Heating and Cooling Curves (updated)

Internal energy of a substance is the sum of kinetic energy and potential energy of the molecules.

• kinetic energy is directly proportional to the temperture of the substance

• potential energy is related to the average spacing between the molecules (PE increases as spacing increases)

  - (for O-level, we can simplify it and treat it as PE will only change when there is a change of state. E.g during melting, the intermolecular forces are overcome and molcules are further now, hence PE increases. During melting, temp remains constant, hence KE remains the same)

• - i.e at any time, there is only either KE or PE change during the heating or cooling curve.

As the substance is heated:

• energy is transferred into the internal store of the substance.

• internal energy increases.

As the substance is cooled:

• energy is transferred out of the internal store of the substance.

• internal energy decreases.