Hey everyone,
I need help on a question for chemistry homework.(Grade 11)

"Why do pure substances have plateaus?"

thanks to anyone that can help me with this.

what exactly does it mean by a 'plateau' exactly? does it mean that it remains at a relatively constant temperature when a state-change is occuring? or is it a plateau in a different context?

I think plateau here means a condition in which a (pure) substance reaches stability.
For example, if you have table salt (NaCl), it'll stay being table salt if you don't alter the condition with chemical reactions.

If I recall correctly (and correct me if I'm wrong ), it's because of the bonding between elements.
All known elements except noble gases tend to reach stability (or have a noble gas structure) by either gaining or losing electron(s), which is often called ionic bond or even sharing electrons (called covalent bond).

its this one, "does it mean that it remains at a relatively constant temperature when a state-change is occuring?"

Ok, it's because at the boiling/melting points, when the state change is occuring, the substance, be it a solid or liquid at the time, absorbs all heat energy until all of the substance has been converted to the new state (liquid or gas). The temperature doesn't rise because all the heat is being absorbed by the substance. That's the simplest way of putting it I spose probably could be worded better too xD. If it doesn't make sense ask me to clarify it