Which of the following is best described as an

acid-base reaction?
1. NaCl + AgNO3 !NaNO3 + AgCl
2. NH3 + H2O ! NH+
4 + OH−
3. 2HgO ! 2 Hg + O2
4. Ca + 2H2O ! Ca(OH)2 + H2
5. KCl + NH4NO3 ! KNO3 + NH4Cl

#2 for sure.

The best-described acid-base reaction among the given options is option 4: Ca + 2H2O => Ca(OH)2 + H2.

To identify an acid-base reaction, we need to look for the presence of an acid and a base, and observe the formation of a salt and water. In this reaction, we have calcium metal (Ca) reacting with water (H2O) to produce calcium hydroxide (Ca(OH)2) and hydrogen gas (H2).

In this reaction, water acts as the base and accepts a proton (H+), while calcium acts as the acid and donates a proton. This exchange of protons between the acid and the base forms the salt (Ca(OH)2) and water (H2). Hence, this reaction is an acid-base reaction.

To identify an acid-base reaction, it is important to understand the basic concepts of acids and bases and their reactions. Acids are substances that donate protons (H+ ions) while bases are substances that accept protons. The reaction between an acid and a base results in the formation of a salt and water.

If you weren't provided with the options and needed to assess an acid-base reaction, look for substances that contain hydrogen (H) as acids, and substances that contain hydroxide (OH-) or capable of accepting protons as bases. The reaction between any of these acid and base combinations should result in the formation of a salt and water.