1. Acids react with metals to produce hydrogen gas. This reaction is known as a redox reaction, and it involves the transfer of electrons from the metal to the hydrogen ions in the acid. For example, when hydrochloric acid reacts with iron, the iron atoms lose electrons to the hydrogen ions, and hydrogen gas is produced.

$$Fe(s) + 2HCl(aq) → FeCl_2(aq) + H_2(g)$$

2. Acids react with bases to produce water and a salt. This reaction is known as a neutralization reaction, and it involves the combination of hydrogen ions from the acid with hydroxide ions from the base to form water. For example, when hydrochloric acid reacts with sodium hydroxide, the hydrogen ions and hydroxide ions combine to form water, and sodium chloride is produced.

$$HCl(aq) + NaOH(aq) → NaCl(aq) + H_2O(l)$$

3. Acids react with certain metal carbonates and bicarbonates to produce carbon dioxide gas. This reaction is known as a decomposition reaction, and it involves the breakdown of the carbonate or bicarbonate into carbon dioxide, water, and the corresponding metal salt. For example, when hydrochloric acid reacts with sodium bicarbonate, carbon dioxide gas is produced, along with water and sodium chloride.

$$2HCl(aq) + NaHCO_3(aq) → CO_2(g) + 2H_2O(l) + NaCl(aq)$$