ac5 white powders

While there is a variety of very exciting white powders out there in the world, we will contain our exploration of white powders to ionic compounds. Though some of the “white” powders are slightly blue and all are legal, you will still get a feel for a few ways forensic scientists and chemists identify unknown ionic compounds.

This lab is in four parts. You’ve already completed one of these portions of this identification lab – flame tests. With flame tests, you reviewed the concept of atomic structure and emission spectra data. The other three parts will be some review, but unlike flame tests, this investigation will introduce new concepts (so that is why I have kept them in a separate lab). We will work on this lab for a number of days, so by the end you will have a better idea of how to (1) name ionic compounds, (2) how to determine the solubility of ionic compounds in solution, (3) how to predict the product of a double replacement reaction, and (4) identify acids and bases. Yes, there are quite a few content concepts in this single activity! The main objective of this lab, though, is to learn how to identify white ionic solids through their chemical and physical properties. As in all labs this unit, we are also working to discover the limitations of each test.

Here is a list of the “white” ionic powders (bolded) and the reagents (underlined) we will use for this lab (including the flame test chlorides which are in italics). In a well organized table, write the name, the chemical formula, and a common use of the compound for each reagent, household white powder, or flame test white powder. To determine a household use, you will need to reference Wikipedia or whatever other source on chemical compounds you choose.

KCl calcium sulfateCaCO3 copper (II) chlorideCaCl2 strontium chlorideBaCl2 silver nitrateNaHCO3 acetic acid (H+ + acetate ion)NaOH sodium chlorideLiCl sodium carbonate

As always, include an objective or essential question for this laboratory activity and share that objective with a teammate or laboratory partner.

Based on Active Chemistry: Forensics

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P R E P A R I N G

WHAT DO YOU THINK? • LEARNING OBJECTIVES P

Activity 5: White PowdersIdentifying White SolidsF

This lab will be set up a bit differently than your previous activities. Since there is so much material packed into one forensic concept (identifying white powders), this experiment will be broken into four sections for experimenting. An overall analysis will be required with the construction of a flow chart under analyzing. The four critically thinking questions will also serve as the portion that unifies all four of these different sections.

P a r t A : F l a m e T e s t s

See previous laboratory journal write up to reference this data. You do not need to enter anything here, but you should remember that the first test you’d run on an unknown white powder is a flame test.

P a r t B : S o l u b i l i t y

In this section you will determine whether each of the household white powders (bolded in the preparing section) is soluble in water. This is a test of one of the physical properties of the white ionic solids.

1. Label each of six small Erlenmeyer flasks with the name of one of the white powders.2. To each flask, add a small scoop (peanut-sized) of the corresponding powder.3. Fill the flask ¾ of the way full with distilled water.4. Stopper and mix the flask.5. In your observation/results section, write what you observe. Specifically, note if the solid

dissolved fully, partially, or not at all. 6. Save these solutions for Part C and D.

P a r t C : R e a c t i o n s w i t h A g N O 3 a n d H C 2 H 3 O 2

Reaction with silver nitrate

In this section you will look at how each of the household white powders reacts with silver nitrate (one of the reagents from the list in the preparing section). This is a test of one of the chemical properties of the white ionic solids.

1. Using a beryl pipet, create a set of six test tubes, each with a small sample of one white powder solutions from Part B, filling the test tube ½ full.

2. Add 5 drops of a 0.10M solution of silver nitrate. SILVER NITRATE MAKES A MESS BY PERMANENTLY STAINING SKIN AND TABLE TOPS! BE CAREFUL PLEASE.

3. Mix the test tube with a few firm flicks.4. Record your observations in the observation/results section.

Reaction with acetic acid

In this section you will look at how each of the household white powders reacts with acetic acid (one of the reagents from the list in the preparing section). This is a test of one of the chemical properties of the white ionic solids.

1. Using a beryl pipet, create a set of six test tubes, each with a small sample of one white powder solutions from part B, filling the test tube ½ full.

2. Add 20 – 30 drops of a 1.0M solution of acetic acid.3. Mix the test tube with a few firm flicks.4. Record your observations in the observations/results section.


