3.9 Separation of Solutions and Mixtures

1. AThe dissolved Cu²⁺ and SO₄²⁻ ions are far smaller than the paper's pores, so they pass through with the water. Correct
   In the solution, CuSO₄ is present as separate Cu²⁺ and SO₄²⁻ ions surrounded by water molecules. These particles are far smaller than the pores in filter paper, so they pass through with the water and the filtrate is just as blue.
2. BThe CuSO₄ has reacted with the water to form one single new compound, which the filter paper cannot break apart.
   A student who thinks dissolving forms a new compound picks this. CuSO₄ dissolves as Cu²⁺ and SO₄²⁻ ions; the CuSO₄ can be recovered by evaporating the water.
3. CA solution is a single pure substance, so no physical process can separate its components.
   A student who thinks a solution is a pure substance picks this. A solution is a mixture; its components can be separated by processes such as distillation that exploit differences in intermolecular interactions, just not by filtration.
4. DThe CuSO₄ is present as fine solid grains, which are small enough to slip through the paper's pores.
   A student who thinks a dissolved solid exists as tiny grains picks this. Dissolved CuSO₄ is present as individual hydrated ions, not as grains of solid.

CED 3.9.A.1 · Read this in Fix

1. ASpot it on paper, stand the paper in a solvent below the spot, and see whether separate colored spots form. Correct
   Paper chromatography separates dissolved dyes by differences in their attractions to the paper and to the solvent. A mixture of dyes gives two or more spots at different distances; a single dye gives one spot.
2. BPour it through filter paper and see whether any of the colored dye is left behind on the paper.
   A student who thinks filtration can separate the components of a solution picks this. Dissolved dye molecules pass through filter paper with the water, so nothing is left behind whether there is one dye or several.
3. CLeave it to stand for a day and see whether layers of different colors form in the container as the dyes separate.
   A student who thinks the components of a solution separate on standing picks this. A solution stays uniform however long it stands, so no layers form either way.
4. DDistill it and see whether liquids of different colors collect, one after another, in the receiving flask.
   A student who thinks distillation separates every component of a solution picks this. The dyes do not vaporize appreciably; water distills off as a colorless liquid and the dyes stay together in the flask.

CED 3.9.A.1.i · Read this in Fix

1. AX is more polar than Y, as polar substances move farther up a chromatography plate.
   A student who thinks polar components always travel farther picks this. On a polar plate developed with a nonpolar solvent, the more polar component is attracted more strongly to the plate and travels less.
2. BX is less polar than Y, as X is attracted less strongly than Y is to the polar silica. Correct
   X traveled much farther than Y, so X spent more of its time dissolved in the hexane and less held by the silica. Because the silica is polar and the hexane nonpolar, the component held back less strongly, X, is the less polar one.
3. CX has a smaller molar mass than Y, as lighter molecules are carried farther by the solvent.
   A student who thinks lighter molecules travel farther picks this. The distance traveled depends on attractions to the stationary and mobile phases, not on molar mass, so the chromatogram says nothing about the masses of X and Y.
4. DX has weaker attractions among its own molecules than Y, as X traveled the greater distance.
   A student who thinks a component's distance reflects the attractions among its own molecules picks this. The separation depends on each component's attractions to the silica relative to the hexane, not on the attractions within the pure substance.

CED 3.9.A.1.i · Read this in Fix

1. ABox 1
   A student who thinks filtration removes a dissolved solute picks this picture of pure water. The Na⁺ and Cl⁻ ions are far smaller than the pores and pass through with the water.
2. BBox 2
   A student who thinks dissolved NaCl exists as tiny grains of solid picks this cluster. In the solution the ions are separated from one another and spread among the water molecules.
3. CBox 3
   A student who thinks NaCl reacts with water when it dissolves picks this. Dissolving separates the Na⁺ and Cl⁻ ions; no bonds form between the ions and the atoms of water.
4. DBox 4 Correct
   The filter paper holds back the sand grains, which are much larger than its pores. The dissolved NaCl is present as separate Na⁺ and Cl⁻ ions among the water molecules, and these pass through the paper, so the filtrate is still NaCl(aq).

