A unknown piece of metal is placed into a container of acid. Hydrogen gas was produced as a result. The unknown metal could have been ? Ti is the answer, but I don't understand why.

any metal more active than hydrogen. See the activity series:

http://www.grandinetti.org/resources/Teaching/Chem121/Lectures/ActivitySeries/ActivitySeries.png

anything higher on the chart will displace anything lower.

To identify the unknown metal in this experiment, we can start by examining the reaction that occurred. The production of hydrogen gas suggests that the metal reacted with acid to form a metal salt and hydrogen gas, following a general equation:

Metal + Acid → Metal Salt + Hydrogen Gas

In this case, the unknown metal reacted with the acid to produce hydrogen gas. Titanium (Ti) is a metal known to react with acids, producing hydrogen gas. Therefore, titanium is a possible answer to this question.

To understand why titanium reacts with acid to produce hydrogen gas, we need to consider its reactivity. Titanium has a moderate reactivity and is capable of displacing hydrogen from acids. When titanium reacts with an acid, such as hydrochloric acid (HCl), the following reaction occurs:

Ti(s) + 2HCl(aq) → TiCl2(aq) + H2(g)

In this reaction, the titanium displaces hydrogen from hydrochloric acid, forming titanium chloride (TiCl2) and releasing hydrogen gas (H2) as a product.

Therefore, based on the production of hydrogen gas and the reactivity of titanium with acids, titanium is a possible answer for the unknown metal in this experiment.