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E X P E R I M E N T I N G

FLAME • SOLUBILITY • REACTIONS • ACID/BASEE

P a r t D : A c i d s a n d B a s e sIn this section you will look at whether the solution of each household white powder is acidic (pH < 7) or basic (pH > 7). Phenolphthalein (PHTH) is a reagent that indicates if the solution is acidic or basic. If after adding PHTH the solution turns pink, it is basic. You will then add Universal Indicator (UI) to another solution of each white powder to determine the pH. This should feel very familiar (think to the Periodic Table Unit). This is a test of one of the physical properties of the white ionic solids (pH of a solution of the white powder).

1. Using a beryl pipet, create two sets of samples of each of the six solutions from part B, filling the test tubes ½ full.

2. To one of the sets of solutions, add 4 drops of PHTH.3. Record your observations in the observations/results section. Name each white power as

acidic or basic in solution.4. To the other set of solutions, add 4 – 8 drops of UI solution to each test tube.5. Record your observations in the observations/results section, using the scale to match the

color to a pH.

R e a d i n g : W h i t e P o w d e r s

Identifying Unknowns in ChemistryForensic chemists are often faced with the challenging task of identifying unknown compounds. There are millions of possible compounds and mixtures of compounds to choose from, forensic chemists must develop tests that identify each one! The use of tests to determine the identity of an unknown compound is called qualitative analysis. In this type of analysis, it is the identity of the unknown – the qualities that make it unique – that is important and not the quantity, or amount, of the substance. In this experiment, you used the results from a series of tests performed on six different compounds to develop a flowchart. This flowchart will allow you to identify one or more unknown white powders found at the chapter challenge crime scene, if the powder or powders are the ones you have tested in this activity.

The key to identifying a white powder is testing properties that allow one powder to be differentiated from another. For example, both sugar and salt dissolve easily in water. Therefore, testing a powder that may be either sugar or salt by dissolving the powder in water would not help tell them apart. But sugar melts at a relatively low temperature, while salt melts at a much higher temperature, so if our white powder melts in a pan on the stove, we know that it cannot be salt. In other words, the tests used in a qualitative analysis depend on the properties of the possible unknown substances. If the number of possible substances is large or some of the possible substances have many physical and chemical properties in common, a scientist might have to conduct several different tests before she can accurately identify the unknown substance.

Chemical FormulasThe white powders you tested have different physical and chemical properties because they are each unique chemical compounds with different names and chemical formulas. A chemical formula is how chemists represent the chemical composition of a compound. They use the atomic symbols for each element to represent the chemical makeup of the compound. For example NaCl is made up of sodium and chloride. The subscripts represent the relative number of one element to another. For example, in H2O there are two atoms of hydrogen for every one atom of oxygen.


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A N A L Y Z I N G

READING • QUESTIONSA


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Ionic CompoundsThe white powders used in this lab are all ionic compounds. Ionic compounds are made of a combination of positive and negative ions, which were introduced in Activity 2. Sodium chloride is an ionic compound, so its chemical formula, NaCl means there is one positive sodium ion (Na+) for every one negative chloride ion (Cl-). Since the overall compound is neutral, we don’t include the charge on the compound like we do on the ion.

Ionic compounds form on the basis that opposite charges attract. The positive sodium ion is attracted to the negative chloride ion. This attraction between the positive ion (also called a cation) and the negative ion (also called an anion) is called an ionic bond, and the substance formed by the bond is the ionic compound. The sodium ion and chloride ion join to form the ionic compound sodium chloride, NaCl, shown below.

Na+ + Cl- NaCl

In calcium chloride, CaCl2, there is one positive calcium ion (Ca2+) for every two negative chloride ions (again, Cl-).

Ca+ + 2Cl- CaCl2

Notice since there are two chloride ions for every one calcium ion there is 2 in front of the chloride ion. You must always have the same number of ions or atoms of each element on both sides of the chemical equation due to the principle of the conservation of matter, which states that matter cannot be created or destroyed. In other words, chloride ions must be equal in the reactants and products, so there must be two on each side.

In an ionic compound the cations and take the name of the element. The anions take the name of the element but the ending is changed to –ide. Notice that metals form positive ions and nonmetals form negative ions.

