3.2 Properties of Solids

1. A1-Butanol's is higher, as its O–H groups form hydrogen bonds between molecules.
   A student who thinks stronger attractions give a higher vapor pressure picks this. 1-Butanol's O–H groups do form hydrogen bonds between its molecules, but that holds them in the liquid and lowers the vapor pressure.
2. BDiethyl ether's is higher, as its molecules cannot hydrogen bond with one another. Correct
   In diethyl ether every H atom is bonded to C, so its molecules cannot hydrogen bond to one another; 1-butanol has an O–H group, so its molecules do. The two isomers are unbranched chains of the same five C and O atoms, so their dispersion forces are comparable; the hydrogen bonding in 1-butanol makes its attractions stronger overall, so fewer of its molecules escape and diethyl ether has the higher vapor pressure.
3. CThey are equal, as molecules of equal molar mass attract one another equally.
   A student who thinks isomers have equally strong attractions picks this. The two molecules have the same molar mass but different structures: only 1-butanol has an H atom bonded to O, so only its molecules hydrogen bond.
4. DDiethyl ether's is higher, as its C–O bonds break far more easily than O–H bonds.
   A student who thinks evaporation breaks covalent bonds picks this. The vapor of each liquid consists of intact molecules; no C–O or O–H bonds break when the liquids evaporate.

CED 3.2.A.1 · Read this in Fix

1. ASolid lines are covalent bonds within molecules; dashed lines are attractions between molecules. Correct
   Each pair of circles joined by a solid line is one X₂ molecule, held together by a covalent bond. The dashed lines join atoms of different molecules and represent the intermolecular attractions (here London dispersion forces) that hold the molecules in the solid.
2. BSolid lines are intermolecular forces within molecules; dashed lines are covalent bonds between molecules.
   A student who confuses 'inter' with 'intra' picks this, placing intermolecular forces inside the molecule and covalent bonds between molecules. The covalent bond is the link within each X₂ molecule; the intermolecular forces act between molecules.
3. CSolid lines and dashed lines are covalent bonds of two lengths, linking the atoms into one network.
   A student who thinks the attractions between molecules are covalent bonds picks this. Only the solid line within each X₂ molecule is a covalent bond; no electrons are shared between neighboring molecules.
4. DSolid lines are covalent bonds within molecules; dashed lines mark spacing between molecules that do not attract.
   A student who thinks nonpolar molecules do not attract one another picks this. Nonpolar molecules attract by London dispersion forces, and these attractions are what hold the X₂ molecules together in the solid.

CED 3.2.A.2 · Read this in Fix

1. AThe same ions are present in the solid and in the liquid, but they move from place to place only in the liquid. Correct
   Both parts of the diagram show K⁺ and Cl⁻ ions. In the solid the ions are in a regular, fixed arrangement and the bulb is off; in the liquid the arrows show the ions moving, and the bulb is lit. The current is carried by the mobile ions.
2. BElectrons are released from the ions when the solid melts, and these electrons move through the liquid to light the bulb.
   A student who thinks only electrons can carry a current picks this. The diagram shows no free electrons: the moving particles in the liquid are the K⁺ and Cl⁻ ions themselves.
3. CThe solid is made of KCl molecules, which the tester cannot detect, and these break up into ions only in the liquid.
   A student who thinks an ionic compound consists of molecules picks this. The diagram shows separate K⁺ and Cl⁻ ions in the solid, each surrounded by ions of opposite charge, not KCl molecules.
4. DCharges from the tester pass through the gaps between the ions, and these gaps are wide enough only in the liquid.
   A student who thinks the charges in a current come only from the battery and pass through gaps in the material picks this. The arrows in the diagram are on the ions: the K⁺ and Cl⁻ ions themselves move and carry the current in the liquid.

