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Grade 7 / Middle 1 (age 12-13)

grade 7 States of Matter

States of Matter

Matter is solid, liquid, or gas. Particle motion changes with temperature, and melting, freezing, vaporization, condensation, and sublimation link the states. During a change heat rearranges particles so temperature stays fixed, and we note whether heat is absorbed or released. The temperature slider changes the particle model: 0°C is solid, 100°C is gas, and liquid in between. The six names are in the triangle below.

Three States of Matter

20℃
💡 Particle Motion Changes
①Solid: ordered arrangement, in-place vibration
②Liquid: free movement but particles stay near each other
③Gas: very fast random motion in all directions
④Higher temperature → more vigorous particle motion

Types of State Changes

🔍 Six State Changes
①Melting: solid → liquid (ice melts)
②Freezing: liquid → solid (water → ice)
③Vaporization: liquid → gas (water → vapor)
④Condensation: gas → liquid (vapor → droplets)
⑤Sublimation: solid ↔ gas (dry ice, frost)

State Changes and Heat

Absorbs Heat
Solid → liquid → gas: absorbs heat
Melting, vaporization, sublimation (solid→gas) absorb heat
Releases Heat
Gas → liquid → solid: releases heat
Condensation, freezing, deposition (gas→solid) release heat
🧊 Temperature During State Change
①During state change, temperature is constant
②Heat goes into rearranging particles, not raising temperature
③Boiling point: liquid → gas (water: 100℃)
④Melting point: solid → liquid (ice: 0℃)

Exam Key Points

Core Summary
State change ← absorption/release of heat
Heating absorbs, cooling releases
🎯 Exam Key Points
①Particle arrangement: solid vs liquid vs gas
②Six state changes and direction (absorb vs release)
③Temperature constant during change (mp, bp)
④Boiling vs evaporation: boiling throughout vs evaporation at surface
⑤Sublimation examples: dry ice, naphthalene, frost

Examples and Unit Test

Example 1
What is the change of state when ice melts into water, and does it absorb or release heat energy?
1
A change from solid to liquid is called melting (fusion).
2
In melting, particle motion becomes more active, so it absorbs heat energy.
▸ Melting / absorbs heat energy
Melting, vaporization, and sublimation (solid to gas) absorb heat; freezing, condensation, and deposition release heat.
Example 2
Why does the temperature stay the same while ice melts (at 0℃) even as you keep adding heat?
1
During a change of state, the added heat energy is used to rearrange the particles.
2
So it is not used to raise the temperature, and the temperature stays constant at the melting point (0℃).
▸ The heat is used to rearrange particles, so the temperature stays constant
During a change of state, heat goes into rearranging particles, so the temperature stays constant (melting/boiling point).
Unit-test style
Which statement about changes of state is correct?
①Vaporization is a change that releases heat energy
②Melting and vaporization are changes that absorb heat energy
③The temperature keeps rising during a change of state
④In a gas, particles vibrate in place
⑤Freezing is a liquid changing into a gas
▸ ② Melting and vaporization are changes that absorb heat energy
1
In melting (solid to liquid) and vaporization (liquid to gas), particle motion becomes more active and heat is absorbed.
2
Vaporization absorbs heat (①), temperature stays constant during a change of state (③), in a gas particles scatter quickly (④), and freezing is a liquid changing into a solid (⑤).
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Density
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Molecular Motion & Diffusion
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