2 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 2
Which of the following processes involves the breaking of covalent bonds?
Answer and reasoning
AMelting a sample of ice at 0°C A student who thinks hydrogen bonds are the O–H bonds inside a water molecule picks this. Melting overcomes some hydrogen bonds between separate water molecules; the covalent O–H bonds within each molecule stay intact.
BBoiling a sample of ethanol A student who thinks boiling breaks the covalent bonds within molecules picks this. Boiling separates whole ethanol molecules from one another; the vapor still consists of ethanol molecules.
CDissolving table sugar in water A student who thinks a dissolving substance breaks up into its atoms picks this. Sugar dissolves as intact molecules surrounded by water molecules; no covalent bonds within the sugar molecules break.
DDecomposing hydrogen peroxideCorrect In 2 H₂O₂ → 2 H₂O + O₂ the atoms are rearranged into new substances: O–O and O–H bonds in H₂O₂ are broken and new bonds form in H₂O and O₂. This is a chemical process; the other three involve only changes in intermolecular interactions.
Working No calculation. Melting ice and boiling ethanol are phase changes: only intermolecular attractions change. Dissolving sugar separates intact sugar molecules. Decomposing hydrogen peroxide, 2 H₂O₂ → 2 H₂O + O₂, breaks O–O and O–H bonds and forms new O–H and O=O bonds.
Which of the following correctly describes the changes in bonds and attractions that occur as solid sodium chloride, NaCl(s), dissolves in water?
Answer and reasoning
AIonic bonds between Na⁺ and Cl⁻ break, and ion-dipole attractions to water formCorrect Solid NaCl consists of Na⁺ and Cl⁻ ions held by ionic bonds. As it dissolves, these ionic bonds are broken and the separated ions are attracted to the polar water molecules: Na⁺ to the O ends and Cl⁻ to the H ends.
BAttractions between NaCl molecules break, and the molecules spread out among water molecules A student who thinks ionic compounds are made of molecules picks this. Solid NaCl contains no NaCl molecules; it is a lattice of Na⁺ and Cl⁻ ions, and these ions separate from one another as it dissolves.
CCovalent bonds in water molecules break, and new NaOH and HCl molecules then form A student who thinks dissolving a salt is a reaction with water that forms new compounds picks this. The water molecules stay intact; the ions separate from the lattice and are surrounded by water molecules, and evaporation recovers NaCl.
DAll bonds in NaCl break, and separate Na and Cl atoms spread out among water molecules A student who thinks a dissolving substance breaks up into separate atoms picks this. The particles in solution are Na⁺ and Cl⁻ ions, the same charged particles that make up the solid, not neutral atoms.
Working No calculation. NaCl(s) is a lattice of Na⁺ and Cl⁻ ions. On dissolving, the ionic bonds between the ions are broken and each ion becomes surrounded by water molecules held by ion-dipole interactions; the water molecules stay intact.
In preparation: 0 of 2 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
4.4.A.1 Chemical process Fix
Chemical process
A process that forms new substances with new properties. Processes in which chemical bonds are broken and/or formed, so that atoms are rearranged, are typically classified as chemical; for example, the decomposition of water into H₂ and O₂.
Physical process
A process in which no new substance forms. Processes that involve only changes in intermolecular interactions, such as phase changes and the dissolving of a molecular solute like sugar, are typically classified as physical; some processes that can be argued to be physical, such as dissolving a salt, also break chemical bonds.
Phase change
A change of state such as melting, vaporization, sublimation or condensation. For a molecular substance, the molecules move apart or closer together as intermolecular attractions are overcome or formed; the covalent bonds within the molecules are not broken.
Chemical bond versus intermolecular interaction
Chemical bonds (covalent, ionic, metallic) hold atoms or ions together within a substance's particles or lattice; intermolecular interactions are the weaker attractions between separate molecules. Breaking covalent bonds within molecules changes the identity of the particles; overcoming intermolecular interactions does not. Breaking ionic bonds when a salt dissolves separates ions that keep their identity.
