The reaction between sodium and chlorine is described in the following paragraph.
Sodium is a silvery-grey metal that reacts violently with water. Chlorine is a green gas that is toxic and dissolves in water to give a weakly acidic solution. When sodium reacts with chlorine, heat is given out and a white powder (sodium chloride) is formed. Sodium chloride dissolves in water to give a neutral solution.
What information in this paragraph tells us that sodium chloride is a compound, and not a mixture?