiggling salt crystal ions. Ions in a salt crystal are at a relatively low level of potential energy relative to what they would have if they were separated solidity.)

SWBAT. . .

. . . explain why and how salt dissolves in water

IMPORTANT: As you draw on our

“Salt Crystal Dissolving Skeleton Notes” worksheet, leave the right side of the sheet blank

until I say it’s okay to draw there.

HELPFUL HINT: Turn to page 451.

Group leaders, on your whiteboards, write a brief description of what is pictured inside the square on your worksheet.

Everyone, on your worksheets, use block letters to label what the thing in the square is supposed to be.

On our worksheet, the salt crystal looks like a square. What shape is a salt crystal in real life?

a cube

Pass the whiteboard to the next person. Next person, draw the nine circles found inside the square. Inside each circle, write the correct elemental symbol & charge.

Everyone, on your worksheets, label your sodium cations and chloride anions.

Cl- Na+ Cl-

Na+ Cl- Na+

Cl- Na+ Cl-

Pass the whiteboard to the next person. Next person, erase your whiteboard. Draw a picture of a water molecule and label each circle in the water molecule with the correct elemental symbol.

Everyone, on your worksheets, label your hydrogen and oxygen atoms.

Cl- Na+ Cl-

Na+ Cl- Na+

Cl- Na+ Cl-

H HO

Pass the whiteboard to the next person. Next person, add the correct symbols to show the partial charges on the atoms in the water molecule.

Everyone, on your worksheets, label your partial charges now. Also, at the very bottom of the worksheet, briefly explain what the + & - symbols mean.Cl- Na+ Cl-

Na+ Cl- Na+

Cl- Na+ Cl-

H HO

-

+ +

+ = partial positive charge- = partial negative charge

Partial electric charges happen when two atoms form a polar covalent bond due to a moderate difference in electronegativity between the two atoms.

Everyone, on your worksheets, extend your salt crystals by adding more sodium and chloride ions until your squares are full.

Cl- Na+ Cl-

Na+ Cl- Na+

Cl- Na+ Cl-

H HO

-

+ +

Cl-

Na+

Cl- Cl-

Na+

Na+ Na+

Pass the whiteboard to the next person in the group. Next person, erase the whiteboard.Draw a water molecule above a sodium ion, positioned correctly so that the water molecule and the sodium ion would attract each other.

Cl- Na+ Cl-

Na+ Cl- Na+

Cl- Na+ Cl-

H HO

-

+ +

Cl-

Na+

Cl- Cl-

Na+

Na+ Na+

Everyone, on your worksheet, draw another water molecule, above a sodium ion on the top of the salt crystal, oriented so that the water molecule and the sodium ion would attract each other.

+ +

-

H HO

TPS: Discuss with your group what story is being told by our drawing. You have 47 seconds to discuss.

Cl- Na+ Cl-

Na+ Cl- Na+

Cl- Na+ Cl-

H HO

-

+ +

Cl-

Na+

Cl- Cl-

Na+

Na+ Na+

+ +

-

H HO

Water molecules wander randomly until they bump into the salt crystal. Salt ions on the outside of the crystal are attracted to the partial electric charges on the water molecules.

Salt ions on the outside of the crystal are pulled off by water molecules, which then wander away with them.

The salt crystal dissolves.

We need to indicate in our drawing that water molecules are stealing ions from the crystal, so let’s draw some circles and arrows and explain what they mean.

Cl- Na+ Cl-

Na+ Cl- Na+

Cl- Na+ Cl-

H HO

-

+ +

Cl-

Na+

Cl- Cl-

Na+

Na+ Na+

+ +

-

H HO

Water molecule pulls sodium ion out & wanders away with it.

Please draw loops & arrows on your worksheet to show how the water molecules are abducting salt ions. Also, write little explanations next to them.

Water molecule pulls chloride ion out &

wanders away with it.

Now we’re going to draw a salt solution on the right side of the paper.

I hope you’ve been keeping it blank like I asked you to earlier.

Pass the whiteboard to the next person in the group. Next person, erase the whiteboard anddraw a sodium ion surrounded by water molecules. Orient the water molecules so that they’d be attracted to the sodium ion. Na+

+ +

-

+

+

-

++

-

++

-

TPS 23 sec: What’s the word for a dissolved ion that is surrounded by water molecules? A: “solvated”

Everyone, draw a “solvated” sodium ion in the upper half of the right side of your worksheet.

