help in chemistry thanks is urgent!

 15 What is the name given to substances that are initially involved in a chemical reaction?

16. True or false? Bases have a pH level below 7.

17. Is sodium hydroxide (NaOH) an acid or base?

18. Atoms of the same chemical element that have different atomic mass are known as?

19. True or false? A proton carries a positive charge.

20. What is the fourth most abundant element in the universe in terms of mass?