3.3 Solids, Liquids, and Gases

1. ASample 2 is a very thick liquid whose particles slowly flow past one another over time.
   A student who thinks that glass and other amorphous solids are slowly flowing liquids picks this. The sample is rigid, and in an amorphous solid, as in a crystalline one, the particles do not move past one another; only their arrangement lacks order.
2. BSample 2 is an amorphous solid; its particles have no regular, repeating pattern. Correct
   Sample 2 is rigid and its particles are in close contact, but unlike those in Sample 1 they are not in rows or any repeating pattern. A rigid sample with an irregular arrangement of particles is an amorphous solid; Sample 1 is crystalline.
3. CSample 2 is a crystalline solid like Sample 1, with its particles less neatly drawn.
   A student who thinks that every solid must be crystalline picks this. The diagram shows no rows or repeating pattern; solids whose particles lack a regular arrangement are amorphous solids, not untidily drawn crystals.
4. DSample 2 is an amorphous solid whose particles have no motion of any kind.
   A student who thinks the particles of a solid are motionless picks this. The arrangement is amorphous, but the particles of every solid vibrate about fixed positions; they are not still.

CED 3.3.A.1 · Read this in Fix

1. AThey are spaced apart, partway between their spacing as a solid and as a gas.
   A student who thinks liquid particles are spread apart picks this. The molecules of a liquid are in close contact; the solid and liquid of a substance typically have similar molar volumes.
2. BThey are in close contact and stay still until the liquid is stirred or poured.
   A student who thinks the particles of a liquid at rest are motionless picks this. The molecules are always moving and colliding; stirring or pouring moves the liquid as a whole.
3. CThey are soft and change their shape to fit the shape of the beaker.
   A student who gives particles the properties of the bulk liquid picks this. The liquid takes the shape of the beaker because its molecules move past one another, not because each molecule changes shape.
4. DThey are in close contact and are continually moving and colliding. Correct
   In a liquid the molecules touch their neighbors at all times, as in a solid, but they are continually moving, colliding and moving past one another, even when the liquid as a whole is at rest.

CED 3.3.A.2 · Read this in Fix

1. AThe two are similar, because the particles are in close contact in both phases. Correct
   In both the solid and the liquid the particles touch their neighbors at all times, so a mole of the substance occupies a similar volume in either phase; typically the values differ by only a few percent.
2. BThe liquid's is much larger, because its particles are spread well apart.
   A student who thinks the particles of a liquid are spread apart picks this. The particles of a liquid are in close contact, so the liquid's molar volume is only slightly different from the solid's.
3. CThe solid's is larger, because solids are typically less dense than their liquids.
   A student who generalizes from ice floating on water picks this. For most substances the solid is slightly denser than the liquid; water is an exception because of the open, hydrogen-bonded structure of ice.
4. DThe solid's is much smaller, because the particles in a solid are smaller.
   A student who thinks particles change size with phase picks this. The particles are the same size in both phases; the molar volumes are similar because the particles are in close contact in both.

CED 3.3.A.3 · Read this in Fix

1. AEqually often, because the molecules are still moving at the very same average speed.
   A student who thinks collision frequency depends only on particle speed picks this. The speed is unchanged, but there are twice as many molecules in each milliliter, so the wall is struck twice as often.
2. BMore than twice as often, because squeezing the gas makes its molecules move faster.
   A student who thinks compression speeds up the molecules even at constant temperature picks this. At constant temperature the average speed is unchanged; only the crowding changes, doubling the collision frequency.
3. CLess often than before, because the molecules now have less room in which to move around.
   A student who thinks crowded particles move less picks this. The molecules move at the same average speed and, being twice as crowded, strike the wall twice as often.
4. DTwice as often, because each milliliter of gas now holds twice as many molecules. Correct
   The same number of molecules is now in half the volume, so each milliliter holds twice as many. At constant temperature their average speed is unchanged, so each square centimeter of wall is struck twice as often, which is why the pressure doubles.

