Answer:
Gas formation is a sign that a chemical reaction is occurring in various types of reactions, including:
1. **Acid-Base Reactions**: When an acid reacts with a base, one of the products formed is often water, and the other is a salt. In some cases, gas formation occurs as well. For example, when hydrochloric acid reacts with sodium bicarbonate (baking soda), carbon dioxide gas is produced:
\[ \text{HCl} + \text{NaHCO}_3 \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \]
2. **Single Replacement Reactions**: In single replacement reactions, one element replaces another in a compound. Some single replacement reactions can produce gas. For example, when zinc metal reacts with hydrochloric acid, hydrogen gas is produced:
\[ \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \]
3. **Double Replacement Reactions**: Double replacement reactions occur between two compounds, where the positive ions in the reactants switch places. In some cases, a gas can form as a product. For example, when calcium carbonate reacts with hydrochloric acid, carbon dioxide gas is produced:
\[ \text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2 \]
Gas formation in a chemical reaction is often observable as bubbles or effervescence, indicating the release of gas molecules. This gas formation is a clear sign that a chemical reaction is occurring.