CED 3.2.A.3 · Read this in Fix

1. ACalcium oxide, CaO
   A student who thinks any hard, high-melting compound with a continuous lattice is a covalent network solid picks this. CaO is a compound of a metal with a nonmetal: an ionic solid made of Ca²⁺ and O²⁻ ions.
2. BSulfur crystals, S₈
   A student who thinks any solid with covalently bonded nonmetal atoms is a network picks this. Solid sulfur consists of separate S₈ molecules held together by dispersion forces, so it is a molecular solid.
3. CSilicon carbide, SiC Correct
   Silicon is a metalloid and carbon is a nonmetal. In SiC the atoms are covalently bonded into a three-dimensional network, which makes it very hard and high melting.
4. DSolid methanol, CH₃OH
   A student who thinks the attractions between molecules are covalent bonds picks this, treating the hydrogen bonds between CH₃OH molecules as a covalent network. Solid methanol consists of separate CH₃OH molecules, so it is a molecular solid.

CED 3.2.A.4 · Read this in Fix

1. ANaphthalene molecules attract one another by dispersion forces, but NaCl is held by attractions between ions. Correct
   Naphthalene is a molecular solid: its nonpolar molecules are held to one another only by London dispersion forces, which are overcome at a low temperature. NaCl is an ionic solid held by the much stronger Coulombic attractions between Na⁺ and Cl⁻ ions.
2. BNaphthalene's atoms are joined by covalent bonds, which are weaker than the ionic bonds in NaCl.
   A student who thinks covalent bonds are weak picks this. The covalent bonds within each naphthalene molecule are strong and do not break on melting; the weak attractions are the ones between molecules.
3. CNaphthalene molecules are nonpolar, but NaCl molecules are polar and attract by dipole-dipole forces.
   A student who thinks NaCl consists of molecules picks this. Solid NaCl is a lattice of ions with no molecules; the attractions are between ions of opposite charge.
4. DNaphthalene molecules are themselves soft and melt easily, but the ions in NaCl are hard and do not melt.
   A student who gives each particle the properties of the bulk solid picks this. Molecules and ions do not melt; melting is a change in the arrangement of many particles.

CED 3.2.A.5 · Read this in Fix

1. ACircles are positive ions in fixed positions; dots are negative ions held in place by the positive ions.
   A student who thinks the bonding in a metal is ionic picks this. A metal contains no negative ions; the negative charge is carried by the mobile valence electrons.
2. BCircles are metal ions in fixed positions; dots are electrons shared by the two ions nearest to each one.
   A student who thinks the electrons in a metal are shared between particular pairs of atoms picks this. The valence electrons of a metal are delocalized over the whole solid, not held between two cores.
3. CCircles are metal cores in fixed positions; dots are valence electrons free to move through the solid. Correct
   In the model of a metallic solid, each circle is a metal core (the nucleus and inner electrons), which has a positive charge and stays in its lattice position. The dots are the valence electrons, which are not held by any one core and are free to move throughout the solid.
4. DCircles are nuclei in fixed positions; dots are electrons that each stay in orbit around their own nucleus.
   A student who thinks each electron stays with its own atom picks this. The valence electrons of a metal are free to move throughout the solid; only the inner electrons stay with each core.

CED 3.2.A.6 · Read this in Fix

1. AHeating breaks covalent bonds along the chain, and the short fragments fold up incorrectly.
   A student who thinks the shape of a biomolecule is held only by covalent bonds picks this. Warming to 70 °C does not break the covalent bonds of the chain; it disrupts the weaker noncovalent interactions that hold the fold.
2. BHeating makes the atoms of the chain expand, and the larger molecule fits its target poorly.
   A student who thinks atoms expand when heated picks this. The atoms stay the same size; the molecule changes shape because the interactions holding the fold are disrupted.
3. CHeating disrupts noncovalent interactions between regions of the chain, and these hold the fold. Correct
   The folded shape of a protein is largely dictated by noncovalent interactions, such as hydrogen bonds, between different regions of the same chain. These are much weaker than covalent bonds and are disrupted by moderate heating, so the chain unfolds and the enzyme loses the shape its function depends on.
4. DHeating separates the enzyme molecules, and a molecule keeps its fold only while joined to others.
   A student who thinks noncovalent interactions act only between separate molecules picks this. The interactions that hold the fold act between different regions of the same molecule.