Working The number of molecules N is fixed and the volume halves, so the number of molecules per unit volume, N/V, doubles. At constant temperature the average speed of the molecules does not change. The frequency of collisions with a given area of wall is proportional to (molecules per unit volume) × (average speed), so it is 2 × 1 = 2 times the original: twice as often. (This is consistent with Boyle's law: halving V at constant T doubles P.)

CED 3.3.A.4 · Read this in Fix

1. AThe ions, which were motionless at 25°C, begin to vibrate about fixed positions.
   A student who thinks the particles of a solid are motionless until heated picks this. The ions in the crystal were already vibrating at 25°C; heating makes the vibrations more energetic.
2. BThe ions each become larger, which pushes their neighbors farther away.
   A student who thinks particles expand when heated picks this. The ions keep their size; the slight expansion of a heated crystal comes from larger vibrations about the fixed positions.
3. CThe KCl molecules vibrate more, but each stays at its position.
   A student who thinks ionic compounds consist of molecules picks this. Solid KCl contains no KCl molecules; it is a lattice of K⁺ and Cl⁻ ions, and it is the ions that vibrate.
4. DThe ions vibrate more energetically about their fixed positions in the crystal. Correct
   Solid KCl is a crystalline array of K⁺ and Cl⁻ ions. In a solid the ions vibrate about fixed positions without moving past one another; heating makes the vibrations more energetic, and because KCl stays solid the ions keep their positions in the lattice.

CED 3.3.A.1 · Read this in Fix

1. AIn ice, hydrogen bonds hold the H₂O molecules in a more open arrangement than in liquid water. Correct
   The structure of a solid depends on the interactions between its particles. In ice each H₂O molecule is hydrogen bonded to its neighbors in a regular arrangement with open space in it, so a mole of ice occupies a slightly larger volume (about 9% larger) than a mole of liquid water.
2. BIn ice, each H₂O molecule is larger than an H₂O molecule in liquid water.
   A student who thinks a sample takes up more space because its particles get bigger picks this: ice is less dense, so its molecules are taken to be larger. The molecules are the same size in both phases; the difference is in how they are arranged.
3. CIn ice, air trapped among hydrogen-bonded H₂O molecules lowers density below the liquid's.
   A student who thinks ice floats because of the air trapped in it picks this. Ice made from water with all dissolved air removed still floats; its lower density comes from the open arrangement in which hydrogen bonds hold the molecules.
4. DEach H₂O molecule in ice has a smaller mass than an H₂O molecule in liquid water.
   A student who thinks the particles differ from one phase to another picks this. Ice and liquid water consist of identical H₂O molecules with the same mass; only their arrangement differs.

CED 3.3.A.1 · Read this in Fix

1. AIt is justified; the particles of every solid lie in a regular, repeating pattern.
   A student who thinks every solid is crystalline picks this. Amorphous solids, such as glass, are solids whose particles have no regular arrangement, so an irregular arrangement does not rule out a solid.
2. BIt is not justified; liquid particles would be drawn spread well apart from one another.
   A student who thinks the particles of a liquid are spread apart picks this. In a liquid the particles are in close contact, so a liquid would also be drawn with touching particles.
3. CIt is not justified; an amorphous solid also has closely packed particles in no regular pattern. Correct
   A diagram of one instant shows only arrangement. Close contact with no regular pattern describes both a liquid and an amorphous solid; they differ in motion (in the solid the particles do not move past one another), which a single snapshot cannot show.
4. DIt is not justified; the sample could equally be a gas, as gas particles also touch.
   A student who thinks gas particles are close together picks this. In a gas the particles are far apart compared with their size, so this diagram cannot represent a gas.

CED 3.3.A.1 · Read this in Fix

1. AMore of the covalent O–H bonds in the molecules break, and the smaller fragments flow more easily.
   A student who thinks heating a liquid breaks the covalent bonds within its molecules picks this. Liquid water at 80°C consists of intact H₂O molecules; heating affects the attractions between molecules.
2. BFaster-moving molecules more easily overcome the hydrogen bonding to their neighbors and slip past them. Correct
   The viscosity falls from 1.00 to 0.35 mPa·s between 20°C and 80°C. As the temperature rises the molecules move faster and more often break free of the hydrogen bonding that holds them to neighbors, so they move past one another more easily and the liquid flows more easily.
3. CThe molecules move far apart, so the hot liquid is mostly empty space that offers very little resistance.
   A student who thinks heating moves the particles of a liquid far apart picks this. Water at 80°C is still a liquid whose molecules are in close contact; its density is only about 3% lower than at 20°C.
4. DEach water molecule itself becomes runnier as it is heated, so it slides along more easily.
   A student who gives particles the properties of the bulk liquid picks this. Flowing is a property of the sample; individual molecules are not runny, and their shape does not change on heating.