Evidence of a new substance
Characteristic properties, such as melting point or solubility, that differ from those of the starting material show that a new substance has formed. Observations such as a gas being released, a change in mass in an open container, or a temperature change occur in both physical and chemical processes, so on their own they do not decide the classification.
Dissolution of a molecular solute
When a molecular solid such as glucose dissolves, intact molecules separate from one another and become surrounded by solvent molecules; intermolecular attractions change, but no covalent bonds within the solute molecules break.
Students often think Boiling or vaporizing a molecular substance breaks the covalent bonds within its molecules, so the vapor consists of separate atoms. In fact No. Vaporization separates whole molecules from one another by overcoming intermolecular attractions. Br₂ molecules remain Br₂ molecules in the gas; breaking the Br–Br bond would be a chemical process.
Students often think Particles expand when a substance is heated or changes to a gas, which is why the substance takes up more space. In fact No. The particles themselves keep their size; heating makes them move faster, and in a gas they are much farther apart. The larger volume of a gas comes from the space between the particles.
4.4.A.2 Dissolution of an ionic solid Fix
Dissolution of an ionic solid
When a soluble salt such as NaCl dissolves in water, the ionic bonds between the cations and anions in the solid are broken and ion-dipole interactions form between the separated ions and water molecules. Covalent bonds within any polyatomic ions remain intact.
Ion-dipole interaction
The attraction between an ion and a polar molecule such as water: the partially negative O end of water molecules points toward a cation, and the partially positive H ends point toward an anion.
Students often think Ionic compounds consist of molecules, so an ionic solid dissolves as intact formula-unit molecules (NaCl, K₂SO₄) moving among the water molecules. In fact No. Solid NaCl is a lattice of Na⁺ and Cl⁻ ions held together by ionic bonds; the formula gives the ratio of ions, not a discrete molecule. When it dissolves, the separated ions are surrounded by water molecules.
Students often think Dissolving a salt is a chemical reaction in which the salt and water break apart and recombine to form new compounds, such as NaOH and HCl from NaCl. In fact No. The water molecules are not broken apart; the ions separate from the lattice and are surrounded by intact water molecules. Evaporating the water recovers NaCl.
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 sealed flask contains a small amount of liquid bromine, Br₂. The flask is warmed until all of the bromine has vaporized. The diagrams show the flask's contents before warming and four possible representations of the contents afterward. Which numbered diagram best represents the contents of the flask after warming?
Answer and reasoning
ADiagram 1 A student who thinks boiling breaks the covalent bonds within molecules picks this. Vaporizing bromine only separates Br₂ molecules from one another; the Br–Br bonds stay intact, so the vapor consists of Br₂ molecules, not Br atoms.
BDiagram 2Correct Vaporization overcomes the intermolecular attractions between Br₂ molecules but does not break the Br–Br bonds. The flask is sealed, so the same six molecules are present; they keep their size and spread out to fill the container.
CDiagram 3 A student who thinks a substance loses particles or mass when it becomes a gas picks this. The flask is sealed, so all six Br₂ molecules remain inside; the gas has the same mass as the liquid did.
DDiagram 4 A student who thinks particles expand when heated picks this. The Br₂ molecules keep their size; the gas occupies more of the flask because the molecules are farther apart, not because they are larger.
Working No calculation. Vaporization is a physical process: only intermolecular attractions are overcome. The six Br₂ molecules stay intact, keep their size and stay in the sealed flask, but spread out to fill it. The diagram with six same-sized Br₂ molecules spread through the box is correct.
Samples of two white solids, X and Y, are each heated strongly in an open crucible and then allowed to cool. The table shows observations made on each sample and on the solid left after it cooled. Which conclusion is best supported by the data?
Answer and reasoning
ABoth changed chemically, because each of the two solids released a gas as it was heated A student who thinks any release of gas means a chemical change picks this. A substance that vaporizes or sublimes also releases a gas; for X the solid left has the same solubility as X, so there is no evidence that a new substance formed.
