Can matter be classified as an element, compound and a mixture at the same time? why or why not?

When hydrochloric acid reacts with potassium hydroxide solution, the following reaction occurs. hcl (aq) + koh (aq) h2o (l) + kcl (aq) the reaction gives off heat energy, so it is an reaction.

What is the most common reason for matter changing its state?

How many moles are in 7.2 x 10^23 carbon molecules?