activity 5 clay -...

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Active Chemistry Artist as Chemist

Activity 5 Clay

GOALSIn this activity you will:

• Identify an unknown hydrate.

• Distinguish between a hydrated and an anhydrous compound.

• Examine and describe the effects of heat on clay.

What Do You Think?Ceramics are materials made from clay and have been used by humans for both practical and artistic purposes dating back almost 13,000 years. However, ceramics are far from being antiquated. In fact, they are used in some of our most “high-tech” materials today.

• List as many ceramic materials and products as you can.

• What are some properties of ceramic materials that make them so useful?

• What are some properties of ceramic materials that can limit their usefulness?

Record your ideas about these questions in your Active Chemistry log. Be prepared to discuss your responses with your group and the class.

Investigate

Part A: Heating a Hydrate

If a substance contains water as part of its crystalline structure, it is called a hydrate. The solid that is left when the water is removed from a hydrate is called an anhydrate. In this part of the activity, you will remove the water from a mystery hydrate. By measuring the amount of water removed, you will be able to identify the hydrate.

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1. You will use heat to drive off the water from a hydrate. Dehydrating the hydrated compound requires intense heating for a period of time. You will use a clean, dry crucible and cover to do this. Set up your equipment as shown in the diagram.

2. You will need to determine the mass of water in your hydrate.

a) What information will you need to collect in order to calculate this? Design a procedure you will follow to obtain the information. Use the following steps to guide you.

b) Construct a data table for recording this information in your Active Chemistry log. Make sure that you identify on your data table which unknown you are testing.

Have your procedure and data table approved by your teacher before starting.

3. Light the burner. Heat the crucible with its cover in the hottest part of the flame for 3 min in order to remove any moisture that might be present in the crucible or cover.

Remove the crucible carefully using tongs. Place it on a heat-resistant surface on your lab table to cool. Turn off the burner.

When the crucible and lid have cooled, determine the mass to the nearest 0.01 g.

4. Obtain your unknown hydrate. You will not need more than 2.00 g of the hydrate. Observe the hydrated compound with a hand lens. Measure the mass of the unknown hydrate, crucible/lid to the nearest 0.01 g.

a) Record the letter of your unknown on your data table.

b) Describe the crystalline structure of the hydrate in your Active Chemistry log.

5. Place the hydrate in the crucible and then on the clay triangle. The cover should tilt slightly, which will allow water vapor to escape as it forms. Light your Bunsen burner and adjust flame.

6. Begin heating gently, gradually increasing the heat until there is no more popping or spattering. Remove the cover using tongs and examine the material in the crucible. If the edges of the solid are turning brown, reduce the heat momentarily and then begin heating again at a slower rate.

7. Use tongs to remove the crucible, lid, and contents. Allow them to cool before making any measurements. Some anhydrous compounds will readily absorb moisture from the air so it is important that you quickly determine the mass as soon as it has cooled down. If a desiccator is available, place your crucible inside the desiccator to let it cool.

Safety goggles and a lab apron must be worn at all times in a chemistry lab.

Follow all safety rules for working with an open flame.

Handle objects with care once they have been heated. Objects look the same whether they are hot or cool.

Activity 5 Clay

clay triangle

crucible with lid

clay triangle

crucible with

lid ajar

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Wash your hands and arms thoroughly after the activity.

8. Using good technique and making accurate measurements will be critical in obtaining accurate calculations. Make sure you record mass to the nearest 0.01 g.

a) How can you be sure that all of the water has been removed from the hydrated compound?

b) Adjust the procedure as needed to ensure that you have removed all of the water.

9. Observe the dehydrated compound with a hand lens.

a) Describe the crystalline structure. Note any changes in your observations before and after heating.

b) Explain what might be responsible for any changes that you noted. Record these observations and thoughts in your Active Chemistry log.

10. Dispose of the materials as directed by your teacher. Clean up your workstation.

Part B: Using Calculations to Find the Formula of a Hydrate

In this part of the activity, you will determine the formula of the unknown hydrate.

