Ou should have observed that Al, Fe, Pb, Sn, and Zn reacted with HCl. List these five metals in order of decreasing reactivity (most reactive metal first)

Al, Zn, Fe, Sn, Pb.

Explanation:

The arrangement can be done using the infomation on electrochemical series of metals.

The electrochemical series is built up by arranging various metals in order of their standard electrode potentials (or reactivity). The most negative E° values are placed at the top of the electrochemical series, and the most positive at the bottom. some of the metals are arranged as follws;  K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, H, Cu, Hg, Ag, Au (decreasing reactivity)

One can use the following to remember the arrangement: king Nathan Can Manage All Zone Freely Since Probably He Can Handle All Ages.

practical process described pest part of scientific investigation.

because practical knowledge is powerful then therotical knowledge.