Question

which pair of substances would most likely result in the production of a solid when reacting with a base?
litmus and phenolphthalein
soluble metal salt and carbon dioxide
acids and water
hydrogen ions and soaps

Explanation:

• Litmus and phenolphthalein are indicators, they change color with base, no solid production.
• Soluble metal salt reacts with base to form metal hydroxide (often solid, e.g., $\ce{FeCl_{3} + 3NaOH -> Fe(OH)_{3}(s) + 3NaCl}$), and $\ce{CO_{2}}$ reacts with base to form carbonate (some metal carbonates are solid, e.g., $\ce{CO_{2} + Ca(OH)_{2} -> CaCO_{3}(s) + H_{2}O}$).
• Acids react with base to form salt and water, no solid (unless salt is insoluble, but acids + water with base is not the right context).
• Hydrogen ions react with base to form water, soaps react with base in different ways, no solid production here.

Answer:

B. soluble metal salt and carbon dioxide