1. Using the original mass of the hydrate and the final mass of the anhydrate, calculate the mass of the water removed.

2. To determine the identity of the unknown hydrate, you will have to know the amount of water and the amount of hydrate in your original sample. You already know the amount as measured in grams. What you will need to calculate is the amount in moles — the wonderful chemistry way of counting.

The mole is a specific unit of measurement that chemists use. A mole is simply a quantity of something. It happens to be a very large quantity. Just like a pair means two items and a dozen means 12 items, a mole means 6.022 � 1023 items! This odd unit is used because it makes calculations easier. Typically, it will refer to a number of particles — atoms, molecules, and ions.

One mole of any element is also equal to its atomic mass in grams. From the periodic table, you find that one mole of hydrogen atoms has a mass of 1.008 g. A mole of carbon atoms has a mass of 12.01 g, and a mole of oxygen atoms has a mass of 16.00 g. You can determine the molar mass of a molecular or ionic compound by simply adding up the atomic masses (in grams) of each element present in the molecules or ions.

Thus, the molar mass of water is 18.0 g. One mole of H2O contains two moles of hydrogen (2.0 g, rounded off) and one mole of oxygen (16.0 g) for a total molar mass of 18.0 g /mol.

a) Calculate the amount of water removed from your sample in moles. If you had removed 18 g of water, that would have been 1 mol. If you had removed 0.18 g, that would have been 0.01 mol.

_ g of water � 1 mol H2O

_________ 18.0 g

� _ mol H2O

3. You do not know which anhydrate you have out of the following four possibilities shown in the table. Therefore, you will need to calculate the number of moles of each possible anhydrate.

a) Prepare a similar table in your log.


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b) Calculate and record the molar mass for each of the possible anhydrates.

Suppose your unknown has the formula MgSO4. The formula tells you several things. The formula indicates that for every one unit of this compound, there is 1 magnesium atom, 1 sulfur atom, and 4 oxygen atoms. You can look up these atomic masses on the periodic table. The first entry is already listed for you, so that you can check your procedure. You can now proceed to calculate the molar masses for the other anhydrates.

c) Calculate and record the amount of anhydrate in moles. The number of moles must be based on the amount of anhydrate you had divided by the molar mass of each anhydrate. For example, if the mass of the anhydrate was 1.2 g and the anhydrate was MgSO4, then the amount in moles is 0.01 mol since 1.2 g/(120 g/mol) = 0.01 mol.

d) Calculate the amount of anhydrate in moles for each of the possible anhydrates. Use the number of grams of the anhydrate that you measured in the activity and the molar mass that you just determined. Place these values in

the second column of the table. The same number of grams will correspond to a different number of moles for each of the four possible anhydrates.

4. You will now make a prediction of your anhydrate based on one more bit of information. Hydrates are formed with specific ratios of the anhydrate and water.

MgSO4·7H2O

1 mol of magnesium sulfate combines with 7 mol of water

Na2CO3·H2O

1 mol of sodium carbonate combines with 1 mol of water

CaSO4·2H2O

1 mol of calcium sulfate combines with 2 mol of water

CuSO4·5H2O

1 mol of copper sulfate combines with 5 mol of water

Using your calculated quantities for moles of water and moles of the possible anhydrates, determine which of the hydrates you began with by computing which of these anhydrates have the correct ratio of anhydrate to water. The ratio is a ratio of moles — the chemist’s way of counting.

a) Record your answer in your Active Chemistry log.

5. Compare your results with other members of your class with the same unknown.

a) Are there differences? What reasons could account for those differences?

Activity 5 Clay

Anhydrate Molar mass(g/mol)

Number of moles

MgSO4 120.3

Na2CO3

CaSO4

CuSO4

__ g of MgSO4 � 1 mol MgSO4 ___________

120.3 g � __ mol MgSO4



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Safety goggles and a lab apron must be worn at all times in a chemistry lab.

Wash your hands and arms thoroughly after the activity.

Part C: Clay

1. Obtain a sample of artist’s clay from your teacher. Observe the hydrated compound with a hand lens.

a) Describe any solid structures you see.

2. Measure the mass of the hydrated clay.

a) Record the mass in your Active Chemistry log.

