Cambridge O Level Chemistry · Syllabus 5070 · States of Matter
Change of state
What is Change of state?
A physical change in which a substance goes from one state of matter to another - solid, liquid or gas - because energy is transferred to or from it. Energy transferred to the substance increases the movement of its particles and allows them to overcome the attractive forces holding them together; energy transferred away allows those attractions to pull the particles back together. The substance itself is unchanged: no new substance is made, the particles keep their size and chemical identity, and the change can be reversed by reversing the energy transfer.
This definition is part of the States of Matter chapter in Cambridge O Level Chemistry.
Change of state in context
A change of state is a physical change in which energy transferred to or from a substance lets its particles overcome, or be pulled back together by, the attractive forces between them. Melting, boiling and evaporating take energy in; condensing and freezing give energy out. The covalent bonds inside molecules are not broken, so steam and ice are both still \(\mathrm{H_2O}\). On a heating curve the flat plateaus mark these changes: the temperature stays constant because the energy being transferred is used to overcome the attractions between particles rather than to increase their average kinetic energy.
Examiner tips on Change of state
- The single most valuable sentence in this chapter: a change of state overcomes the attractive forces between particles. It does not break the covalent bonds inside a molecule. Steam is still \(\mathrm{H_2O}\); ice is still \(\mathrm{H_2O}\). Nothing chemical has happened.
Questions students ask about Change of state
Why does the temperature stay constant while a substance melts or boils?
The plateau on a heating curve does not mean heating has stopped: energy is still being transferred the whole time. During a change of state that energy is used to overcome the attractive forces holding the particles together and move them further apart, rather than to increase the average kinetic energy of the particles. Because temperature depends on the average kinetic energy, the temperature does not rise until the change of state is complete. The first plateau is the melting point and the second is the boiling point.