States of Matter & Phase Changes — Practice · Grade 8 Science

Chapter: Phases of Matter

What this practice covers

By the end of States of Matter & Phase Changes you should be able to:

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

Vocabulary used: solid, liquid, gas, melting point, boiling point, condensation, thermal energy, model, evidence, system boundary, assumption.

Problem 1

In which state do particles vibrate about relatively fixed positions in the introductory particle model?
  1. Gas
  2. Liquid
  3. Solid
  4. Plasma
Answer and explanation

Solid

Solids have tightly packed particles that vibrate in place rather than moving around freely.

Problem 2

While a pure liquid boils at fixed pressure, what happens to its temperature during the phase change?
  1. It stays the same until all the liquid has boiled away
  2. It rises steadily
  3. It drops suddenly
  4. It becomes negative
Answer and explanation

It stays the same until all the liquid has boiled away

At fixed pressure, added energy changes particle interactions during boiling rather than raising temperature while liquid and vapor coexist.

Problem 3

Which phase change describes water vapor turning into liquid water on a cold glass?
  1. Melting
  2. Evaporation
  3. Condensation
  4. Sublimation
Answer and explanation

Condensation

Gas turning into liquid is condensation.

Problem 4

Explain the difference between evaporation and boiling.
Answer and explanation

Evaporation happens slowly at the surface of a liquid at almost any temperature, while boiling happens throughout the entire liquid at one specific temperature, the boiling point.

Problem 5

A heating curve shows a solid's temperature rising, then flattening out at 0°C while it melts, then rising again as a liquid. What temperature does that flat section occur at?
Answer and explanation

0 °C

The flat section on a heating curve occurs at the melting point, given here as 0°C.

Problem 6

Dry ice (solid carbon dioxide) turning directly into a gas without becoming a liquid is an example of…
  1. melting
  2. sublimation
  3. condensation
  4. freezing
Answer and explanation

sublimation

Sublimation is the direct change from solid to gas.

Problem 7

Ice and liquid water coexist at about 0°C at normal atmospheric pressure while heat is added. Why does the temperature stay nearly constant? Explain the reasoning in your own words.
Answer and explanation

Energy is used in melting while both phases remain; temperature rises after the ice has melted if heating continues.

Identify melting: the substance is changing from solid to liquid.

Added energy changes particle interactions instead of mainly increasing average kinetic energy.

Response criteria

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

Problem 8

A puddle disappears at 18°C. Explain why this does not require boiling. Show the reasoning, not just the conclusion.
Hints

The particles remain the same chemical substance through the change.

Answer and explanation

Evaporation occurs at the surface below boiling temperature as some particles escape. The liquid need not reach its boiling point.

Problem 9

Ice melts in a sealed container. Must its molecules split into hydrogen and oxygen?
Hints

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

Answer and explanation

No. Melting changes arrangement and interactions; water molecules retain their chemical identity.

Problem 10

Evaporation and boiling can both make water vaporEvaporation and boiling can both make water vaporEvaporation: surfaceBoiling: vapor bubbles in bulkBoth retain water identity. Visible fog above a cup is mainly liquid droplets.Schematic selected molecules, not literal bubble size or an experiment to perform.
Evaporation selects surface particles escaping from liquid and can occur below boiling. In the specified boiling-water model, vapor bubbles form in the liquid’s bulk. Both are phase changes retaining water identity. Bubble size and selected particles are schematic, and neither drawing says visible white fog is invisible gas molecules or that all bubbling processes are boiling.
For this unit’s illustrated case, which feature distinguishes these two modeled phase changes, and why would a bubble from an unknown mixture need other evidence?
Answer and explanation

Surface-only escape versus vapor-bubble formation within the liquid distinguishes these cases. Unknown bubbles could arise from other processes, so gas identity and conditions must be established.

Bulk means the material away from only its outer surface. Water vapor is gaseous water and is invisible; visible mist consists mainly of small liquid droplets. The model separates these ideas without asking the learner to boil water.

Surface-only escape versus vapor-bubble formation within the liquid distinguishes these cases. Unknown bubbles could arise from other processes, so gas identity and conditions must be established.

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 11

A labeled pure-water sample has a nearly constant temperature during melting while energy still enters. What changes at particle and whole-sample levels, and which identity stays the same?
Answer and explanation

More of the sample becomes liquid and particle arrangement/freedom of movement changes. Water molecules remain water; a plateau does not mean zero energy input or motionless particles.

Interpret the plateau with the stated phase-change conditions.

Track substance identity separately from arrangement and phase fraction.

More of the sample becomes liquid and particle arrangement/freedom of movement changes. Water molecules remain water; a plateau does not mean zero energy input or motionless particles.

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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