2 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 2
The diagram at the top represents a small sample of liquid water. When the water boils, bubbles form in the liquid. Which numbered diagram best represents the particles inside one of the bubbles?
Answer and reasoning
ADiagram 1 A student who thinks boiling breaks water molecules into hydrogen and oxygen picks this. Boiling separates whole H₂O molecules from one another; the bonds within each molecule are not broken, so no H₂ or O₂ forms.
BDiagram 2 A student who thinks particles expand when they are heated picks this. The molecules in the vapor are the same size as in the liquid; the gas takes up more space because the molecules are much farther apart.
CDiagram 3Correct Boiling is a physical change: water molecules escape from the liquid and are far apart in the vapor, but each is still an H₂O molecule of the same size. The bubble is water vapor.
DDiagram 4 A student who thinks the bubbles in boiling water are air picks this. Bubbles form throughout the boiling liquid as water turns to vapor, and they carry the water away, so they contain H₂O molecules, not nitrogen and oxygen.
Working Boiling is a change of phase, a physical change: H₂O molecules leave the liquid intact and are far apart in the vapor; they do not split into H₂ and O₂, do not grow larger, and are not replaced by air.
A student heats a white solid in a sealed glass tube and wants to find out whether the heating changed the composition of the solid. Which measurement, made before and after heating, would answer the question?
Answer and reasoning
AThe total mass of the sealed tube and its contents A student who thinks mass changes in a chemical reaction picks this. Atoms are rearranged, not created or destroyed, so the total mass of the sealed tube stays the same whether the change is physical or chemical.
BThe color of the solid under the same lighting A student who thinks a color change always shows a chemical change picks this. A change in color is possible evidence only; the color of a solid can change, for example with particle size, without any change in composition.
CThe physical state of the solid at room temperature A student who thinks a change of phase produces a new substance picks this. A substance can change phase without any change in composition, so the state of the sample does not show whether its composition changed.
DThe percent by mass of each element in the solidCorrect Composition is the proportion of each element in a substance. If the percent by mass of an element differs after heating, the solid is a new substance and a chemical change occurred; if it is the same, the composition did not change.
Working Composition is the proportion of each element in a substance, so the needed quantity is the percent by mass of each element in the solid before and after heating. The total mass of a sealed tube does not change in either a physical or a chemical change, and color or phase can change without a change in composition.
In preparation: 0 of 2 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
4.1.A.1 Physical change Fix
Physical change
A change in which a substance's properties change but its composition does not: the same particles (atoms, molecules, or ions) are present before and after, only arranged or spaced differently.
Change of phase
A change between the solid, liquid, and gas phases, such as melting, boiling, or condensing. The particles of the substance are unchanged; only their spacing, arrangement, and motion change, so a change of phase is a physical change.
Formation and separation of mixtures
Mixing substances (including dissolving one in another) and separating them again (for example by filtration, evaporation, or distillation) are common physical changes: each substance keeps its own composition.
Distillation
A separation in which a liquid mixture is boiled and the vapor is condensed and collected. A dissolved solid such as NaCl, which does not vaporize at the boiling point of water, stays behind, so the collected liquid is the solvent.
Students often think Changes of phase of water break or form the bonds within its molecules: boiling splits water into hydrogen and oxygen, and condensation forms water from hydrogen and oxygen in the air. In fact No. When water boils, the H₂O molecules separate from one another but stay intact; when water vapor condenses, the same H₂O molecules come closer together. Only the forces between molecules are overcome or re-formed; the bonds within each molecule are unchanged.
Students often think The particles of a substance expand when it is heated, so the molecules in a gas are larger than the molecules in the liquid. In fact No. The particles themselves stay the same size; in a gas they are much farther apart and move faster. The larger volume of a gas comes from the space between the particles.
4.1.A.2 Chemical change Fix
Chemical change
A change in which substances are transformed into new substances, typically with different compositions, because atoms are rearranged into new combinations.
Composition
The kinds and proportions of the elements in a substance, for example its percent by mass of each element. A change in composition shows that a new substance has formed.
Possible evidence of a chemical change
Production of heat or light, formation of a gas, formation of a precipitate, and a change in color are observations that may indicate a chemical change. Each can also accompany a physical change (boiling forms a gas, condensing releases heat, dilution changes a color), so the deciding question is whether new substances have formed.
Precipitate
An insoluble solid that forms when solutions are mixed; its formation is possible evidence of a chemical change.
Students often think A change in color always shows that a new substance has formed, so any change in color is a chemical change. In fact No. A color change is possible evidence of a chemical change, but some physical changes also change color; diluting a colored solution, for example, makes its color paler while the dissolved substances stay the same.
Students often think Because color is a physical property, a change in color is always a physical change and can never be evidence of a chemical change. In fact No. Color is a physical property of a substance, but a change in color can result from forming a new substance with a different color, as when a black solid forms from gray iron and yellow sulfur. Whether the change is chemical depends on whether new substances form.
