Table 2.7 Abbreviations for StatesOne mark of a valid theory is itsability to explain observations. Inthis section, you will investigate theproperties of ionic compounds andmolecular compounds. You will usethe theory you learned in section 1.3to explain what you observe. Then,you will investigate the propertiesof two other important classesof substances.

Bonding and PropertiesChemists can infer many features of the structure and bonding of differentsubstances. They do this by observing their physical and chemical properties. Forexample, physical properties such as structure, melting and boiling points, andelectrical conductivity give important clues about bonding in ionic compoundsand molecular compounds. It is important to know the state of a substancebecause substances have different properties in different states. For example,substances have different properties when they are dissolved in water. To avoidconfusion, chemists use subscripts to show the state of a substance when writingits formula. Table 2.7 shows these subscripts.

Properties of Ionic CompoundsYou can see some important properties of sodium chloride, NaCI(s), in Figure2.4. Note that a red light indicates when a substance is conducting electricity.

. In the solid state, ionic compounds have regular crystalline shapes with

flat sides. This property suggests that the ions in the crystal are arrangedin a regular, repeating pattern. As you know, scientists call this patterna crystal lattice.

~ Like most ionic compounds, sodiumchloride (A) has a relatively high melting point(801'C). It conducts electricity in solution (8), anddoes not conduct electricity in its solid state (C).

56 MHR . Unit I Energy and Matter in Chemical Change

. Ionic compounds tend to have fairly high melting points. This propertysuggests that the attraction between positively and negatively charged ionsin the lattice is strong and extends throughout the crystal. A great deal ofenergy is required to break the strong ionic bonds.

. In the solid state, ionic compounds do not conduct electricity. This propertysuggests that the ions in the lattice are not free to move.

. When melted or dissolved in water, ionic compounds are electrolytes. Theyconduct electricity. This property suggests that melting and dissolving breakdown the crystal lattice and allow ions to move freely and carry charge.

Properties of Molecular CompoundsProperties of molecular compounds can also be used to build up a pictureof their structure. Figure 2.5 shows glucose solid and solution being testedfor electrical conductivity.

. Most molecular compounds have fairly low melting points. This propertysuggests that the attractive forces between molecules must be weak and easyto overcome.

. Once melted, molecular compounds can usually be heated further withoutdecomposing into other compounds or elements. This property suggeststhat the covalent bonds within molecules must be strong.

. Molecular substances tend to be non-electrolytes. They do not conductelectricity in any state as pure substances. This property suggests thatmolecular substances are not made of ions. They do not conduct electricityeven in liquid form, because the liquid contains only neutral molecules.

. When dissolved in water, most molecular substances do not conduct electricity.However, some molecular substances do conduct electricity when dissolved inwater. This observation suggests that some molecular substances can form ionswhen they are dissolved in water. When a substance that is dissolved in waterconducts electricity, you cannot be sure whether it is an ionic compound ora molecular compound. You must perform further tests.

Like most molecular compounds, glucosedoes not conduct electricity either in solid form (A) orin an aqueous solution (8). Glucose is a solid at roomtemperature, but it has a relatively low melting point(146°C). Many molecular compounds, such as sulfurdioxide, 502(g), are gases at room temperature. Manyothers are liQuids.

In the investigation that follows, you will design a procedure to identifysubstances as ionic or molecular, based on their properties. You will explainwhy theories of ionic and covalent bonding help to describe the differentproperties of substances.

Chapter 2 Names, Formulas, and Properties. MHR 57

Scientists often work in me same way as detectives. Like detectivessearching for clues, chemists classify substances by matchingobservations against meories about ionic and molecular compounds.In this investigation, you will try to determine me type of bondingin several common substances by observing meir properties.

Question

Which substances in this investigation are composed of molecules,and which are composed of ions?

Prediction I

Read Procedure steps 1 to 6. Before you write your procedure, examine thesubstances you will be investigating. Describe their physical appearance. Predictwhether your tests will determine whether each one is ionic or molecular.

Procedure0 With your group, decide on a set of at least

five observations, including a conductivity test,which will help you deteffi1ine whether eachsubstance is ionic or molecular. You may includetests using any of the apparatus listed. Qualitativeobservations are fine, too. For example, you candescribe the melting point as "low" or "high,"depending on whether the substance melts ina beaker sitting on a hot plate at low heat. Youcan also describe the odour of the substances.

Safety Precautions

mng~g. This may be the first time you work with chemicals

in this course. If so, review the general laboratorysafety procedures outlined on page xiv of thistextbook. Your teacher may provide additionalguidelines specific to your school.

. Review with your teacher the safety hazards of thechemicals you will be using. Be sure you understandthe MSDS information for these chemicals, and userecommended handling and disposal procedures.

. If you have medical conditions, such as allergies orasthma, inform your teacher and find out appropriateprecautions to take before beginning the investigation.

. Be sure to wear gloves, safety glasses, and protectiveclothing while you are working in the lab.

. Write a step-by-step procedure for your tests.CAUTION: Uyou are going to use smell asone of your tests, be sure to include the correctprocedure for doing so. CAUTION: Nevertaste any substances in the laboratory.. Design a data table to record your

Materials9 labelsdistilled watersolid honey (paraffin

wax is an alternative)

Epsom saltslauric acidwashing soda

e Have your procedure and data table approvedby your teacher.

Apparatusmagnifying lens5 beakers (100 mL)

scoopulastirring rodconductivity testerhot platetest tube rack4 test tubestest tube holder

58 MHR . Unit 1 Energy and Matter in Chemical Change

observations. Use a grid format, withspace for observations of each substancein a separate row.

