Figure: Heating curve for a pure substance

The heating curve shows a flat section between X and Y. What is happening in this section?
Energy is used to break bonds inside the molecules
Energy is used to overcome forces between the particles
The particles stop moving until the state has changed
A chemical reaction forms a new substance
84 exam-style questions on Edexcel GCSE Chemistry 3.1 States of matter, covering 3.1.1 The three states of matter and changes of state. Each one has a worked solution and a mark scheme showing where the marks go.