CED 3.3.A.2 · Read this in Fix

1. AThe H atom of one HCl molecule is most often hydrogen bonded to the Cl atom of a neighbor.
   A student who thinks any H atom in a polar bond can form a hydrogen bond picks this. Hydrogen bonding needs an H atom bonded to N, O or F; HCl molecules attract one another by dipole-dipole and dispersion forces.
2. BSeparate H⁺ ions and Cl⁻ ions, rather than HCl molecules, move about the liquid.
   A student who pictures pure HCl as the ions of hydrochloric acid picks this. Pure liquid HCl consists of HCl molecules; HCl ionizes when it dissolves in water.
3. CThe H end of one HCl molecule is most often near the Cl end of a neighboring molecule. Correct
   HCl molecules are polar, with δ+ on H and δ− on Cl. Although the molecules move constantly, dipole-dipole attractions make orientations in which the δ+ H end of one molecule is near the δ− Cl end of a neighbor more common than other orientations.
4. DMolecules point in random directions, since attractions act only in solids.
   A student who thinks attractions between particles vanish when a solid melts picks this. The particles of a liquid still attract one another, which keeps them in close contact and favors orientations of polar molecules in which opposite partial charges are near each other.

CED 3.3.A.2 · Read this in Fix

1. AThe gas consists of particles different from, and larger than, those in the liquid.
   A student who thinks each phase has its own kind of particle picks this. Gaseous X consists of the same X molecules as solid and liquid X; the large molar volume of the gas comes from the space between them.
2. BParticles attract one another in the solid and liquid but repel one another in the gas.
   A student who thinks gas particles repel one another picks this. Gas particles attract one another weakly; they are far apart because they are in constant motion and the attractions are too weak to hold them together.
3. CParticles touch their neighbors in the solid and the liquid but are far apart in the gas. Correct
   The solid and liquid molar volumes (41.2 and 44.0 cm³/mol) differ by less than 7%, so the particles must be in close contact in both. The gas molar volume is about 680 times the liquid's, so in the gas the particles are far apart and most of the volume is empty space.
4. DThe spaces between particles in the gas are filled with air, which adds to its volume.
   A student who thinks the space between gas particles must be filled with something picks this. The space between the particles of gaseous X is empty; no air is present in a sample of pure X.

CED 3.3.A.3 · Read this in Fix

1. ADiagram 1 Correct
   This diagram shows intact H₂O molecules far apart from one another and spread through the whole container, as the particles of a gas are; a gas has no definite volume, so it fills its container.
2. BDiagram 2
   A student who thinks boiling breaks the covalent bonds within water molecules picks this diagram of H₂ and O₂. Water vapor consists of intact H₂O molecules; vaporization overcomes only the attractions between molecules.
3. CDiagram 3
   A student who thinks gas particles settle at the bottom of a container picks this diagram. Gas molecules are in constant random motion and spread through the whole container.
4. DDiagram 4
   A student who thinks particles expand when heated or vaporized picks this diagram of enlarged molecules. H₂O molecules keep their size; in the gas they are far apart, with mostly empty space between them.

CED 3.3.A.4 · Read this in Fix

1. ADiagram 1
   A student who thinks gas particles settle at the bottom of a container picks this diagram. A gas has no definite volume: however few molecules remain, they spread through the whole flask.
2. BDiagram 2 Correct
   The five remaining molecules are still in constant random motion with only weak attractions between them, so they spread through the whole flask. The average spacing is larger than before because fewer molecules share the same volume.
3. CDiagram 3
   A student who thinks gas particles are close together, touching one another, picks this diagram. Gas molecules are far apart compared with their size and move independently through the whole flask.
4. DDiagram 4
   A student who thinks the space between gas particles is filled with something picks this shaded diagram. The flask contains only N₂ molecules; the space between them is empty.