Pass the whiteboard to the next person in the group. Next person, erase the whiteboard anddraw a chloride ion surrounded by water molecules. Orient the water molecules so that they’d be attracted to the chloride ion.

Everyone, draw a “solvated” chloride ion in the lower half of the right side of your worksheet.

Cl-

+ +

-

+

+

-

++

-

++

-

TPS 23 sec: What would you call the mixture of dissolved ions and water molecules on the right side of the page?

Now that we know how a salt crystal dissolves . . . TPS 59 seconds: List some things that would help a salt crystal dissolve.

Salt crystals dissolving requires water molecules to wander randomly so that they can bump into the outside surfaces of the salt crystals.

Therefore, if the water is hotter, water molecules will wander faster and hit the salt crystal harder and more often.

If the water is hotter, ions in salt crystals will jiggle more violently, making it easier for them to break away from their attractive ion neighbors.

If the water is stirred, the salty water around a dissolving crystal will get replaced with fresh water, speeding the dissolving process by slowing the re-crystallization process.

If the salt is ground into a fine powder, the increase in surface area will allow it to dissolve faster than if it were still large crystals.

Since salt ions need to be solvated, there had better be plenty of water molecules to surround them. More solvent allows more solute to dissolve.

Okay. Now it’s time for a little quiz.Flip your worksheet over.

Take the quiz.

Your score on the quiz will not affect your grade, but finding out how you did will help you figure out what you know and what you don’t know.

When you’re done, bring your answer document to the teacher’s desk so you can have it scanned.

We’ll go over the questions when everybody is done.

Until then, if you’re done with the quiz, answer the essay question on the back of your worksheet, below the multiple choice questions.

Okay. Let’s go over the quiz.

I encourage you to take notes on the back of your worksheet as we discuss the answers.

The slides after this one are all construction zone stuff.

Students, don’t bother moving forward beyond this slide.

Cl- Na+

+ +

-

++

-

Cl-

Cl- Cl-

Na+

Na+ Na+

Cl- Na+ Cl-

Cl- Cl-

Na+

Na+ Na+

Na+

Cl-

+ +

-

+ +

-

+

+

-

+

+

-

++

-

++

-

++

-

sodium ion chloride ion

water molecule

The square object above is a

(It’s actually not supposed to be a square. It’s supposed to be a _________________.)

Name: Seat:

Period: Date:

Daniel R. BarnesChemistry04/20/23 02:55 AM

Salt Crystal Dissolving Skeleton Notes

(Formative assessment multiple choice quiz on back)

The square object above is a

(It’s actually not supposed to be a square. It’s supposed to be a _________________.)

Name: Seat:

Period: Date:

Daniel R. BarnesChemistry04/20/23 02:55 AM

Salt Crystal Dissolving Skeleton Notes

(Formative assessment multiple choice quiz on back)

For each of the following instructions,Each individual writes/draws on his worksheetThe next member at your table also draws on the mini-whiteboard

* Label the square object with the circles in it. Use block letters. Get artistic if you can, but don’t feel stress if your letters are ugly.

* Determine the identity of the circles inside the square; Inside each circle, label the circle w/its elemental symbol & charge For one example of each particle, draw an arrow pointing to that particle and write the full name of the particle at the tail of the arrow.

* Extend the salt crystal by adding ions to fill the entire square.

* Label the water molecule’s atoms w/elemental symbols

* Label the partial charges on the water molecule’s atoms using + and -.

* Beneath your drawing, briefly explain what + and - mean.

* Draw another water molecule, this one pulling out a sodium ion from the top of the salt crystal.

* On the right side of the sheet, draw a solvated sodium ion & a solvated chloride ion. Make sure to indicate the charges on each atom.

* Explain why you expect the bonds in a water molecule to be covalent and why you expect them to be polar

* Explain why the sodium and chlorine atoms have the charges that they do.

* Explain why the sodium and chloride ions have the sizes that they do.

* Explain how water molecules pull ions out of the salt crystal.

* Explain how water molecules surround dissolved ions, “solvating” them.

* List some things that you think should help a salt crystal dissolve in water.

NEW IDEA: The concentration at which dissolving happens just as fast as crystallization is the maximum possible concentration of the solute. This special concentration is the “solubility” of the solute.

3D MANIPULABLE MODEL IDEA:Styro balls w/ x, y, & z axis holes drilled in themPainted with poster paint to be

green chlorine and light grey sodiumVelcro squares x 6/ball (hooks = neg; fuzz = +) sewn

to coaxial opposites by thread @ their midpointsto next door neighbors w/thread at their corners