Answer and Explanation:
18. An experiment in which only one variable is changed is a(an) Â controlled experiment .
19. The combination of chemical reactions through which an organism builds up or breaks down  materials as it carries out its life processes is called  metabolism.
20. Neutrons and  protons are found in the nucleus of an atom.
21. A chemical bond formed by the transfer of electrons is a(an) Â ionic bond .
22. The pH scale is a measurement system that indicates the concentration of  hydrogen ions in a solution.