CED 3.3.A.4 · Read this in Fix

1. AThey stay spread evenly at the same average spacing, as the volume is fixed. Correct
   The container is rigid and sealed, so the volume and the number of molecules are fixed and the average spacing cannot change. The molecules move faster and collide more often, but they stay spread evenly through the container.
2. BThey spread farther apart from one another, because a gas expands when heated.
   A student who thinks a heated gas always expands picks this. A gas expands on heating only when its volume can change; in a rigid container the spacing stays the same and the pressure rises instead.
3. CThey gather near the top of the container, because the heated gas rises upward.
   A student who applies 'hot air rises' to a closed container picks this. The whole sample is heated, so there is no cooler gas for it to rise through; the molecules stay spread evenly.
4. DThey stay in contact with one another but jostle more, since they gain more energy.
   A student who thinks gas molecules are close together picks this. The N₂ molecules are far apart before and after heating; they gain energy and move faster.

CED 3.3.A.4 · Read this in Fix

1. AThe molecules repel one another, with each push driving them farther apart toward the walls of the container.
   A student who thinks gas particles repel one another picks this. The molecules attract one another weakly; they spread because they are moving, not because they push one another away.
2. BThe molecules move constantly in random directions, with attractions too weak to hold them together. Correct
   Gas molecules are in constant motion and attract one another only weakly, so nothing keeps them in one region; they travel in all directions until they are spread through the whole container. This is why a gas has neither a definite volume nor a definite shape.
3. CEach molecule expands in the larger space until, together, the molecules fill the container.
   A student who thinks particles change size picks this. The molecules keep their size; they spread out, so the space between them increases.
4. DThe empty space pulls on the molecules, its suction drawing them outward to the walls.
   A student who thinks a vacuum pulls matter into it picks this. An empty space exerts no force; the molecules move into it because of their own random motion.

CED 3.3.A.4 · Read this in Fix

1. AIt will be lower, because ethanol has less mass as a gas than it has as a liquid.
   A student who thinks gases have little or no mass picks this. The same ethanol molecules are present, with the same total mass; only their spacing has changed.
2. BIt will be higher, because the ethanol vapor takes up more space than the liquid did.
   A student who thinks taking up more space means more mass picks this. The vapor occupies more volume, but the number of molecules, and so the mass, is unchanged.
3. CIt will be the same, because each molecule splits into atoms whose masses add to its own.
   A student who thinks evaporation breaks the covalent bonds within molecules picks this. The prediction is right but the reason is wrong: ethanol vapor consists of intact C₂H₅OH molecules.
4. DIt will be the same, because the flask still contains the same number and kind of molecules. Correct
   Evaporation changes the spacing and motion of the ethanol molecules but not their number or identity, and the sealed flask lets nothing in or out, so the mass is unchanged.

CED 3.3.A.4 · Read this in Fix

1. AAir molecules are soft and get squashed by the added masses, but water molecules are hard.
   A student who gives particles the properties of the bulk sample picks this. Air molecules are not squashed; the volume of air decreases because the empty space between its molecules decreases.
2. BWater molecules are motionless, so the masses are unable to move them closer.
   A student who thinks the particles of a liquid at rest are motionless picks this. Water molecules are always moving; water is hardly compressed because its molecules are already in close contact.
3. CMolecules in air are far apart, but molecules in water are already in close contact. Correct
   The air's volume fell from 40.0 mL to 22.9 mL as the mass on the plunger increased, while the water's stayed at 40.0 mL. Air is easily compressed because its molecules are far apart; water is not measurably compressed because its molecules are already in close contact.
4. DAir has no mass, so it gives way under the masses, while water's mass holds it in place.
   A student who thinks gases have no mass picks this. The air in the syringe has mass, and mass does not decide how compressible a sample is: the air compresses because its molecules are far apart.

CED 3.3.A.2 · Read this in Fix