DidYouKnow?, Assessing the Risk: Toxic Properties of SubstancesIndustrial workers who handle the molecular substance ethanol, C2HsOH(t'),are warned that the chemical can be harmful. The MSDS for pure ethanolstates that it can cause mutations, damage unborn children, irritate the skinand lungs, cause unconsciousness, and damage the liver and nervous system.Despite these hazards, people expose themselves to ethanol solutions daily.They do so every time they drink an alcoholic beverage.

Life in our modern society can involve the use of many chemicals withproperties that produce pronounced physical and psychological effects. Society'sattitude to these substances varies. Some of them are widely accepted. Caffeine,for example, is a powerful stimulant that is found in coffee, tea, cocoa, andchocolate (Figure 2.6). It is a diuretic, meaning that it causes more frequenturination. It also increases blood pressure by constricting blood vessels. Caffeineis not classified as a hazardous substance. Its use is not regulated. Controllingyour caffeine intake is a personal decision.

On the other hand, governments and other bodies strictly regulate theuse of some substances. Many performance-enhancing drugs, for example,are strictly banned from athletic competitions. The use and sale of manypsychoactive drugs, such as cocaine, heroin, and ecstasy (MDMA), are illegal.People under a certain age cannot legally purchase alcoholic beverages andcigarettes. In the last decade, many towns and cities have decided to bansmoking in public places.

Knowing about chemistry can help people make informed personal decisionsabout whether to include potentially harmful chemicals as part of their lifestyle.Table 2.8 outlines some information about the chemicals in alcoholic beveragesand tob:\cco products and their effects. In the next investigation, you will havea chance to find similar information about another chemical that you wishto research.

~ Caffeine is present invarying amounts in all these items.

Table 2.8 Ethanol and Nicotine Products

60 MHR . Unit I Energy and Matter in Chemical Change

Nicotine-like chemicals are beinginvestigated for their potentiallyuseful medicinal effects. Theymay be useful in the treatment ofAlzheimer's disease, Parkinson'sdisease, some mental illnesses,and adult ADD. Some chemicalsthat are related to nicotine appearto improve memory and learningin rats, and may protect rat braincells from some toxins. Otherchemical relatives of nicotine areeffective painkillers. Nicotineitself is a "dirty drug," whosenegative effects often counteractits possible benefits. Tobaccoproducts, such as cigarettes, withtheir mix of potent chemicals,have no known medical use.

" Creatively present your findings and actionplan. For example, you may present your workas a poster, a slide show, or a web page.

Think About ItDeciding to use chemicals always involves balancingrisks and benefits based on what you know abouttheir properties. In this investigation, you will createan action plan for regulating the use of a particularchemical so that people can enjoy its benefits (if any)while minimizing its risk to human health and to theenvironment. Try to create an action plan that isimaginative, feasible, and likely to be effective.

What to Do0 As a group, choose a widely used chemical to

investigate. Some possibilities are listed below:. chlorine or fluoride chemicals added to

drinking water. formaldehyde-emitting glues used in

plywood and chipboard. a medicine

. "road salt" used to melt ice

. addictive chemicals discussed in this section

. MSG (monosodium glutamate) or another

food additive

. Gather information on the benefits of usingthe chemical and the specific hazards it presentsto human health and to the environment. Suchrisks may arise while in producing, distributing,using, recycling, or disposing of the chemical.Use Internet and print resources. The MerckIndex is a useful starting point for informationabout chemicals and their properties.

. Find out whether use of the chemical is nowregulated in Canada. If its use is regulated,find out how.

-I 0 Brainstorm ways to reduce the hazards associatedwith the chemical. You could, for example, decideto ban it completely, control how it is distributed,require a permit to purchase or use it, or restrictthe quantity that is allowed in products. Useyour ideas to create an action plan for usingthe chemical.

Chapter 2 Names, Formulas, and Properties. MHR 61

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r \

I'i ,

Section 2.2 Summary

In this section, you learned about some of the properties of molecularcompounds and ionic compounds. You used your knowledge of bondingto explain some of these properties. You have also explored some of the risbassociated with chemicals and their properties. In section 2.3, you will learnabout the properties of acids and bases. These substances have some dangerousproperties, but they are also very useful.

Check Your Understanding

1. Compare and contrast each term in the pairs below.(a) crystal lattice - molecule

(b) electrolyte - non-electrolyte

2. Explain why ionic compounds tend to have very high melting points,while molecular compounds tend to have relatively low melting points.

3. Apply Examine the table on the left. The table shows melting points forsodium fluoride, sodium chloride, and sodium bromide. By examining thesemelting points, what can you infer about the bonding in each compound?

4. Apply You have been given two solid substances. One of them isan ionic compound, and one is a molecular compound.

(a) Describe four safe tests you could perform to try to identify thesubstances as ionic compounds or molecular compounds.

(b) For each test, explain what you would expect to see for an ioniccompound, and what you would expect to see for a molecular compound.

5. Thinking Critically Examine the observations in the table below.

(a) Use the observations to classify each substance as an ionic compoundor a molecular compound. Give reasons to justify your choice.

(b) Based on the information in the table, which of the five tests (odour,

hardness, ability to dissolve in water, conductivity, or melting point)is the least useful when classifying compounds as ionic or molecular?Explain your answer.

62 MHR . Unit 1 Energy and Matter in Chemical Change