**Correct Option: B. They have a sour taste and turn blue litmus paper red.** ### Detailed Explanation: 1. **Understanding Acids:** - Acids are substances that can donate protons (H⁺ ions) in a solution. This property is fundamental to their behavior in chemical reactions and their interaction with indicators. 2. **Characteristic Properties of Acids:** - One of the most recognizable properties of acids is their taste. Acids typically have a sour taste; for example, citric acid in lemons and acetic acid in vinegar are both acids that are known for their sour flavor. - Additionally, acids affect pH indicators. A common indicator used in chemistry is litmus paper. When an acid is present, it will turn blue litmus paper red, which is a classic test for acidity. 3. **Why Option B is Correct:** - The statement in option B accurately describes two well-known characteristics of acids: their sour taste and their ability to change the color of litmus paper from blue to red. This is a direct observation that can be made in a laboratory setting, making it a reliable characteristic property of acids. ### Why the Other Options are Incorrect or Weaker: - **Option A: They feel slippery to the touch.** - This statement is actually a characteristic property of bases, not acids. Bases, such as sodium hydroxide, feel slippery because they can react with the oils on your skin to form soap-like substances. Therefore, this option is incorrect. - **Option C: They produce hydroxide ions (OH⁻) in solution.** - This statement is also incorrect for acids. Acids produce hydrogen ions (H⁺) when dissolved in water, not hydroxide ions. Hydroxide ions are characteristic of bases. Therefore, this option misrepresents the fundamental behavior of acids. - **Option D: They react with metals to produce carbon dioxide gas.** - This statement is misleading. While some acids do react with certain metals to produce hydrogen gas (H₂), not carbon dioxide (CO₂). Carbon dioxide is typically produced when acids react with carbonates or bicarbonates, not with metals. Thus, this option does not accurately describe a general property of acids. ### Summary of Key Points: - Acids have a sour taste and turn blue litmus paper red. - They donate protons (H⁺) in solution, which is a defining characteristic. - Bases feel slippery and produce hydroxide ions (OH⁻), distinguishing them from acids. - Acids react with metals to produce hydrogen gas, not carbon dioxide. This thorough understanding of acids and their properties will help you identify them in various chemical contexts and differentiate them from bases and other substances.