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

The correct answer is soluble metal salt and carbon dioxide.

Explanation:

Both the carbon dioxide and the soluble metal salt may react with a base to produce a solid or what is known as a precipitate. An example of soluble metal salt like calcium nitrate when reacts with a base like sodium hydroxide, leads to the generation of solid calcium hydroxide and sodium nitrate, the reaction is known as the precipitation reaction.  

Ca(NO₂)₂ (aq) + NaOH (aq) ⇒ Ca(OH)₂ (s) + NaNO₃ (aq)

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