CED 3.2.A.7 · Read this in Fix

1. ALiquid Z, as its vapor pressure is the lowest at each temperature on the graph Correct
   At each temperature the curve for Z is the lowest (for example about 150 torr at 60 °C, compared with about 350 torr for Y). A low vapor pressure means that few molecules escape from the liquid, which shows that the attractions between the molecules of Z are the strongest.
2. BLiquid X, as its vapor pressure is the highest at each temperature shown
   A student who thinks stronger attractions give a higher vapor pressure picks this. X does have the highest curve, but a high vapor pressure shows that molecules escape easily, so X has the weakest attractions.
3. CLiquid X, as its molecules have the greatest molar mass of the three
   A student who thinks molar mass decides the strength of the attractions picks this. X has the greatest molar mass (72 g/mol) but also the highest vapor pressure at every temperature, so its molecules are held least strongly.
4. DLiquid Y, as its molecules have the lowest molar mass of the three liquids
   A student who thinks smaller molecules get closer and so attract more strongly picks this. Y has the smallest molar mass (46 g/mol), but its vapor pressure is higher than that of Z at every temperature, so its attractions are weaker than those in Z.

Working Read the three curves at one temperature. At 60 °C the vapor pressure of Z is about 150 torr and that of Y is about 350 torr; the vapor pressure of X is already about 600 torr at 30 °C and is off the scale by 40 °C. At every temperature on the graph the order of vapor pressure is X > Y > Z. A lower vapor pressure at a given temperature means that fewer molecules escape from the liquid, so the attractions between molecules are strongest in Z. The molar masses (X 72, Y 46, Z 60 g/mol) do not follow this order.

CED 3.2.A.1 · Read this in Fix

1. AIt is half right: melting point is the true measure, so 2,2-dimethylpropane attracts more.
   A student who thinks melting overcomes the attractions completely picks this, taking melting point as the direct measure. Vaporizing, not melting, overcomes the attractions completely, so the boiling points are the better guide, and they show that pentane has the stronger attractions.
2. BIt fails: isomers attract equally strongly, so neither property depends on attraction.
   A student who thinks isomers have equally strong attractions picks this. The isomers differ in shape and so in contact area, and their different boiling points show that their attractions differ.
3. CIt is limited: melting only rearranges the molecules, so how they pack also matters. Correct
   The boiling points show that pentane molecules attract one another more strongly, as expected from their larger contact area. Yet 2,2-dimethylpropane has the higher melting point. Vaporizing overcomes the attractions completely, but melting only rearranges them, so melting point also depends on other factors such as how well the molecules pack in the solid. The model works for boiling point but not reliably for melting point.
4. DIt is misapplied: boiling and melting break covalent bonds, not attractions between molecules.
   A student who thinks phase changes break covalent bonds picks this. Both substances melt and boil as intact C₅H₁₂ molecules; only the attractions between molecules are rearranged or overcome.

CED 3.2.A.1 · Read this in Fix

1. ADiagram 1
   A student who thinks boiling breaks the covalent bonds within molecules picks the diagram of separate N atoms. Nitrogen gas consists of N₂ molecules; the strong bond within each molecule is not broken at −196 °C.
2. BDiagram 2
   A student who thinks the particles of a gas stay close together picks the diagram in which the molecules still touch one another. After boiling, the molecules are far apart and spread through the whole container.
3. CDiagram 3
   A student who thinks particles expand when a substance is heated or becomes a gas picks the diagram with enlarged molecules. The gas takes up more space because the molecules are farther apart, not because each molecule is bigger.
4. DDiagram 4 Correct
   Boiling overcomes the attractions between N₂ molecules completely, so the molecules end up far apart and spread through the container. The molecules themselves stay intact and stay the same size.

