How Materials Change State · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Why the Temperature Line Goes Flat

Science · Matter & Materials · ages 11-12
Name ______________________   Date ____________

  1. What is it called when a solid turns directly into a gas, without becoming a liquid first, like dry ice does?

    • Condensing
    • Sublimation
    • Freezing

  2. What happens to particles when a liquid freezes?

    • They slow down and lock into a fixed pattern
    • They speed up and spread further apart
    • They join together to make bigger particles

  3. True or false: while ice is melting, the temperature keeps rising steadily.

    Circle one:   True   False

  4. Look at the ice-heating graph. The line goes flat a second time near the end. What boiling point, in °C, does that flat section show?

    0 min02 min04 min506 min1008 min100

    Answer: ______________

  5. This graph shows a substance being heated. The line stays flat at 20°C for a while, rises, then stays flat at 100°C. What is the melting point of this substance?

    0 min202 min204 min606 min1008 min100
    • 20°C
    • 60°C
    • 100°C

  6. Why doesn't the temperature go up while water is boiling, even though you keep heating it?

    • The energy is being used to pull particles apart into a gas, not to speed them up further
    • The pan is losing heat as fast as it gains it
    • The thermometer stops working at 100°C

  7. A student says: 'Boiling and evaporation are the same thing because both turn liquid into gas.' Is the student correct?

    Circle one:   True   False

  8. Dry ice (solid carbon dioxide) placed in a warm room disappears completely, leaving no puddle at all. What must be happening to the particles?

    • They're melting into a liquid that quickly evaporates
    • They're moving directly from a fixed solid arrangement into a free-moving gas, skipping the liquid stage
    • They're freezing further and shrinking
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Why the Temperature Line Goes Flat W1-mt_CyV7crZ8hl-s1

  1. Sublimation · Sublimation skips the liquid step completely: solid particles gain enough energy to break free straight into a gas.
  2. They slow down and lock into a fixed pattern · Freezing doesn't create new particles, it just slows them down until they lock into a fixed, repeating pattern.
  3. False · While ice is melting, the added heat is used to break the bonds between particles, so the thermometer reading doesn't move.
  4. 100 · The second flat part of the graph, at 100°C, is where the liquid boils away into a gas.
  5. 20°C · The first flat part of the graph shows where the temperature stops rising while the solid melts, at 20°C.
  6. The energy is being used to pull particles apart into a gas, not to speed them up further · The heat keeps flowing in, but it's used to pull particles apart into a gas rather than speed them up, so the temperature holds steady.
  7. False · Evaporation happens only at the surface and at any temperature, while boiling happens throughout the liquid and only at the boiling point.
  8. They're moving directly from a fixed solid arrangement into a free-moving gas, skipping the liquid stage · Dry ice sublimates: its particles jump straight from a fixed solid arrangement to a free-moving gas without ever becoming liquid.
Worksheet · LightMySky