Magnesium ribbon reacts vigorously with hydrochloric acid. The test tube gets hot and bubbles are produced. When a metal reacts with an acid in a single replacement reaction, we can expect the production of A.) oxygen gas. B) water vapor. C) hydrogen gas. D) carbon dioxide gas.

The answer is C, hydrogen gas. This is because in single replacement reactions, the single element (here Magnesium) replaces whichever element in the compound it corresponds to. Because Mg loses electrons since it’s a metal, it will replace the element which also loses electrons, which is Hydrogen here. So when they switch places, MgCl2 and H2 are made - and H2 is the hydrogen gas.