Key ConceptsDifferent substances are made from various atoms, ions, or molecules, which interact with water in different ways. Since dissolving relies on the interaction in between water and the substance gift dissolved, each substance has actually a properties solubility.
Students will observe a solubility test between salt and sugar. They will then be gift with 4 known crystals and also an unknown. Based on the solubility demonstration, the class will style a solubility test to find the identification of the unknown.
Students will be able to identify and also control variables when making a solubility test. Students will have the ability to explain why various substances dissolve to different extents in water.
Download the student task sheet, and also distribute one per student once specified in the activity. The task sheet will serve together the “Evaluate” component of every 5-E great plan.
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Be sure you and also the students wear correctly fitting goggles.
Materials for the DemonstrationsGram balanceSimple balanceGraduated cylinderWater4 clean plastic cups2 tiny plastic cupsSalt Sugar cereal balls (Kix job-related well)Zip-closing plastic bag (quart-size, storage-grade)
Materials for Each GroupSalt (sodium chloride)Epsom salt (magnesium sulfate)MSG (monosodium glutamate)Sugar (sucrose)Coarse kosher salt (sodium chloride) WaterBlack construction file Masking tapePen or long-term markerMagnifierGram balance5 small plastic cup 5 clear plastic cupsGraduated cylinderPaper towel
About this Lesson
This lesson will take two or three course periods.
Do a demonstrate to present that different substances have different solubilities.
Tell students that in this demonstration, you will pour salt and sugar right into water to find out which dissolves better. In stimulate to do this test fair, friend will use the same amount (mass) that salt and also sugar, the exact same amount the water in ~ the same temperature, and you will certainly swirl every in the same method for the same length of time.
Question to Investigate
Which dissolves in water better, salt or sugar?
Materials because that the DemonstrationBalance that measures in gramsGraduated cylinderWater2 clear plastic cups2 tiny plastic cupsSalt Sugar
Teacher PreparationLabel 1 clear plastic cup and also 1 tiny cup Salt.Label the other clear plastic cup and another little cup Sugar.Measure 5 grams of salt and also 5 grams that sugar and place castle in the pair of little labeled cups. Pour 5 mL that room-temperature water right into the pair of larger empty cups.
ProcedureAt the very same time, pour the salt and sugar right into the water in the matching cups. Swirl every cup in ~ the exact same time and also in the same means for about 20 seconds. Walk approximately the room to show students the quantity of salt and also sugar left in the bottom of every cup. If you have actually an overhead projector, location the cup on the projector so the the entire class can to compare what is left undissolved in each cup. Ask students even if it is one substance seems to dissolve far better than the other. Swirl again because that 20 seconds and also observe. Climate swirl for 20 more seconds and also have students make their last observations. Slowly and also carefully pour the solution from each cup ago into its empty labeled cup. Shot not to let any undissolved crystal get in these cups. Present students the cups so the they can compare the lot of undissolved decision remaining.
Much much more sugar will certainly dissolve 보다 salt. There will be an ext undissolved salt than sugar left in the cups.
Note: Solubility is usually measured through the variety of grams that a substance that dissolves in a specific volume the water in ~ a provided temperature. The ahead demonstration provides this conventional way to measure solubility. An additional approach might be to compare the variety of molecules or ions of each substance the dissolves in water. This would require a method to “count” the molecules or ions in each substance.Explore
Have students shot to identify the unknown based upon appearance.
Let students recognize that prior to doing a solubility test, they will look carefully at the crystals to watch if lock might be able to get part clues around the identification of the unknown by appearance alone. Have actually students follow the procedure below and record your observations about the crystals on the task sheet. Let students know that they can look in ~ the crystals and touch them but they have to not taste them.
Question to Investigate
Can you identify the unknown decision by the means it looks?
Materials for Each GroupBlack construction paperMasking tapePen or irreversible marker5 little plastic cupsSaltEpsom saltMSGSugarUnknown (Coarse kosher salt)Magnifier
Teacher PreparationLabel the 5 little plastic cup Salt, Epsom salt, MSG, Sugar, and Unknown. Add at the very least two teaspoons of each decision to its labeled cup.
ProcedureUse masking tape and a pen to label 4 corners that a item of black color construction record Sugar, Salt, Epsom salt, and MSG. Brand the center Unknown.Place little samples the Epsom salt, table salt, sugar, MSG, and the unknown on the labeled locations of the building paper.
Use a magnifier to look carefully at each form of crystal.
All that the crystals space white, yet some are much more transparent or opaque 보다 others. Each type of crystal is additionally a various size and also shape.
