Changes of State and Energy
When substances change state, energy is transferred. This energy either helps break bonds (when heating) or form bonds (when cooling). These changes are physical, not chemical.
Energy during state changes
- During melting and boiling, energy is taken in to overcome forces between particles
- During freezing and condensing, energy is released as particles form stronger bonds
- The temperature remains constant during the change – energy goes into breaking or making bonds, not raising temperature
Heating and cooling curves
Key features:
- Sloped sections = temperature rising (particles gaining kinetic energy)
- Flat sections = change of state (energy used to break bonds, not increase temperature)
Examples of energy transfer
Change of State | Energy Transfer | Particles |
Solid → Liquid | Energy taken in | Particles vibrate more, break free |
Liquid → Gas | Energy taken in | Particles gain enough energy to escape |
Gas → Liquid | Energy released | Particles slow down and bond again |
Liquid → Solid | Energy released | Particles lose energy and settle into a fixed position |
Why temperature stays the same
- Even though heat is still being added, temperature doesn’t rise
- This is because all the energy is used to overcome attractions, not increase particle speed
- Once the state change is complete, temperature starts rising again
Questions
- What happens to energy during melting?
- Why does temperature stay constant during boiling?
- What kind of change is a state change – chemical or physical?
- Does freezing release or absorb energy?
- What is the energy used for during a state change?
Summary
- Energy is transferred during state changes
- Melting and boiling require energy input to break bonds
- Freezing and condensing release energy to form bonds
- Temperature stays constant during the actual change of state
- These are physical changes – the substance is the same but in a different form