CED 3.2.A.2 · Read this in Fix

1. AIt fits both properties, as each copper atom is itself malleable and is itself a conductor.
   A student who gives each atom the properties of the bulk metal picks this. Malleability and conductivity are properties of a large collection of particles; they arise from how the cores can rearrange and from the mobile valence electrons.
2. BIt fits malleability only, as rows of identical spheres can shift but nothing in it carries charge. Correct
   Rows of identical spheres can slide to new positions and still be surrounded by identical neighbors, which is consistent with malleability. The model shows no mobile valence electrons, so it contains nothing that could carry charge through the solid and cannot account for conductivity.
3. CIt fits both properties, as the spheres can shift and can also travel through the solid carrying charge.
   A student who thinks the metal atoms themselves move through the solid to carry current picks this. In a solid metal the cores stay in the lattice; the charge is carried by mobile valence electrons, which the model does not show.
4. DIt fits conductivity only, as charges from a battery pass through the gaps between the spheres.
   A student who thinks the charges come only from the battery and pass between the atoms picks this. The charge carriers are the metal's own mobile valence electrons, which the model leaves out; the model can, however, represent rows of atoms shifting.

CED 3.2.A.2 · Read this in Fix

1. AThe attractions between ions are weak, so a small force is enough to pull one layer away from the next.
   A student who thinks brittleness shows weak attractions picks this. The attractions between the ions are strong; the crystal splits because the shift brings ions of like charge together.
2. BEach ion is bonded to one partner ion, and the shift pulls the two ions of each pair away from each other.
   A student who thinks an ionic solid is made of pairs of ions picks this. Each ion is attracted to all of its oppositely charged neighbors; the diagram shows a continuous lattice, not pairs.
3. CIons of like charge are now next to each other, and the repulsion between them pushes layers apart. Correct
   After the shift, cations in the upper layers sit next to cations in the lower layers, and anions next to anions. The repulsion between these like charges replaces the attraction that held the layers together, so the crystal splits. This is why ionic solids are brittle.
4. DThe ions are themselves brittle, so the force cracks the individual ions in the layers that are shifted.
   A student who gives each ion the properties of the bulk solid picks this. Brittleness is a property of the crystal; the ions are unchanged by the force, and the crystal splits between layers.

CED 3.2.A.3 · Read this in Fix

1. AThe solid, the liquid and the solution each conduct well.
   A student who thinks the presence of ions is enough for conduction picks this. The ions in solid KCl cannot move from place to place, so the solid does not conduct.
2. BThe solution conducts, but the solid and the liquid do not.
   A student who thinks ions are formed only when an ionic compound dissolves in water picks this. The ions are present in the solid and become mobile when it melts, so molten KCl also conducts.
3. CNeither the solid, the liquid nor the solution conducts.
   A student who thinks only electrons can carry a current picks this, reasoning that KCl has no free electrons in any state. Mobile ions carry the current in the molten compound and in the solution.
4. DThe liquid and the solution conduct, but the solid does not. Correct
   KCl is an ionic compound. In the solid the K⁺ and Cl⁻ ions are held in fixed positions, so the solid does not conduct. Melting the solid or dissolving it in water makes the ions mobile, so both the liquid and the solution conduct.

CED 3.2.A.3 · Read this in Fix

1. ALondon dispersion forces, applied to the polarizability of each formula unit
   A student who thinks ionic solids are held together by intermolecular forces picks this. NaF and MgO are lattices of ions held by ion-ion attractions, which are far stronger than dispersion forces between molecules.
2. BBond order, applied to a double bond in each MgO molecule and a single bond in each NaF molecule
   A student who thinks ionic compounds consist of molecules picks this, picturing Mg=O and Na–F molecules. Neither solid contains molecules; each ion is attracted to all of its oppositely charged neighbors in the lattice.
3. CCoulomb's law, applied to the charges of the ions and the distance between them Correct
   Ionic solids are held together by Coulombic attractions between ions, F ∝ q₁q₂/r². The distances are similar, but the product of the charges is four times as great for Mg²⁺ and O²⁻ as for Na⁺ and F⁻, so the ions in MgO attract far more strongly and MgO melts at a much higher temperature.
4. DMolar mass, applied to the mass of one formula unit of each of the two compounds
   A student who thinks mass decides the melting point picks this. The molar masses of NaF (41.99 g/mol) and MgO (40.30 g/mol) are nearly equal, so mass cannot account for the difference; the charges of the ions do.