CED 3.9.A.1 · Read this in Fix

1. AIt is mostly acetone, because the covalent bonds in acetone molecules break at a lower temperature than water's.
   A student who thinks boiling breaks covalent bonds picks this. The claim is right, but distillation separates intact molecules: the acetone in the distillate is still CH₃COCH₃, and only intermolecular attractions are overcome.
2. BIt is mostly acetone, because the weaker attractions among acetone molecules give acetone the higher vapor pressure. Correct
   Water molecules are held together by hydrogen bonds, which are stronger than the dipole-dipole and dispersion attractions among acetone molecules. Acetone molecules escape from the liquid more readily, so acetone has the higher vapor pressure and the vapor, and the first distillate, is mostly acetone.
3. CIt is mostly water, because water's lighter molecules escape from the liquid more easily than acetone's.
   A student who thinks the liquid with lighter molecules always vaporizes first picks this. Water's molecules are lighter (18 g/mol versus 58 g/mol), but their hydrogen bonds hold them in the liquid more strongly, so water is the less volatile liquid.
4. DIt is mostly water, because stronger attractions among water molecules give water the higher vapor pressure.
   A student who thinks stronger attractions raise vapor pressure picks this. Stronger attractions hold molecules in the liquid, so water has the lower vapor pressure and distills later.

CED 3.9.A.1.ii · Read this in Fix

1. ABox 1
   A student who thinks a boiling mixture gives a vapor with the same composition as the liquid picks this. The component held by weaker attractions, P, escapes more readily, so the vapor is richer in P.
2. BBox 2
   A student who thinks stronger attractions give a higher vapor pressure picks this. Stronger attractions keep Q molecules in the liquid, so the vapor contains fewer Q molecules, not more.
3. CBox 3 Correct
   P molecules are held in the liquid by weaker attractions, so they escape into the vapor more readily than Q molecules: P has the higher vapor pressure. The vapor is therefore richer in P than the liquid is, though it still contains some Q, and all the molecules stay intact. This enrichment is what distillation uses.
4. DBox 4
   A student who thinks boiling breaks covalent bonds picks this picture of separated P atoms. Boiling overcomes only the attractions between molecules; P molecules enter the vapor intact.

CED 3.9.A.1.ii · Read this in Fix

1. AThe molar mass of each dye, because lighter dye molecules are carried faster
   A student who thinks lighter molecules travel farther picks this. Separation depends on the attractions of each dye to the paper and to the solvent; dyes of similar mass can separate widely and dyes of very different mass can travel together.
2. BThe strength of the attractions among the molecules of each dye on its own
   A student who thinks a component's distance depends on the attractions among its own molecules picks this. The dyes are dissolved and spread apart; what decides their movement is their attraction to the paper relative to the solvent.
3. CThe relative strength of each dye's attractions to the paper and to the solvent Correct
   Each dye molecule moves only while it is dissolved in the solvent and stops while it is held to the paper. A dye attracted more strongly to the paper, relative to the solvent, spends more time stationary and travels a shorter distance, so dyes with different relative attractions end up at different heights.
4. DThe size of each dye's particles, because the paper holds back the larger ones
   A student who thinks chromatography paper acts as a filter picks this. Dissolved dye molecules are far smaller than the spaces between the paper's fibers; they are held back by attractions to the paper, not by size.

CED 3.9.A.1.i · Read this in Fix

1. AIts molecules are too heavy for the ethanol to carry them up the paper.
   A student who thinks heavier molecules are harder for the solvent to carry picks this. A component that dissolves well in the solvent moves whatever its mass; this one stays because it is held by the paper.
2. BIt has reacted with the paper and is now covalently bonded to the paper's surface.
   A student who thinks sticking to a surface means forming covalent bonds picks this. The component is held by intermolecular attractions; with a solvent it is attracted to more strongly, it would move, unchanged.
3. CIts particles are too large to pass through the tiny pores between the paper fibers.
   A student who thinks chromatography paper filters components by size picks this. Dissolved molecules are much smaller than the spaces in the paper; components are held back by attractions to the paper, not by size.
4. DIt is attracted much more strongly to the paper than to the ethanol molecules. Correct
   A component moves only while it is dissolved in the mobile phase. A component whose particles are attracted far more strongly to the paper than to ethanol molecules hardly dissolves in the moving ethanol, so it stays at the origin.

CED 3.9.A.1.i · Read this in Fix