BBoth changed chemically, because each lost 0.42 g of mass when heated in the open crucible A student who thinks a change in mass shows a chemical change picks this. Any gas that escapes from an open crucible lowers the mass, including the vapor of an unchanged substance, so the equal mass losses cannot show which solid formed a new substance.
CY changed chemically, because the solid left from Y is insoluble, although Y was solubleCorrect A new substance has different characteristic properties. Y was soluble in water, but the solid left after heating Y is insoluble, so heating Y produced a new substance. For X the data show no change in properties, so they give no evidence that a new substance formed.
DNeither changed chemically, because neither solid changed color as it was heated A student who thinks a chemical change must show an obvious sign such as a color change picks this. Many chemical changes produce no color change; the solid left from Y is insoluble while Y was soluble, so Y did form a new substance.
Working No calculation. Gas release and a mass loss of 0.42 g occur for both solids, and neither changes color, so these observations do not separate the two. The solid left from X has the same solubility as X (no evidence of a new substance); the solid left from Y is insoluble although Y was soluble, so a new substance formed from Y: a chemical change.
A 0.100 mol sample of potassium sulfate, K₂SO₄(s), dissolves completely in water. How many moles of solute particles are present in the solution?
Answer and reasoning
A0.300 molCorrect Dissolving K₂SO₄ breaks the ionic bonds between the ions: K₂SO₄(s) → 2 K⁺(aq) + SO₄²⁻(aq). The covalent bonds within each sulfate ion stay intact, so 0.100 mol gives 0.200 mol K⁺ and 0.100 mol SO₄²⁻, a total of 0.300 mol of particles.
B0.100 mol A student who thinks ionic compounds consist of molecules picks this, counting each K₂SO₄ as one dissolved particle. Dissolving separates the ions: each formula unit gives two K⁺ ions and one SO₄²⁻ ion.
C0.200 mol A student who reads the subscript in K₂ as a single bonded unit picks this, counting one K₂²⁺ ion and one SO₄²⁻ ion per formula unit. The two K⁺ ions are separate ions, so each formula unit gives three ions.
D0.700 mol A student who thinks a dissolving substance breaks up into its separate atoms picks this, counting 2 K + 1 S + 4 O = 7 particles per formula unit. The S–O covalent bonds within SO₄²⁻ are not broken; the sulfate ion stays intact in solution.
Working K₂SO₄ is ionic: each formula unit contains two K⁺ ions and one SO₄²⁻ ion. Dissolving breaks the ionic bonds between these ions but not the covalent S–O bonds within SO₄²⁻. K₂SO₄(s) → 2 K⁺(aq) + SO₄²⁻(aq), so 0.100 mol gives 0.200 mol K⁺ + 0.100 mol SO₄²⁻ = 0.300 mol of solute particles.
A student wants to find out whether dissolving a white molecular solid, Z, in water produces a new substance. Which procedure would provide the most useful evidence?
Answer and reasoning
AStir Z into the water and check whether the solid can no longer be seen A student who thinks a solid that can no longer be seen has disappeared or become a new substance picks this. A dissolved molecular solid is still present as intact molecules too small to see, so disappearance from view does not show whether a new substance formed.
BCheck whether the mass of solution equals the masses of Z and water used A student who thinks mass data show whether a change is chemical or physical picks this. Mass is conserved in a closed or nonvolatile system whether or not a new substance forms, so equal masses cannot decide the question.
CEvaporate the water and compare the melting point of the solid left with that of ZCorrect If dissolving only changes intermolecular interactions, evaporating the water recovers Z itself, with the same melting point. A different melting point would show that a new substance had formed. Comparing a characteristic property tests the question directly.
DMeasure whether the temperature of the water changes as Z dissolves A student who thinks a temperature change is a sign of a chemical change picks this. Dissolving often warms or cools the water even when no new substance forms, so a temperature change does not settle the question.
Working No calculation. A new substance would have different characteristic properties. Recovering the solute by evaporating the water and comparing a characteristic property (melting point) with that of the original Z tests directly whether Z is unchanged. Disappearance from view, mass totals and temperature changes occur whether or not a new substance forms.
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