Working The solids are ionic, so the interactions to compare are the Coulombic attractions between ions: F ∝ q₁q₂/r². NaF contains Na⁺ and F⁻ (charges 1+ and 1−); MgO contains Mg²⁺ and O²⁻ (charges 2+ and 2−). With similar r, the product of the charges is (2)(2) = 4 for MgO and (1)(1) = 1 for NaF, so the attraction between neighboring ions is about four times as strong in MgO, consistent with its much higher melting point.

CED 3.2.A.3 · Read this in Fix

1. AWhether the melting point is higher than that of table salt
   A student who thinks the type of solid can be read from how high the melting point is picks this. Both ionic and covalent network solids have high melting points over wide, overlapping ranges, so comparing with NaCl does not distinguish them.
2. BWhether the substance conducts electricity when it is molten Correct
   The properties in the table fit both types of solid. An ionic solid conducts when molten because its ions become mobile; a covalent network solid contains no ions and does not conduct when molten. The conductivity of molten Q therefore distinguishes the two.
3. CWhether the solid is hard enough to scratch a steel blade
   A student who thinks brittle ionic solids are weakly bonded and therefore soft picks this. Ionic solids and covalent network solids are both hard, so a hardness test does not reliably distinguish them.
4. DWhether the solid dissolves in a nonpolar solvent like hexane
   A student who thinks a covalent network solid consists of small molecules picks this, expecting it to dissolve in a nonpolar solvent. A covalent network solid would have to break covalent bonds to dissolve, so neither type of solid dissolves in hexane and the test gives no information.

Working Given: melting point 1400 °C (high), does not conduct as a solid, hard, insoluble in water. These properties fit both an ionic solid and a covalent network solid. The two types differ when molten: an ionic solid contains ions, which become mobile on melting, so the liquid conducts; a covalent network solid contains no ions, so it does not conduct when molten. The quantity needed is the electrical conductivity of molten Q.

CED 3.2.A.3 · Read this in Fix

1. AGraphite's covalent bonds are weak and break when it is rubbed; diamond's covalent bonds are much stronger.
   A student who thinks graphite is soft because its covalent bonds are weak picks this. The covalent bonds within a graphite layer are strong; graphite is soft because whole layers slide past each other.
2. BGraphite's carbon atoms are themselves soft and wear away; diamond's carbon atoms are themselves hard.
   A student who gives each atom the properties of the bulk solid picks this. Both solids are made of identical carbon atoms; the difference in hardness comes from how the atoms are bonded and arranged.
3. CGraphite's atoms are spaced far apart and can be pushed closer; diamond's atoms are packed tightly together.
   A student who thinks hardness depends on how tightly the atoms are packed picks this. Graphite is soft because its layers slide, not because its atoms can be pushed closer together; the atoms within each layer are held by strong covalent bonds.
4. DGraphite's layers attract one another weakly and can slide; diamond's bonds hold atoms at fixed angles. Correct
   In graphite the covalent bonds act within each layer, and adjacent layers, held to one another by much weaker attractions, can slide past each other relatively easily. In diamond every atom is covalently bonded to four others in a three-dimensional network with fixed bond angles, so no part of the solid can shift without breaking covalent bonds.

CED 3.2.A.4 · Read this in Fix

1. ASiO₂ molecules have more electrons than CO₂ molecules, so their dispersion forces are stronger.
   A student who thinks SiO₂ consists of small molecules like CO₂ picks this. SiO₂ contains no separate molecules; its atoms form a covalent network, which is why it melts about 1800 °C higher than CO₂ sublimes.
2. BSiO₂ is a network of covalently bonded atoms, but solid CO₂ consists of separate molecules. Correct
   In SiO₂ the Si and O atoms are covalently bonded into a three-dimensional network, so strong covalent bonds must break for it to melt. Solid CO₂ is a molecular solid: only the weak intermolecular forces between CO₂ molecules have to be overcome for it to sublime.
3. CSiO₂ is a lattice of Si⁴⁺ and O²⁻ ions, but solid CO₂ consists of covalently bonded molecules.
   A student who thinks a hard, high-melting solid is ionic picks this. Silicon is a metalloid and oxygen a nonmetal; they are covalently bonded in a network, and SiO₂ contains no ions.
4. DSiO₂ melts when Si–O bonds break, but CO₂ sublimes when its weaker C=O bonds break.
   A student who thinks phase changes of a molecular substance break covalent bonds picks this. CO₂ sublimes as intact molecules; its C=O bonds do not break (and they are stronger than Si–O single bonds).

