In a gas evolution reaction, a gas is produced. For example, when sulfuric acid reacts with lithium sulfide, it produces H2S gas and lithium sulfate. Some gas evolution reactions form an intermediate product. For example, when hydrochloric acid reacts with sodium hydrogen carbonate, carbonic acid is produced as an intermediate product. The carbonic acid can decompose to produce water and carbon dioxide gas. We'll review four gas evolution reactions. In the first three reactions, we're combining an ionic compound with a strong acid. In the last reaction, we combine an ionic compound with a strong base. In the first reaction, we do not produce an intermediate product. In the last three reactions, we do produce an intermediate product, which decomposes to produce water and a gas. Let's practice writing balanced chemical equations for gas evolution reactions. We'll start by writing the equation for the reaction of sodium bisulfite with hydroiodic acid. When we react a sulfite or bisulfite salt with a strong acid, we produce the intermediate sulfurous acid. All of our intermediates decompose to produce water and a gas. If we remove H2O from sulfurous acid, SO2 remains, so we will produce sulfur dioxide gas. We also need to include the counter ions which are sodium and iodide. Sodium iodide is a soluble ionic compound, so it remains aqueous. To balance this reaction, the acid is donating an H+ to the bisulfite ion to produce sulfurous acid. Bisulfite only needs one H+, so we only need one hydroiodic acid. All of our coefficients are ones, so this reaction is already balanced. If we react sodium sulfite with hydroiodic acid, we still produce the same intermediate, sulfurous acid. So we also produce the same products. The only difference comes in balancing the reaction. Sulfite requires two H+ ions to produce sulfurous acid, so we need two HI. This also means we will produce two sodium and two iodide, so we need two sodium iodide. We still only produce one water and one sulfur dioxide gas because we still only had one sulfurous acid decompose. Now let's write the balanced chemical equation for the reaction of ammonium bromide with sodium hydroxide. Our intermediate is ammonium hydroxide. If we remove H2O from ammonium hydroxide, we have NH3 left over, so our gas is ammonia. We also have sodium bromide left over. This equation is already balanced with coefficients of one.