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 apparatus shown to boil a solution of sodium chloride, NaCl, in water until about half of the liquid has boiled away. Which prediction about the liquid that collects in the receiving flask is correct?
Answer and reasoning
AIt is water with no NaCl in it, and it boils at the same temperature as pure waterCorrect Boiling and condensing are physical changes. Water vaporizes, travels through the condenser, and condenses as liquid water in the receiving flask; NaCl does not vaporize at this temperature and stays in the boiling flask. The collected liquid is water and has the properties of water.
BNo liquid collects, because boiling decomposes the water into hydrogen and oxygen A student who thinks boiling breaks water molecules apart picks this. The vapor is H₂O, which condenses back to liquid water in the condenser; decomposing water into H₂ and O₂ is a chemical change that boiling does not cause.
CIt is a solution of NaCl, because the dissolved NaCl evaporates along with the water A student who thinks a dissolved solid leaves with the water vapor picks this. NaCl does not vaporize at the boiling point of water, so it stays behind in the boiling flask and the condensed liquid contains no NaCl.
DIt is a new liquid compound that boils at a different temperature from water A student who thinks a change of phase produces a new substance picks this. Water vapor and liquid water are both H₂O; condensing the vapor gives back water with the same composition and boiling point.
A student adds water to a dark blue solution of copper(II) sulfate, and the solution becomes pale blue. The student claims that a chemical change has occurred because the color of the solution changed. Which statement best evaluates the student's claim?
Answer and reasoning
AThe claim is correct, because a change in color shows that a new substance has formed A student who thinks any color change proves a chemical change picks this. A color change is possible evidence only; here the paler color comes from the lower concentration of the same dissolved substance.
BThe claim is correct, because the added water reacts with the dissolved copper(II) sulfate A student who thinks water reacts with whatever it dissolves or dilutes picks this. Adding water only spreads the dissolved particles through a larger volume; evaporating the water would recover the same copper(II) sulfate.
CThe claim is incorrect, because color is a physical property, so color changes are physical A student who thinks every color change is physical because color is a physical property picks this. The claim is wrong, but this reason is not: a color change can be evidence of a chemical change when a new colored substance forms.
DThe claim is incorrect, because the dissolved particles are the same, only farther apartCorrect Diluting forms a more dilute mixture: the same dissolved ions are present, spread through a larger volume, so the color is paler. No new substance forms, so the change is physical even though the color changed.
A student mixes gray iron filings with yellow sulfur powder and then heats the mixture strongly. It glows red, and when it cools, a uniform black solid remains that is not attracted to a magnet, although iron is. Which statement best explains these observations?
Answer and reasoning
AMixing combined the iron and sulfur into a compound, and heating then changed its color A student who thinks mixing substances forms a compound picks this. In a mixture each substance keeps its own properties (the iron could still be pulled out with a magnet); the iron and sulfur combine only when heating makes them react.
BHeating made the iron and sulfur combine to form a new substance with different propertiesCorrect Heating caused a chemical change: iron and sulfur atoms combined into a new substance, a black solid that is not magnetic. Its properties differ from those of both iron and sulfur because its composition is different.
CHeating destroyed the iron, so the black solid is the sulfur that was left behind A student who thinks a reactant can be destroyed in a chemical change picks this. The iron atoms are not destroyed; they are present in the black product, combined with sulfur atoms.
DThe iron is still present as iron metal, and heating changed its color and magnetism A student who thinks reactants remain the same substances with altered properties picks this. The black solid is a new substance; its atoms of iron are combined with sulfur, and iron metal is no longer present.
On a humid day, drops of liquid water form on the outside of a glass of ice water, and energy is released as heat as the drops form. A student claims that the formation of the drops is a chemical change. Which statement best evaluates the student's claim?
Answer and reasoning
AThe claim is correct, because the release of heat shows that a new substance has formed A student who thinks any release of heat proves a chemical change picks this. Physical changes also release heat; condensing water vapor releases heat while the H₂O molecules stay the same.
BThe claim is correct, because hydrogen and oxygen from the air combine to form the water A student who thinks condensation forms water molecules from hydrogen and oxygen picks this. The water was already present in the air as H₂O vapor; condensing it brings H₂O molecules together without forming new bonds within molecules.
CThe claim is incorrect, because water molecules from the air only move closer togetherCorrect The drops form when water vapor in the air condenses on the cold surface. This is a change of phase: the H₂O molecules are unchanged and only move closer together, so it is a physical change even though heat is released.
DThe claim is incorrect, because the water passes through the glass from the inside A student who thinks the drops seep through the glass picks this. Glass is not porous, and drops form on any cold surface in humid air; they come from water vapor in the air.
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
