The correct option is **C. They react with metals to produce hydrogen gas.** ### Detailed Explanation: 1. **Understanding Acids:** Acids are substances that can donate protons (H⁺ ions) in a chemical reaction. They are characterized by their sour taste, ability to conduct electricity when dissolved in water, and their reactivity with certain metals. 2. **Why Option C is Correct:** - When acids react with certain metals, they undergo a chemical reaction that produces hydrogen gas (H₂) and a salt. This is a classic reaction that can be represented by the general equation: \[ \text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen gas} \] - For example, when hydrochloric acid (HCl) reacts with zinc (Zn), the reaction can be written as: \[ 2 \text{HCl} + \text{Zn} \rightarrow \text{ZnCl}_2 + \text{H}_2 \uparrow \] - The upward arrow indicates the production of hydrogen gas, which can be observed as bubbles forming during the reaction. 3. **Why the Other Options are Incorrect:** - **Option A: They turn litmus paper blue.** - This statement is incorrect because acids actually turn blue litmus paper red. This is a key characteristic of acids and is used as a simple test to identify them. Bases, on the other hand, turn red litmus paper blue. - **Option B: They have a bitter taste.** - This statement is misleading. Acids typically have a sour taste, not a bitter one. Bitter tastes are more commonly associated with bases (alkalis). For example, substances like vinegar (acetic acid) taste sour, while baking soda (a base) has a bitter taste. - **Option D: They feel slippery to the touch.** - This statement is also incorrect. The slippery feeling is characteristic of bases, not acids. Bases can feel slippery because they react with the oils on your skin, creating a soap-like effect. Acids, in contrast, can feel harsh or irritating to the skin. ### Summary of Key Points: - Acids react with metals to produce hydrogen gas, which is a defining characteristic. - Acids turn blue litmus paper red, while bases turn red litmus paper blue. - Acids have a sour taste, whereas bases have a bitter taste. - Bases feel slippery, while acids can be irritating to the skin. ### Revision Summary: - Acids produce hydrogen gas when reacting with metals. - Acids turn blue litmus paper red. - Acids taste sour, while bases taste bitter. - Bases feel slippery, while acids can irritate the skin.