Evaporation, Condensation & a Wet Cold Cup — Practice · Grade 8 Science

Chapter: Phases of Matter

What this practice covers

By the end of Evaporation, Condensation & a Wet Cold Cup you should be able to:

12 questions, written answer, each with a worked explanation.

Vocabulary used: evaporation, condensation, water vapor, humidity, dew point, model, evidence, system boundary, assumption.

Problem 1

Why compare with a room-temperature cup?
Answer and explanation

It helps test the role of cooling while keeping the cup construction similar. Clear outside water on the cold cup supports an air-water source rather than a colored leak.

It helps test the role of cooling while keeping the cup construction similar. Clear outside water on the cold cup supports an air-water source rather than a colored leak.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 2

Does a drying cloth need to reach 100 °C?
Answer and explanation

No. Surface evaporation occurs below boiling. Water transfers from the cloth into the air, rather than vanish.

No. Surface evaporation occurs below boiling. Water transfers from the cloth into the air, rather than vanish.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 3

Which supplied cup is more likely to condense water?
Answer and explanation

The 8 °C surface is below the supplied 12 °C dew point; the 18 °C surface is above it. The prediction depends on those air conditions.

The 8 °C surface is below the supplied 12 °C dew point; the 18 °C surface is above it. The prediction depends on those air conditions.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 4

Why is greater loss from a sunny wide tray not proof of an area effect alone?
Answer and explanation

Area, illumination and possibly temperature differ. A comparison changing only intended area is needed to isolate its effect.

Area, illumination and possibly temperature differ. A comparison changing only intended area is needed to isolate its effect.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 5

Illustrative records show a cold intact cup with clear outside droplets. An open tray loses 6 g while a covered matched tray loses 1 g in an hour. Explain both observations and identify a limit of the mass comparison.
Answer and explanation

The droplets can be condensed water from surrounding air; the tray loss can be water evaporating into that air. The open tray loses 5 g more under the stated comparison. Covering changes exchange and can also change temperature or airflow, so the result does not isolate every mechanism. Neither observation requires water creation or destruction.

The droplets can be condensed water from surrounding air; the tray loss can be water evaporating into that air. The open tray loses 5 g more under the stated comparison. Covering changes exchange and can also change temperature or airflow, so the result does not isolate every mechanism. Neither observation requires water creation or destruction.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 6

A white cloud above warm water is called visible water vapor. Correct the statement.
Answer and explanation

The gas water vapor itself is normally invisible. The visible white cloud mainly consists of small condensed liquid droplets, and its appearance alone does not identify every particle present.

The gas water vapor itself is normally invisible. The visible white cloud mainly consists of small condensed liquid droplets, and its appearance alone does not identify every particle present.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 7

Evaluate and correct: Cold cups create new water on their outside.
Answer and explanation

Existing water vapor from the air can condense on the cold surface; identify its source and energy transfer.

Existing water vapor from the air can condense on the cold surface; identify its source and energy transfer.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 8

Choose one prediction from this unit. Explain a comparison that could test it, specifying observations, changed and matched conditions, and a result that would challenge it.
Answer and explanation

Evaporation can cool the remaining liquid because escaping particles remove energy. Water and energy accounts therefore need separate arrows: a water arrow indicates where matter moves; an energy arrow shows a transfer, not a stream of a material called heat. Area, illumination and possibly temperature differ. A comparison changing only intended area is needed to isolate its effect.

Evaporation can cool the remaining liquid because escaping particles remove energy. Water and energy accounts therefore need separate arrows: a water arrow indicates where matter moves; an energy arrow shows a transfer, not a stream of a material called heat. Area, illumination and possibly temperature differ. A comparison changing only intended area is needed to isolate its effect.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 9

Air dew point is supplied as 15 °C. An intact cup surface is 18 °C and another is 9 °C. Predict the condensation comparison and account for water origin.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

The 9 °C surface is below the dew point and more likely to collect droplets; the 18 °C surface is above it. Existing water vapor from air can supply the condensate. The prediction uses the stated air conditions.

The 9 °C surface is below the dew point and more likely to collect droplets; the 18 °C surface is above it. Existing water vapor from air can supply the condensate. The prediction uses the stated air conditions.

Problem 10

A white cloud above warm water is called visible water vapor. Correct the statement. Explain a condition that would change this conclusion.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

The gas water vapor itself is normally invisible. The visible white cloud mainly consists of small condensed liquid droplets, and its appearance alone does not identify every particle present.

The gas water vapor itself is normally invisible. The visible white cloud mainly consists of small condensed liquid droplets, and its appearance alone does not identify every particle present.

Problem 11

Evaporation occurs at the liquid surfaceEvaporation occurs at the liquid surfaceLiquid: many interacting particlesSelected surface particles escapeBelow boiling is possibleRemaining average motion may fallParticle motion varies; the drawing does not give every particle one speed.
Selected particles leave the surface while the remaining liquid still contains interacting particles. Evaporation can occur below the boiling temperature and often removes particles with above-average kinetic energy, lowering the remaining average. This is a schematic selected-particle view, not a universal rule that every particle moves upward or that molecules break into new atoms.
For this unit’s illustrated case, why can a puddle dry at room temperature without the whole puddle boiling?
Answer and explanation

Some surface particles can escape into the gas phase under those conditions. Boiling is a different bulk process involving vapor bubbles throughout the liquid.

A surface is the boundary between liquid and its surroundings. Molecules in a liquid have a range of motions; an escaping molecule retains its chemical identity. Matter leaving also carries energy, linking evaporation to cooling.

Some surface particles can escape into the gas phase under those conditions. Boiling is a different bulk process involving vapor bubbles throughout the liquid.

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

An uncovered sample loses 2 g and a covered sample 0.3 g over the same interval. Explain the supported matter-loss comparison and why these balance readings alone cannot establish the whole energy account.
Answer and explanation

The uncovered sample loses 1.7 g more. Escaping matter can carry energy, but temperature, evaporation energy and other transfer paths need additional evidence for an energy balance.

Compare mass changes for the same boundary and interval.

Keep the matter measurement distinct from an energy measurement.

The uncovered sample loses 1.7 g more. Escaping matter can carry energy, but temperature, evaporation energy and other transfer paths need additional evidence for an energy balance.

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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