3. Create an object from the clay. You may wish to create something that you will be able to use in your museum display or a replica of another object made of clay. Or you may want to just be creative and make something unique with your clay.

4. Let your object air-dry overnight, or, if available, set it in a low-heat drying oven overnight.

You now have dehydrated clay. (We limit the use of the word anhydrate for pure compounds. This clay

is composed of many different compounds.) Observe your dried object with a hand lens. Note any changes compared to dehydrated clay, the hydrated clay, and the dehydrated clay object.

a) How might you explain any differences that you noted?

5. Measure the mass of the dehydrated clay object.

a) Record the mass in your Active Chemistry log.

6. Calculate the mass percent of water in your sample:

mass of water removed __________________ starting mass of hydrate

� 100% � _% water

a) Record the mass percent of water in your log.

b) If you had fired your clay in a kiln, how might that affect the calculated percentage of water?

7. Dispose of the materials as directed by your teacher. Clean up your workstation.


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HYDRATES AND ANHYDRATES

Defining Hydrate and Anhydrate

Many compounds form as a result of reactions that occur in water solutions. These compounds appear to be dry, but when they are heated, water is released. The water molecules are a part of the crystalline structure and are weakly bonded to the ions or molecules that make up the compound. When a potter fires his clay in a kiln, water is removed from the clay, resulting in a change in the nature of the clay. If a substance contains water as part of its crystal structure, it is called a hydrate. The solid that is left when the water is removed from a hydrate is called an anhydrate.

The Chemistry Way of Counting – Moles

Since atoms are so incredibly small and samples of matter typically contain so many atoms, chemists needed to establish a method for counting atoms. The method is the same one employed by food manufacturers everywhere. No one at the factory would want to have to count the candies that go in a 454-g bag each time. Instead, if they know that 10 candies have a mass of 8.9 g, they can estimate that there will be about 510 candies in a 454-g bag. They can simply use the mass to determine the number of items in the bag.

Chemists routinely use a unit of measurement called the mole. A mole is simply a quantity of something, just like an octet is a specific quantity of something. An octet means eight and a mole happens to mean 6.022 � 1023.

Chem Wordshydrate: a compound that has water attached to it.

anhydrate: a compound that does not have any water attached to it.

mole: the number equal to the number of carbon atoms in exactly 12 g of pure 12C.

Activity 5 Clay

1 dozen = 12 1 mole = 6.022 � 1023

1 dozen pots � 12 pots 1 mole pots � 6.022 � 1023 pots

1 dozen atoms � 12 atoms 1 mole atoms � 6.022 � 1023 atoms

1 dozen molecules � 12 molecules 1 mole molecules � 6.022 � 1023 molecules

6.022 � 1023 � 602,200,000,000,000,000,000,000



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Chem WordsAvogadro’s number: the number equal to the number of carbon atoms in exactly 12 g of pure 12C, 6.022 � 1023 units.

Furthermore, if a student has two dozen brushes, you know she has 24 brushes. You carry out the mathematics of this in your head with no trouble. The math looks like this:

2 dozen brushes � 12 brushes ____________ 1 dozen brushes � 24 brushes

If you have 2 mol of atoms:

2 mol atoms � 6.022 � 1023 atoms

______________

1 mol atoms � 2(6.022 � 1023 atoms) � 1.204 x 1024 atoms

By definition, one mole is equal to the number of carbon atoms in exactly 12 g of pure carbon-12 (the isotope

carbon-12, or 12C). Various techniques have been used to determine this number as 6.022 � 1023. This number is called Avogadro’s number to honor Amedeo Avogadro's contributions to chemistry.

It is also true that 12 g of 12C contains 6.022 � 1023 atoms. One mole of any substance will always contain 6.022 � 1023 chemical units of that substance. The masses of one mole of two different substances will not be the same because the atoms making up those

substances have different atomic masses.

The mass of one mole is equal to an element’s atomic mass expressed in grams.