Some Common Ions

Groups of atoms may act as a single ion in an ionic compound, like in calcium carbonate, CaCO3. There is one calcium cation (Ca2+) for every one carbonate anion (CO3

2-). We call ions like carbonate, (CO3

2-), polyatomic ions. Sulfate (SO42-), hydroxide (OH-), nitrate (NO3

2-), acetate (C2H3O2

-) and bicarbonate (HCO3) are other polyatomic anions found in the white powders and reagents used in this lab.


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SolubilityIn Part A, you placed a sample of each white powder in water to see if it would dissolve. This property of matter is called solubility. The powders that dissolved in water are said to be soluble. When a solid dissolves in water the mixture of the dissolved solid and the water is called a solution. The powders that did not dissolve are said to be insoluble. Whether or not a particular powder dissolves in water is a complex matter that depends on many factors that are not discussed here. Fortunately, the solubility of many substances in water has been investigated. The results of some of the investigations are summarized in the table below.

Table of Solubility Rules

This table can be used to predict if a given solid will dissolve in water. To determine if a solid is soluble, look at the chemical formula for the solid, and identify which of the five negatively charged ions listed in the above table the solid contains. Then read the rule for solids containing that ion and the exceptions to the rule. For example, sodium carbonate, Na2CO3, contains the ion carbonate, CO3

2-. According to the table, all carbonates are insoluble except for the ones containing Group I metals. Sodium is a Group I metal, so sodium carbonate is an exception to the general rule that carbonates are insoluble. Therefore, sodium carbonate is soluble and dissolves in water. Other examples are given below.

•KCl is soluble because all chlorides are soluble except those containing ions of silver, mercury (I), and lead (II), and KCl does not contain any of the exceptions.

•Mg(OH)2 is insoluble because hydroxides are insoluble except for those containing Group I metals and magnesium is not a Group I metal.

Reactions of Ionic Compounds: Double Replacement ReactionsIonic compounds can form insoluble solids in chemical reactions. For example, when silver nitrate solution is added to sodium chloride solution a milky white solid is formed. The chemical equation for this reaction is shown below.

silver nitrate + sodium chloride sodium nitrate + silver chloride

AgNO3 + NaCl NaNO3 + AgCl

By examining the reaction, you can see that the metals, sodium (Na) and silver (Ag), exchange places to form two new compounds. This reaction is called a double replacement reaction. You can determine the identity of the milky white solid, called a precipitate, by looking at the two products and using the solubility rules. The first product, sodium nitrate, dissolves in water because all nitrates are soluble. The second product, silver chloride, is insoluble because all chlorides are soluble with the exception of mercury (II), lead (II), and silver. Because of silver chloride’s insolubility,


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instead of dissolving and being invisible like the sodium nitrate, the silver chloride that is formed is visible as a milky white precipitate.

More examples of double replacement reactions are shown below:

potassium chloride + silver nitrate potassium nitrate + silver chloride

calcium chloride + sodium carbonate calcium carbonate + sodium chloride

Another example of a double replacement reaction from your lab is the reaction of acetic acid solution with sodium hydrogen carbonate solution.

2HC2H3O2 + Na2CO3 2NaC2H3O2 + H2CO3

The hydrogen ion in acetic acid trades places with the sodium ion in the sodium carbonate. The carbonic acid product, H2CO3, immediately decomposes to produce carbon dioxide gas (the bubbles in part A) and water.

H2CO3 H2O + CO2

So whenever carbonic acid is a product in a chemical equation, we replace it with water and carbon dioxide.

2HC2H3O2 + Na2CO3 2NaC2H3O2 + H2O + CO2

Generally, acetic acid and other acids react with carbonates and hydrogen carbonates to form carbon dioxide gas. The gas is in the bubbles that form in the solution. A carbonate is a compound that contains the CO3

2- ion. Some common carbonates include sodium carbonate (Na2CO3) and calcium carbonate (CaCO3). Hydrogen carbonates are compounds that contain the bicarbonate ion HCO3

-. The most common bicarbonate is sodium bicarbonate, NaHCO3, commonly known as baking soda. Bicarbonates react with acids in a manner similar to carbonates.