Have student teams share their concepts for a solubility test and consider just how each plan controls variables.
As each team presents your plans, have the class identify just how each solubility check controls variables. All groups will likely indicate that they use the very same volume of water in ~ the very same temperature in the same kind of containers, and also the same amount of every crystal. But there might be some disagreement in exactly how to measure up the very same amount of each crystal. Some students may suggest they usage the very same volume that each decision while rather may imply the exact same mass of every crystal. If no one argues using mass, explain that in the demonstration, you used an equal mass of salt and also sugar—5 g that each.
Ask students:Is it better to use the very same volume (like a teaspoon or 5 mL) or the very same mass (like 5 g) of every crystal? Why? tell students that you will carry out a demonstration that will assist them check out whether they need to use a volume or mass measure up so that they deserve to dissolve the very same amount that each crystal in water.
Do a show to display that massive is far better than volume in measure up equal quantities for a solubility test.
Question to Investigate
Is it much better to measure the very same volume or very same mass of every crystal when conducting a solubility test to identify an unknown?
Materials for the Demonstration2 clean plastic cupsCereal balls (Kix work-related well)Zip-closing plastic bag (quart-size storage-grade)Balance
Fill 2 clear plastic cups completely with cereal balls. Both cups must be identical and also contain the very same amount of grain balls. Test these cup on a balance come make sure that this cups have the very same mass.
ProcedureHold the cup filled with cereal up so the students can see that both have around the same amount of grain in them. Place the cup in the center of each finish of a straightforward balance to prove to her students the both contain the same amount of cereal.
Ask students to make a prediction:I to be going come crush the grain balls in one cup. Carry out you suppose the elevation of cereal in this cup to be higher, lower, or the exact same as in the other cup? student will probably say the the crushed grain will no take increase as lot room in the cup.Pour the grain from one of the cups right into a storage-grade, zip-closing plastic bag. Get as lot air out as feasible and seal the bag. Ar the bag ~ above the ground, and also crush the grain thoroughly through your foot. Once the cereal is pulverized, open up the bag, and pour the crushed cereal earlier into the cup.
Ask students:Which cup contains more cereal?Students will realize that both cups contain the exact same amount of cereal, but some may have an advice to say the the cup through the cereal balls contains an ext cereal.Was any cereal included or removed from either cup?Point the end that also though the crushed cereal takes up much less space, that is quiet the exact same amount of issue (cereal) as was in the cup before it to be crushed.How can you prove the these 2 cups save the exact same amount that matter? college student should suggest placing the cup on a balance together you did before.
Place the cups on opposite end of a balance to prove the the mass of cereal in every cup is the same.
Even though the volume of grain balls is greater than the volume of crushed cereal balls, the cups will certainly balance on the scale.
Relate student observations in the demonstration to the five crystals they will certainly dissolve in water.
Tell students to imagine the the large cereal balls represent huge crystals and the crushed cereal represents small crystals. Define that the size and shape that the crystals may be different, however the balance reflects that your mass is the same. Repeat students the mass is a measure of the quantity of matter. Due to the fact that the huge and small crystals (cereal) have actually the exact same mass, both cups contain the exact same amount the matter. Finish that, in order to measure equal amounts, it is far better to measure the massive of substances 보다 the volume.
Ask students:In the solubility test you will certainly do, you will have to measure equal quantities of the 5 crystals. Exactly how will friend measure equal amounts? after ~ this demonstration, students have to realize the measuring mass because that a solubility check is far better than measuring by volume.
Based on what college student observed when they looked closely at the different crystals along with evidence native the demonstration, they have to realize that different crystals have actually a slightly various size or shape. This will cause them come pack differently in the spoon therefore that an ext granules the one will be in the spoon 보다 the other.
Note: The adhering to explanation may be too complicated for students but is consisted of here for you to think around and comment on with students if girlfriend think the is appropriate. Also if salt and also sugar granules were precisely the exact same size and shape and also packed precisely the same means in a spoon, that still would not be a great idea to use a tespoon to measure equal quantities for a solubility test. Here’s why: Salt is about 25% more dense 보다 sugar. As such a teaspoon of salt weighs an ext than a tespoon of sugar by nearly 25%. Her dissolving test would certainly not be accurate since you would certainly be starting out through a bigger mass the salt than sugar.
Have students weigh 5 grams of every of the crystals because that the solubility test.