This means that:

• 1 mol of He contains 6.022 � 1023 atoms of He and has a mass of 4.003 g

• 1 mol of Al contains 6.022 � 1023 atoms of Al and has a mass of 26.98 g

• 1 mol of U contains 6.022 � 1023 atoms of U and has a mass of 238.0 g

• 2 mol of U contains 1.2044 � 1024 atoms of U and has a mass of 476.0 g

• 1 mol of H2O contains 6.022 � 1023 molecules of H2O and has a mass of 18 g:

2 [H (1 g)] � 1 [O (16 g)] � 18 g

• 1 mol of SiO2 contains 6.022 � 1023 molecules of SiO2 and has a mass of 60 g:

1 Si (28 g) � 2 [O (16 g)] � 60 g

• 1 mol of ZnSO4·2H2O contains 6.022 � 1023 formula units of ZnSO4·2H2O and has a mass of 197 g:

Amedeo Avogadro


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• 1 Zn (65 g) � 1 S (32 g) � 4 [O (16 g)] � 2 [H2O (18 g)] � 197 g

See the pattern? You just add up the masses of all the elements present in a compound to determine the mass of one mole of that compound. This is called the molar mass.

This means that anything less than that mass will be only part of a mole.

For example, if you weighed out 2.00 g of ZnSO4·2H2O, you would have 0.0102 mol of ZnSO4·2H2O, as shown below.

2.00 g ZnSO4·2H2O � 1 mol ZnSO4·2H2O

_______________ 197 g � 0.0102 mol ZnSO4·2H2O

If you have 500.0 g of ZnSO4·2H2O, that would be:

500.0 g ZnSO4·2H2O � 1 mol ZnSO4·2H2O

_______________ 197 g � 2.54 mol ZnSO4·2H2O

In order to determine the identity of your unknown hydrate, you have to understand a few things about chemical formulas. Suppose your unknown has the formula CaSO4·2H2O. The formula tells you several things. First, the formula indicates that for every one unit of this compound, there is 1 calcium atom, 1 sulfur atom, 6 oxygen atoms (4 from the CaSO4 and 2 from the H2O), and 4 hydrogen atoms. Most commonly, this compound would be described as a dihydrate. That is, there are two water molecules for every calcium sulfate group. Its name is therefore calcium sulfate dihydrate. The formula also indicates that for every 1 mol of this compound, there will be 1 mol of calcium atoms, 1 mol of sulfur atoms, 6 mol of oxygen atoms, and 4 mol of hydrogen atoms.

Chem Wordsmolar mass: the mass of one mole of a pure substance.

Activity 5 Clay

Number of Moles of Each Atom in CaSO4·2H2O

Hydrogen atoms

Oxygen atoms Sulfur atom Calcium

atom CaSO4·2H2O



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Putting Hydrates and Anhydrates to Work

Gypsum is a natural mineral whose chemical name is calcium sulfate dihydrate. The formula is represented as CaSO4·2H2O. This tells you that each molecule of calcium sulfate has two water molecules attached to it.

If you have ever had a broken bone, the doctor may have made a cast out of gypsum to fit over the broken area. Improved materials have replaced gypsum in many places. Gypsum is also called plaster of Paris from gypsum quarries located near Paris. To make plaster of Paris you start with powdered gypsum by heating the hydrated calcium sulfate to about 160ºC to drive off some of the water to form the hemi-hydrate calcium sulfate (CaSO4·

1 __ 2 H2O). Hemi means 1 __ 2 . Now when you mix

it with water it makes a paste that you can apply to the area that has the broken bone. A hard cast is formed when the hemi-hydrate (CaSO4·

1 __ 2 H2O) reacts with the water to return to the original

dihydrate (CaSO4·2H2O).

2CaSO4· 1 __ 2 H2O � 3H2O → 2CaSO4·2H2O

This process takes about 30 min to set.

Plaster has many uses in the world of art as well. Plaster is used to produce intricate details in interior architecture. Plaster can be poured into casts to create sculptures or other pieces of art. Plaster is often used as an intermediate stage for large bronze sculptures. Plaster can also have other substances like cement, sand, and even wood fibers added to it to give it more strength.

Checking Up1. What is a mole?