CED 3.2.A.4 · Read this in Fix

1. AThe mixture was stirred too hard, so sucrose molecules were split into ions.
   A student who thinks dissolving or stirring splits molecules into ions picks this. Sucrose molecules stay intact in solution however much it is stirred, so no ions come from the sucrose.
2. BToo much sucrose was added, so crowded molecules passed electrons along.
   A student who thinks shared electrons can pass from molecule to molecule picks this. The valence electrons of sucrose are held within the covalent bonds and lone pairs of each molecule, however concentrated the solution.
3. CToo little sucrose was added, so the water itself carried the current.
   A student who thinks water itself is a good conductor picks this. Water without dissolved ions is a very poor conductor and would not light the bulb.
4. DTap water was used in place of distilled water, so dissolved ions were present. Correct
   Sucrose dissolves as neutral molecules, so a solution in distilled water does not conduct. Tap water contains dissolved ions, which are mobile charged particles and can carry a small current.

CED 3.2.A.5 · Read this in Fix

1. AA covalent network solid, as molecules this large make up one continuous network
   A student who thinks very large molecules make a covalent network picks this. In a network solid the covalent bonds run throughout the solid and the melting point is very high; polyethylene's chains are separate molecules, which is why it softens near 120 °C.
2. BA molecular solid, in which separate chains are attracted to one another by dispersion forces Correct
   Polyethylene softens at a low temperature and can be remolded because its chains are separate molecules held to one another only by London dispersion forces. Its valence electrons are held in covalent bonds within each chain, so it does not conduct. Some molecular solids are made of very large molecules or polymers.
3. CA covalent network solid, in which covalent bonds link each chain to the chains next to it
   A student who thinks the attractions between molecules are covalent bonds picks this. The chains attract one another by dispersion forces; if covalent bonds linked the chains, the solid could not soften and be remolded at 120 °C.
4. DA molecular solid, in which softening breaks the chains into small molecules that flow
   A student who thinks melting breaks covalent bonds picks this. When polyethylene softens, the chains stay intact and slide past one another; only the attractions between chains are overcome.

CED 3.2.A.5 · Read this in Fix

1. AThe metal cores are still surrounded by mobile electrons, so they are attracted as before. Correct
   The attraction in a metal is between the cores and the mobile valence electrons around them, and it does not depend on which cores are neighbors. After the layers shift, every core is still surrounded by mobile electrons, so the solid holds together in its new shape. This is why metals are malleable and ductile.
2. BThe attractions between the particles of a metal are weak, so layers slide with very little resistance.
   A student who thinks metals are malleable because their bonding is weak picks this. The attractions in most metals are strong; layers can shift because the attraction between cores and mobile electrons persists in the new arrangement.
3. CThe metal atoms are themselves soft, so each atom is flattened by the force and stays in contact.
   A student who gives each atom the properties of the bulk metal picks this. The atoms do not change shape; the layers of cores move to new positions.
4. DThe metal cores flow freely through the solid, as they do whenever the metal carries a current.
   A student who thinks the metal cores move through the solid picks this. The cores shift by layers when a force is applied, but they do not flow, and they do not move when the metal conducts; the mobile electrons carry the current.

