1 question, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 1
Equal volumes of hexane, C₆H₁₄, and water are shaken together in a test tube and then allowed to stand for several minutes. Hexane is less dense than water. Which of the numbered boxes in the diagram best represents the contents of the test tube?
Answer and reasoning
ABox 1 A student who thinks any two liquids mix completely if shaken picks this. Shaking only breaks the liquids into droplets; because their intermolecular interactions are so different, hexane and water separate again into layers.
BBox 2 A student who thinks liquids mix by reacting to form a new compound picks this. Hexane and water do not react; the molecules of each stay unchanged and separate into two layers.
CBox 3 A student who thinks hexane and water molecules repel each other picks this picture of layers pushed apart. Water and hexane molecules attract each other weakly, and the two layers are in contact at the boundary.
DBox 4Correct Water molecules attract one another strongly by hydrogen bonding, while hexane molecules interact only through dispersion forces. Mixing would replace strong water-water attractions with much weaker water-hexane attractions, so the liquids separate into two layers that touch at a boundary, with the less dense hexane on top.
In preparation: 0 of 1 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
3.10.A.1 Miscible Fix
Miscible
Describes two liquids that mix in all proportions to form a single homogeneous phase, such as ethanol and water. Liquids that are not miscible form separate layers.
Solubility
The extent to which a substance (the solute) dissolves in a given solvent at a given temperature. A substance can be very soluble in one solvent and only slightly soluble in another.
Similar intermolecular interactions ('like dissolves like')
Substances with similar intermolecular interactions tend to be miscible or soluble in one another: polar and hydrogen-bonding substances tend to mix with water, and nonpolar substances tend to mix with nonpolar solvents. This is a tendency, not a rule without exceptions.
Particulate view of dissolving
For a solute to dissolve, attractions between solute particles and between solvent particles must be overcome and replaced by solute-solvent attractions. Mixing is favored when the new solute-solvent attractions are comparable in strength to the attractions they replace.
Ionic solutes in water
A soluble ionic compound dissolves in water because ion-dipole attractions between the ions and the polar water molecules (O toward cations, H toward anions) are strong enough to separate the ions. Nonpolar solvents such as hexane cannot interact this strongly with ions, so ionic compounds are essentially insoluble in them.
Molecules with polar and nonpolar parts
In a molecule such as an alcohol, R–OH, the O–H group can hydrogen bond with water but the hydrocarbon part interacts mainly through dispersion forces. As the hydrocarbon part grows, it increasingly dominates, and solubility in water falls.
Students often think Solubility is set by molar mass: heavier molecules are less soluble and lighter ones more soluble, whatever their intermolecular interactions. In fact No. Solubility depends on how the intermolecular interactions of the solute and solvent compare. Glucose (180 g/mol) is very soluble in water, while hexane (86 g/mol) is not.
Students often think Substances dissolve in or mix with a solvent because they react with it to form a new compound. In fact Not in general. When sugar or ethanol dissolves in water, the molecules stay intact, and when NaCl dissolves, its ions separate but are otherwise unchanged. The particles are held among the solvent particles by attractions between particles, and the solute can be recovered by evaporating the solvent.
4 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 4
A student uses the model 'polar substances dissolve in polar solvents' to predict that every alcohol, R–OH, mixes with water in all proportions. Methanol, CH₃OH, and ethanol, C₂H₅OH, do mix with water in all proportions, but 1-hexanol, CH₃(CH₂)₅OH, is only slightly soluble in water. Which statement best describes how well the model accounts for these observations?
Answer and reasoning
AIt is limited: in 1-hexanol the long nonpolar chain, not the O–H group, dominates how the molecules interact.Correct The O–H group of every alcohol can hydrogen bond with water, but the hydrocarbon part interacts mainly through dispersion forces. In methanol and ethanol the O–H group dominates; in 1-hexanol the six-carbon chain dominates, so its interactions are much less like water's. A two-category polar/nonpolar model cannot capture this balance.
BIt is limited: the long chain of 1-hexanol repels water molecules, which the model does not take into account. A student who thinks nonpolar groups repel water picks this. Water molecules are weakly attracted to the hydrocarbon chain; 1-hexanol is only slightly soluble because water molecules attract one another far more strongly than they attract the chain.
