Physical vs. Chemical Properties — Practice · Grade 8 Science

Chapter: Properties of Matter

What this practice covers

By the end of Physical vs. Chemical Properties you should be able to:

12 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: physical property, chemical property, physical change, chemical change, density, model, evidence, system boundary, assumption.

Problem 1

Which of these is a chemical property, not a physical property?
  1. color
  2. flammability
  3. hardness
  4. density
Answer and explanation

flammability

Flammability describes how a substance reacts (burns), which is a chemical property.

Problem 2

Which is an example of a physical change?
  1. tearing paper
  2. burning paper
  3. a nail rusting
  4. baking a cake
Answer and explanation

tearing paper

Tearing paper changes its shape and size, but it's still the same paper — no new substance forms.

Problem 3

A rock has a mass of 45 g and a volume of 15 cm³. What is its density?
Answer and explanation

3 g/cm³

Density = mass ÷ volume = 45 ÷ 15 = 3 g/cm³.

Problem 4

Which observation is the best evidence that a chemical change has happened?
  1. a change of shape
  2. a change in temperature only
  3. the formation of a new substance with new properties
  4. a change of state
Answer and explanation

the formation of a new substance with new properties

The clearest sign of a chemical change is a genuinely new substance forming, with properties different from what you started with.

Problem 5

Explain why dissolving salt in water is a physical change, not a chemical change.
Answer and explanation

The salt doesn't become a new substance — it's still salt, just spread out among the water molecules. If you evaporated the water, you'd get the same salt back.

Problem 6

A metal sample has a mass of 158 g and a volume of 20 cm³. What is its density?
Answer and explanation

7.9 g/cm³

Density = mass ÷ volume = 158 ÷ 20 = 7.9 g/cm³.

Problem 7

A student tears paper and another burns paper. Classify each change and explain using composition. Explain the reasoning in your own words.
Answer and explanation

Tearing is physical; burning is chemical. New substances, rather than size alone, distinguish the changes.

Tearing changes shape while the paper remains the same material.

Burning rearranges atoms into substances such as gases and ash.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

Copper is stretched into wire. Classify the change using the information that it remains copper. Show the reasoning, not just the conclusion.
Hints

Ask whether a new substance was demonstrated, rather than whether the sample looked different.

Answer and explanation

Physical: shape changes while the substance remains copper. No new composition is described.

Problem 9

An unknown powder changes color when heated. Is that enough to identify a chemical reaction? What else would help?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Color is evidence to investigate, not proof alone. Compare composition or characteristic properties before and after and rule out physical causes.

Problem 10

Plan and, if safe materials are available, conduct the sand/salt comparison. Use the supplied recovery record to argue whether it is consistent with a physical mixture, and explain why a mass shortfall is not automatically a reaction.
Hints

Use the illustrated study titled Mixtures and physical separation. Keep the observations, model and conditions separate.

Answer and explanation

The separate-component tests and recovery of sand and salt are consistent with a physical combination. The illustrative recovered masses are 4.8+2.9=7.7 g versus 8.0 g originally added, leaving 0.3 g unaccounted for in that collection. Transfer residues, remaining dissolved salt or reading uncertainty can explain a shortfall. Better rinsing, complete collection and repeated matched tests can improve the account; the data do not establish a new compound.

The separate-component tests and recovery of sand and salt are consistent with a physical combination. The illustrative recovered masses are 4.8+2.9=7.7 g versus 8.0 g originally added, leaving 0.3 g unaccounted for in that collection. Transfer residues, remaining dissolved salt or reading uncertainty can explain a shortfall. Better rinsing, complete collection and repeated matched tests can improve the account; the data do not establish a new compound.

Response criteria

  • Specify components and a property-based method.
  • Use recovered identity and mass evidence.
  • Explain a limitation or controlled improvement.

Problem 11

A material property needs a controlled comparisonA material property needs a controlled comparisonMass (g)0153045051015Volume (cm³)One uniform material, same conditions: mass/volume = 3 g/cm³.
The illustrative samples have masses 15, 30 and 45 g at volumes 5, 10 and 15 cm³. Their mass-to-volume ratio is the same, 3 g/cm³. A larger sample has more mass without having a new density. This line models one uniform material at matched conditions; it does not identify that material by density alone.
For this unit’s illustrated case, for a 12 cm³ sample following this pattern, predict mass. Can that result alone identify the substance?
Answer and explanation

Predicted mass is 12×3 = 36 g. Density alone is insufficient because different substances can share similar values; compare additional characteristic properties.

The slope of this mass-versus-volume graph is density, not speed. Both axes name measured properties. Use several properties, with conditions stated, before deciding whether a substance’s identity has changed.

Predicted mass is 12×3 = 36 g. Density alone is insufficient because different substances can share similar values; compare additional characteristic properties.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 12

Two colorless samples have the same density but melt at different temperatures under comparable conditions. Does matching density establish identical substance identity? Explain the role of a second property.
Answer and explanation

No. Their distinct melting behavior provides another characteristic-property comparison. One matching density is not unique identification.

Compare multiple characteristic properties under stated conditions.

Separate an observed property from an inference of identity.

No. Their distinct melting behavior provides another characteristic-property comparison. One matching density is not unique identification.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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