CED 3.2.A.6 · Read this in Fix

1. AHigher, as the small atoms push the metal atoms apart and weaken their attraction
   A student who thinks interstitial atoms loosen the lattice picks this. The small atoms fit into the spaces and act as obstacles to the sliding of layers, which makes the lattice more rigid.
2. BUnchanged, as the small atoms fill spaces that were empty in the lattice
   A student who thinks atoms in the empty spaces have no effect picks this. The small atoms get in the way when layers of metal atoms shift, so malleability decreases.
3. CLower, as the small atoms take up the mobile electrons that hold the metal together
   A student who thinks alloying removes the mobile electrons picks this. Alloys typically retain a sea of mobile electrons and remain conducting; the decrease in malleability comes from the more rigid lattice.
4. DLower, as the small atoms make it harder for layers of metal atoms to shift Correct
   The alloy is an interstitial alloy. The small atoms in the spaces between the metal atoms make the lattice more rigid, so layers of metal atoms cannot slide past one another as easily, and the alloy is less malleable and less ductile than the pure metal.

CED 3.2.A.6 · Read this in Fix

1. ACopper ions and zinc ions are both free to move through the solid and carry the charge.
   A student who thinks the metal ions themselves move through a solid metal picks this. The cores stay in their lattice positions; the mobile valence electrons carry the charge.
2. BCopper atoms and zinc atoms both contribute valence electrons to a sea of mobile electrons. Correct
   Brass is a metallic solid. Alloys typically retain a sea of mobile electrons: both the copper atoms and the zinc atoms contribute valence electrons that are free to move throughout the solid, so brass conducts.
3. CCopper atoms are themselves conductors, and the current passes along chains of touching copper atoms.
   A student who gives each atom the properties of the bulk metal picks this. Conduction is not a property of a single atom; it arises from the valence electrons that are mobile throughout the solid, whichever atoms contributed them.
4. DZinc atoms transfer electrons to copper atoms, and the ions that are formed carry the charge.
   A student who thinks the bonding in an alloy is ionic picks this. Brass contains no anions; both metals contribute electrons to the mobile electron sea.

CED 3.2.A.6 · Read this in Fix

1. AHammer a thick bar of iron containing carbon and a thin bar of pure copper by hand, then compare how far each flattens.
   A student who accepts a comparison in which several factors differ picks this. The samples differ in metal and in thickness, and the force of a hand-held hammer is not controlled, so a difference cannot be attributed to the carbon.
2. BHammer two identical bars of iron containing carbon, one lightly and one heavily, then compare how far each bar flattens.
   A student who thinks the factor under test should be kept the same picks this. Both bars contain carbon, so the procedure shows the effect of the hammering, not the effect of adding carbon.
3. CConnect identical bars of pure iron and of iron containing carbon to a battery, then compare the current in each bar.
   A student who thinks any measured property tests the claim picks this. The current shows conductivity, which depends on the mobile electrons; the claim is about malleability, which must be measured by deforming the bars.
4. DPress identical bars of pure iron and of iron containing carbon with the same force, then compare how far each flattens. Correct
   The procedure changes only the factor in the claim (whether carbon is present), keeps the size of the bars and the force the same, and measures malleability directly as the amount each bar flattens.

CED 3.2.A.6 · Read this in Fix

1. AUnbranched chains in A lie close together, so dispersion forces act along more of their length. Correct
   The shape of the chains decides how they interact. The unbranched chains in A can lie side by side, in contact along their whole length, so the total attraction between neighboring chains is large. The branches in B keep the chains apart, so the attractions between them are weaker and B softens at a lower temperature.
2. BUnbranched chains in A are joined to one another by covalent bonds, which hold them in place.
   A student who thinks the attractions between molecules are covalent bonds picks this. The chains in both samples are separate molecules attracted by dispersion forces; no covalent bonds link one chain to the next.
3. CCovalent bonds along the chains in A are stronger, so more heating is needed to break the chains.
   A student who thinks softening breaks covalent bonds picks this. Both samples have the same repeating unit and so the same covalent bonds, and softening leaves the chains intact.
4. DUnbranched chains in A expose more H atoms, so more hydrogen bonds form between the chains.
   A student who thinks any hydrogen atom can form hydrogen bonds picks this. Every H atom in a hydrocarbon polymer is bonded to carbon, so no hydrogen bonds form between the chains; they attract one another by dispersion forces.

CED 3.2.A.7 · Read this in Fix