Answer:
steam
Step-by-step explanation:
Let us consider the sentence again:
'As water molecules heat up, water changes from a liquid to steam or vapor.'
Now, the meaning of this sentence is clearly understood if the student can understand the meaning of the word 'steam' because that is where his/her confusion lies.
If the meaning of 'steam' is understood, the student can now verify the meaning of the sentence. This is clear from the observation that the action expressed by the sentence is the change of water to steam.