Chemical changes

Two clear liquids are mixed. Data are recorded. Before: Liquid A: clear, no odor, pH 6 Liquid B: clear, sharp odor, pH 2 After mixing: clear, no odor, pH 4, temperature unchanged, no gas, no solid Best conclusion?

• Chemical reaction occurred because the odor disappeared

• Chemical reaction occurred because the pH changed

• Not enough evidence for a chemical reaction; data mostly suggest physical mixing/dilution

• Chemical reaction occurred because both were liquids

Answer explanation

The data indicates a physical mixing or dilution rather than a chemical reaction. The odor disappeared, but the pH change from 6 to 4 suggests dilution of the acidic liquid, not a reaction, as no gas or solid was produced.

Tags

NGSS.MS-PS1-2

A solid is added to water and stirred. Before: white solid; water is clear After: solid disappears; liquid remains clear; can be recovered by evaporating the water What does the evidence support?

• Chemical reaction occurred because the solid disappeared

• Physical change occurred because dissolving is reversible here

• Chemical reaction occurred because evaporation is involved

• No change occurred

Answer explanation

The solid dissolves in water, making it a physical change since it can be recovered by evaporation. This indicates that the process is reversible, supporting the choice of a physical change.

Tags

NGSS.MS-PS1-2

A student heats a blue solid. Before: blue solid, insoluble in water, not magnetic After: black solid, insoluble in water, now attracted to a magnet Best conclusion?

• Chemical reaction likely occurred because a new substance with new properties formed

• Physical change occurred because only color changed

• Phase change occurred because solids turn black when heated

• No change occurred because solubility stayed the same

Answer explanation

A chemical reaction likely occurred because the blue solid transformed into a black solid with new properties, such as being attracted to a magnet, indicating a new substance formed.

Tags

NGSS.MS-PS1-2

• 4 only

• 1 and 4

• 2 and 3

• 4 and 2

Answer explanation

The formation of a solid from two liquids (2) and the appearance of a new odor (3) indicate that new substances are created, which are strong evidence of a chemical reaction. The other observations do not confirm a chemical change.

Tags

NGSS.MS-PS1-2

A student crushes a tablet into powder. Before: tablet mass 2.0 g After: powder mass 2.0 g; color unchanged; can still dissolve in water the same way Best conclusion?

• Chemical change because shape changed

• Physical change because size/shape changed but properties stayed the same

• Chemical change because mass stayed the same

• No change because it can dissolve

Answer explanation

The process of crushing the tablet is a physical change. The mass remains the same, and the properties, such as color and solubility, are unchanged, indicating that only the size and shape have altered.

Two trials are run. Trial 1: mixing Chem A + Chem B causes bubbles and temperature increases Trial 2: mixing Chem A + Chem C causes no bubbles and no temperature change Which statement is best supported?

• Chem A reacts chemically with both Chem B and C

• Chem A reacts chemically with B but not with C

• A never reacts chemically because it is the same substance in both trials

• Chem C must be a catalyst

Answer explanation

Trial 1 shows a reaction with bubbles and temperature increase, indicating a chemical reaction between Chem A and B. Trial 2 shows no reaction with Chem C, supporting that Chem A reacts with B but not with C.

Tags

NGSS.MS-PS1-2

NGSS.MS-PS1-4

A student mixes two colorless liquids and a white solid forms immediately. Which is the best claim?

• A chemical reaction occurred because a precipitate formed immediately

• No chemical change occurred because the liquids were colorless

• It is a physical change because only the state of matter changed

• A gas must have formed even though none was observed

Answer explanation

A precipitate is a solid that forms from a chemical reaction between two liquids. The immediate formation of a white solid indicates a chemical change occurred, making the first option the best claim.

Tags

NGSS.MS-PS1-2

A student records flammability before and after mixing two liquids. Before: both liquids are flammable After mixing: resulting liquid is not flammable; no gas or solid observed; temperature unchanged Best conclusion?

• Chemical reaction likely occurred because a chemical property changed

• Physical change because no gas or solid formed

• No change because the mixture stayed a liquid

• Phase change because flammability changed

Answer explanation

A chemical reaction likely occurred since the flammability, a chemical property, changed after mixing. The absence of gas or solid and unchanged temperature does not negate the chemical change.

Tags

NGSS.MS-PS1-2

A student mixes two powders. Before: Powder A is white; Powder B is yellow After: mixture is pale yellow; all other properties unchanged; powders can be separated by careful sorting Best conclusion?

• Chemical reaction occurred because the color changed

• Physical change; likely just mixing

• Chemical reaction occurred because powders touched

• Not enough evidence to decide; mixing powders is impossible

Answer explanation

The change in color indicates a physical change, as the powders can still be separated and their properties remain unchanged. This suggests they were simply mixed, not chemically reacted.

Tags

NGSS.MS-PS1-2

A student mixes two liquids and measures temperature. Temperature (°C): - start: 22 - after 30 seconds: 29 - after 2 minutes: 27 No gas, no solid, no color change. Best conclusion?

• Chemical reaction definitely occurred because temperature changed

• Temperature change alone suggests energy transfer; it may indicate a reaction

• No reaction occurred because color didn’t change

• A phase change occurred

Answer explanation

The temperature change indicates energy transfer, which may suggest a reaction. However, the lack of gas, solid, or color change means we cannot definitively conclude a chemical reaction occurred.

Tags

NGSS.MS-PS1-2

NGSS.MS-PS1-4

NGSS.MS-PS3-4