Acids and BasesIn Part D, you added phenolphthalein (PHTH) to your sample. Phenolphthalein is one of a class of compounds known as acid-base indicators. Acids are compounds that form H+ ions in solution. Acids taste sour (citric and maleic acids are used in sour candies), react with metals (corrosive), neutralize bases, and react with some indicators to produce a color change. Acid-base indicators are substances that change color when exposed to an acid or a base. Acids do not cause phenolphthalein to change color. Some common acids and their uses include:

Name Formula UseAcetic Acid HC2H3O2 VinegarCarbonic Acid H2CO3 Carbonate SodasHydrochloric Acid HCl Stomach Acid, Serial Number Etching

Bases are compounds that form hydroxide ions (OH-) in solution. Bases taste bitter, are corrosive, feel slippery, saponify fats (turns fats into soaps), neutralize acids, and cause certain indicators to change color. Bases cause phenolphthalein to turn bright pink. Some common bases and their uses are listed below.

Name Formula UseSodium Hydroxide (Lye) NaOH Making soap, drain cleaners


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solidsoluble

solid soluble

gas

Ammonia NH3 Fertilizers and cleanersMagnesium hydroxide Mg(OH)2 AntacidsSodium Carbonate Na2CO3 Luminol blood test

Flow Charts

Now that you can identify each compound based on its solubility in water and reaction with the other three reagents, you need a way to represent this simply. In this portion of the lab, you will learn how to use a type of map called a flowchart. Flowcharts can be used to illustrate a deductive reasoning process (remember Activity 1?!), so they can be very handy to a forensic scientist.

The following flowchart (next page) can be used to identify the six modes of transportation shown. Choose a mode of transportation and see if the flowchart identifies it correctly. Choose a second one and try it again. On this sheet, fill in all the blanks. Is each blank unique? If so, the flowchart works.

Notice that a flowchart does not need to be based on yes-or-no questions to work. For example, one of the questions could have been, “How many wheels does it have?” Answers to this could have been, “Two,” “Four,” and “More than four.”


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No

Q u e s t i o n s : W h i t e P o w d e r s

1. Which of the following compounds are insoluble in water?K2CO3, Na2SO4, MgCO3, Ba(OH)2, FeCl3, Cu(NO3)2, PbCl2

2. Use the list of ions below to answer the questions.Cl-, Na+, Al3+, SO4

2-, MnO4-, NH4

+, O2-, Fe2+

a.Which ions are cations?b.Which ions are anions?c.Which ions are polyatomic ions?

3. Identify the double replacement reactions. For those that are not double replacement reactions, write what type of reaction they are based on what we learned in the periodic table unit.

a.AgNO3 + NaBr → NaNO3 + AgBr

b.CaCO3 → CaO + CO2

c. FeCl3 + 3KOH → 3KCl + Fe(OH)3

d.Zn + CuSO4 → ZnSO4 + Cu

4. Complete the word equations (in words!) for the following double replacement reactions.a.potassium chloride + lead (II)nitrate →

b.iron (III) chloride + potassium hydroxide →

c. sodium hydroxide + calcium nitrate →

5. What are the precipitates that form in question number 4? Write their chemical formulae.

6. Name these acids or bases. What color would PHTH turn in a solution of each compound?

a.HCl

b.H2SO4

c. KOH

d.H2SO3

e.Mg(OH)2


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W h a t d o e s t h e a c t i v i t y m e a n ? Chemistry explains the macroscopic phenomenon (what you observe) with and explanation of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Explain the meaning of this activity by completing the MNS table.

M A C R O N A N O S Y M B O L I C

What are the observable changes that take place to indicate a double replacement reaction has happened?

To the best of your ability, describe what a solution of an ionic compound looks like at the nanoscale. Looking at a picture of a solution on Odyssey may be VERY helpful.

Chemists use formulas as symbols to represent elements and compounds. Explain the meaning of the formula NaOH. How do chemists name this ionic compound?

H o w d o I k n o w ?

Using terms such as chemical change, physical change, and the types of changes, explain how someone could test to know they have discovered baking soda at a crime scene.

W h y d o I b e l i e v e ? Are these tests presumptive or confirmatory tests? Explain by comparing and contrasting to other activities in the unit.

W h y s h o u l d I c a r e ? Create a flow chart to identify the 12 white powders in this activity (see preparing). The flow chart should be used for the qualitative analysis of these white powders. It will be a tool for you to use when you solve your crime at the end of the unit.


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C R I T I C A L L Y T H I N K I N G

MEAN? • KNOW? • BELIEVE? • CARE?CT

ac5 white powders

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