CIt fails: solubility depends on molar mass, not on the intermolecular interactions of the molecules. A student who thinks molar mass decides solubility picks this. Solubility falls along the series because the nonpolar part grows, not because of mass itself; glucose (180 g/mol) is far heavier than 1-hexanol (102 g/mol) and is very soluble in water.
DIt fails: methanol and ethanol mix with water because they react with it, not because they are polar. A student who thinks liquids mix because they react picks this. Methanol and ethanol molecules stay intact in water; they mix because they hydrogen bond with water molecules.
The diagram shows the structures of four liquids. Which liquid is most likely to be miscible with water in all proportions?
Answer and reasoning
ALiquid 1 A student who thinks any molecule with polar bonds is polar picks CCl₄. Its four identical C–Cl bond dipoles point to the corners of a tetrahedron and cancel, so CCl₄ is nonpolar and forms a separate layer with water.
BLiquid 2Correct Methanol has an O–H group, so methanol molecules can hydrogen bond with water molecules, both as H donors and as acceptors. Its interactions are similar to water's, and its small CH₃ group interacts only weakly, so methanol mixes with water in all proportions.
CLiquid 3 A student who thinks any H atom can hydrogen bond picks hexane. Its H atoms are all bonded to C, so it cannot hydrogen bond with water; it interacts only through dispersion forces and does not mix with water.
DLiquid 4 A student who thinks every polar liquid mixes with water picks CH₂Cl₂. Its bond dipoles do not cancel, so it is polar, but it cannot hydrogen bond with water, and it forms a separate layer.
Solid NaCl dissolves readily in water but does not dissolve in hexane, C₆H₁₄. Which statement best explains this difference at the particulate level?
Answer and reasoning
AWater molecules attract one another more strongly than hexane molecules do, so water dissolves any solute better. A student who thinks strong solvent-solvent attractions make a solvent better for every solute picks this. Strong attractions among water molecules make it harder, not easier, to dissolve solutes that cannot interact strongly with water, such as I₂; NaCl dissolves because its ions attract water molecules strongly.
BNaCl reacts with water to form new compounds that dissolve, but NaCl cannot react with the hexane molecules. A student who thinks dissolving is a chemical reaction picks this. NaCl dissolves as Na⁺ and Cl⁻ ions surrounded by water molecules; evaporating the water gives back NaCl.
CWater molecules have partial charges that attract Na⁺ and Cl⁻ ions strongly enough to separate them; hexane molecules do not.Correct Each Na⁺ ion attracts the partially negative O ends of water molecules, and each Cl⁻ ion the partially positive H ends. These ion-dipole attractions are strong enough to compensate for separating the ions. Nonpolar hexane molecules can attract ions only weakly, so the ions stay in the crystal.
DWater separates NaCl into neutral Na and Cl atoms, which mix with water molecules but not with hexane molecules. A student who thinks an ionic compound separates into neutral atoms picks this. NaCl dissolves as Na⁺ and Cl⁻ ions; it is the attraction between these charged ions and polar water molecules that holds them in solution.
A few crystals of I₂ are added to each of two test tubes, one containing water and the other containing hexane, C₆H₁₄, and both tubes are shaken. Which claim about the result, with its reasoning, is correct?
Answer and reasoning
AMore I₂ dissolves in hexane, because I₂ and hexane molecules both interact through London dispersion forces.Correct I₂ and hexane are both nonpolar, so the I₂-hexane attractions are similar to the attractions within each pure substance, and I₂ dissolves. Water molecules hydrogen bond strongly to one another, and nonpolar I₂ cannot replace those attractions, so very little I₂ dissolves in water.
BMore I₂ dissolves in hexane, because water molecules repel nonpolar I₂ molecules and push them out of the water. A student who thinks nonpolar molecules and water repel each other picks this. The claim is right, but water and I₂ attract each other weakly; I₂ dissolves poorly in water because water molecules attract one another much more strongly than they attract I₂.
CMore I₂ dissolves in water, because the strong attractions among water molecules make water the better solvent. A student who thinks strong solvent attractions make a solvent better for any solute picks this. Water's strong hydrogen bonding keeps nonpolar I₂ out; I₂ is far more soluble in hexane.
DEqual amounts of I₂ dissolve in both, because shaking makes a solid dissolve in any liquid given enough time. A student who thinks shaking makes any solid dissolve in any liquid picks this. Shaking speeds up dissolving but does not change how much can dissolve; I₂ is far more soluble in hexane than in water.
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
