The correct option is **C. They turn blue litmus paper red.** ### Detailed Explanation: 1. **Understanding Acids:** - Acids are substances that, when dissolved in water, increase the concentration of hydrogen ions (H⁺) in the solution. This is a fundamental property of acids and is key to understanding their behavior in aqueous solutions. 2. **Analyzing the Options:** - **Option A: They increase the concentration of hydroxide ions (OH⁻).** - This statement is incorrect. Acids do not increase the concentration of hydroxide ions; rather, they increase the concentration of hydrogen ions (H⁺). In fact, when an acid is added to water, it can react with water molecules to produce H⁺ ions, which can lead to a decrease in the concentration of OH⁻ ions due to the water dissociation equilibrium shifting. - **Option B: They react with metals to produce hydrogen gas.** - While this statement is true for many acids (like hydrochloric acid reacting with zinc), it does not universally apply to all acids. Not all acids react with metals to produce hydrogen gas. For example, some acids, like sulfuric acid, may not produce hydrogen gas when reacting with certain metals under specific conditions. Therefore, while this is a characteristic of some acids, it is not a defining characteristic of all acids in aqueous solutions. - **Option C: They turn blue litmus paper red.** - This statement is correct. One of the classic tests for acidity is the use of litmus paper. Blue litmus paper turns red in the presence of an acid due to the increase in H⁺ ions, which causes a change in the color of the dye used in the litmus paper. This is a reliable and straightforward method to identify acidic solutions. - **Option D: They have a pH greater than 7.** - This statement is incorrect. Acids have a pH less than 7. The pH scale ranges from 0 to 14, where a pH of 7 is neutral (pure water). A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic (alkaline) solution. Therefore, this option does not accurately describe the characteristic of acids. 3. **Summary of Why C is Correct:** - The ability of acids to turn blue litmus paper red is a direct result of their chemical nature, specifically their ability to donate protons (H⁺ ions) in solution. This property is a fundamental characteristic of acids and is widely used in laboratory settings to identify acidic solutions. ### Common Pitfalls: - Students often confuse the properties of acids and bases. Remember that acids produce H⁺ ions, while bases produce OH⁻ ions. - It’s important to remember that not all acids react with metals, so generalizing this property can lead to misunderstandings. - The pH scale is crucial for understanding acidity and basicity; always remember that acids have a pH less than 7. ### Revision Summary: - Acids increase the concentration of H⁺ ions in aqueous solutions. - Blue litmus paper turns red in acidic solutions. - Acids typically have a pH less than 7. - Not all acids react with metals to produce hydrogen gas; this is not a universal characteristic.