Materials for Each GroupGram balance5 little plastic cups 5 clear plastic cupsMasking tape and also pen or long-term markerSaltEpsom saltMSGSugarUnknown (coarse kosher salt)Water
ProcedureUse masking tape and a pen to label the 5 little plastic cup Salt, Epsom salt, MSG, Sugar, and Unknown. Label the 5 bigger clear plastic cup the same way. Sweet 5 g of each crystal and place each in its little labeled cup.
If you perform not have enough time, you can stop here and have students store the crystals and conduct the test another day. If you do have time to conduct the test, the procedure follows.
Have student dissolve the 4 known crystals and the unknown in room-temperature water.
The quantity of water used in the procedure is particular and have to be used since it gives clear results. Swirling the crystals in water is a an excellent way of mix them to assist them dissolve. Lead the course so that all groups pour their crystal samples into the water at the same time. Also tell students when to swirl the water and crystals and when to stop and also observe. There will certainly be three 20-second intervals.
Question come Investigate
Can you determine an unknown using a solubility test?
Materials because that Each GroupGraduated cylinder5 g every of salt, Epsom salt, MSG, sugar, and unknown (coarse kosher salt)5 clean plastic cupsWater
ProcedureUse a i graduated cylinder to add 5 mL of room temperature water to every empty clear plastic cup. Match up each pair the labeled cup so that each cup of decision is close to its corresponding cup of water.
When your teacher tells you to, work-related with your lab partners to to water the weighed amount of each crystal right into its cup that water in ~ the exact same time.
Slowly and carefully to water the equipment from every cup back into its tiny empty cup. Try not come let any undissolved crystal get in the little cup. To compare the amount of crystal continuing to be in every clear plastic cup.
Results might vary. However, sugar should dissolve the most, complied with by Epsom salt. MSG should appear to dissolve a bit an ext than salt and the unknown. The salt and also the unknown should show up to dissolve come a similar degree.
Discuss student observations and the feasible identity of the unknown.
Ask students:Are there any type of crystals that you might rule the end as more than likely not the unknown? based on their observations, students room most likely to get rid of sugar and Epsom salt together the unknown.Which cup or cups seem come have around the same amount of decision left undissolved together the unknown?The unknown, salt, and MSG appear to have comparable amounts of crystal that did not dissolve. What carry out you think is the identification of the unknown? Students might conclude the the unknown is salt, but in some situations might think the could additionally be MSG.What proof do you have to support your conclusion?Students should point out the amount of crystal left behind in every cup as evidence that the unknown is one of two people salt or MSG. If who in the class had a very different conclusion and also had very different observations, what perform you think may have led to these differences? Students should mention possible errors in weighing the crystals, in measure the quantity of water used, stirring in a different way, or accidentally pouring the crystals into the wrong cups.
Tell students the their test proved that different substances have different solubilities. In fact, solubility is a characteristic home of a substance. Explain to students that this form of solubility check can aid eliminate some of the crystals, but may not be accurate enough to determine the unknown. Since they may have actually some doubt around the identity of the unknown, college student will perform a recrystallization test with the crystal options made throughout the solubility test.
Have college student conduct one more test to confirm the identification of the unknown.
Explain to students that they might be able to get much more clues about the identity of the unknown if they permit the services of the liquified crystals to recrystallize. Enabling the substances come recrystallize from your solutions can show similarities and differences the were no as easily seen in the initial crystals.
Materials notes: The recrystallization test must be done automatically after the solubility test v the solutions made during the solubility test. Students will reuse the huge clear plastic cups and solutions native the dissolving part of the activity, follow to the procedure.
Question to Investigate
Will the crystals that type when the solutions evaporate help identify the unknown?
Materials because that Each GroupFive remedies made in the activity, each in a little plastic cup5 clear plastic cup from the activityMagnifierWaterPaper towel
ProcedureRinse every large, clear plastic cup v water come remove any kind of remaining crystal. Dry each with a document towel. Very closely pour the solution from each tiny cup right into its equivalent large, clean plastic cup. Allow the services to sit overnight. The next day, use a magnifier to carefully observe the crystals from both the top and bottom the the cup.
Salt and also the unknown look really similar. Epsom salt, MSG, and also sugar look different from each other and also different from salt and also the unknown. The sugar might not have actually recrystallized yet, however given much more time that will form crystals.
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Ask students:Describe the crystals in every cup. What perform you think is the identification of the unknown? students should discuss the shape and size the the various crystals and an alert that both salt and the unknown look really similar.
Tell students the the unknown is rough circuit kosher salt. The is chemically the exact same as continuous salt, but the procedure for making plain table salt and kosher salt is different and also this is why they look different.