2. What is molar mass?

3. What is the difference between a compound that is hydrated or anhydrous?

4. How many grams are there in 1.5 mol of water?


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Activity 5 Clay

What does it mean?

Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your Active Chemistry log.

How do you know?

Draw a model of the copper sulfate pentahydrate (CuSO4·5H2O) and copper sulfate anhydride using diamonds to represent the anhydride and circles to represent H2O molecules.

Why do you believe?

When measuring anhydrous compounds on the balance, why must you always remember to immediately replace the lid on the reagent container?

Why should you care?

Would it be possible to incorporate an anhydrate/hydrate chemical system into a work of art? That is, can you imagine a scenario where a color change takes place in a piece of art? Design a simple system based on this concept to be used in some art form.

MACRO NANO SYMBOLICHow did the mass measurement of the unknown compound that you investigated convince you that the compound was hydrated?

Explain how an anhydrous compound becomes hydrated at the nano level.

Use formulas to show the difference between a hydrated and a dehydrated compound.

What Do You Think Now?At the beginning of the activity you were asked:

• What are some properties of ceramic materials that make them so useful?

• What are some properties of ceramic materials that can limit their usefulness?

What is a potter doing when she is firing her pottery? What effect does this have on the pottery?



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Reflecting on the Activity and the ChallengeSo far, you have examined several different artistic media, and some of the chemistry behind each one. In this activity you investigated hydrated compounds and saw the changes they undergo upon dehydration and re-hydration. You learned that removing water from a hydrated compound affects the crystalline structure of the compound and in turn its properties. You showed these changes by writing formulas and determining the molar masses of the hydrated and anhydrous compounds.

Since your challenge is to create a museum display (including a work of art) and to explain the chemistry related to creating it, consider how you might use the knowledge gained from this activity in your display.

1. How is knowing the percentage by mass of water different from knowing the mole ratio of water to the hydrate?

2. Why do chemists use the unit moles?

3. What is the gram-molecular mass of NaN3(s)?

4. What is the total number of moles of NaN3(s) in a 52-g sample of the compound?

5. A sample of a compound contains 65.4 g of zinc, 12.0 g of carbon, and 48.0 g of oxygen. What is the mole ratio (simplest formula) of zinc to carbon to oxygen in this compound?

6. A sample of a substance containing only magnesium and chlorine was tested in the laboratory and was found to be composed of 74.5% chlorine by mass. If the total mass of the sample was 190.2 g, what was the mass of the magnesium?

7. Kaolinite is a type of clay commonly used for making china. Its formula is Al2Si2O5(OH)4. Suppose you were given approximately 250 g of kaolinite for creating an object. How many moles of kaolinite did you have?

8. You may have noticed that most electronic equipment comes packed with small packets of desiccants (drying agents that pick up water molecules). Speculate about what you think these desiccants might be made of and why they are placed in packaged electronic equipment.

9. In Part A, if the empty crucible was not heated long enough to completely dry it prior to weighing, what would be the effect on the calculated percent water in the unknown (larger or smaller)? Explain.


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Activity 5 Clay

10. Explain how determining the percent composition of water might be useful to a potter.

11. Calculate the mass of water removed in forming the anhydrate from 3.22 g of Na2SO4·(10H2O).

12. Preparing for the Chapter Challenge

Your challenge for this chapter involves the creation of a work of art. In Part C of this activity you made an object out of clay. How could you use this object in your final work of art? Consider the object and come up with some ideas for how you might add another element to the object to make it more useful or aesthetically pleasing. Take one of your ideas for adding on to the clay object and then do it.

Inquiring Further

1. History of ceramics

Research the history of ceramics. What can archaeologists discern about a culture or a society from its pottery? How does the history of ceramics also mirror the advancements in technology for a culture? What are some of the newest uses for ceramics today? What is it about the properties of clay that make it so useful?

2. Desiccants

Desiccants are used for a variety of purposes because of their tendency to absorb moisture. Conduct some research, both in and out of the lab, to determine if there is a relationship between the percent of water in a hydrate and its water-absorbing capacity. Based on your findings, make some recommendations about which anhydrous salts make the best desiccants.


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