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Reactions Between Acids and Metals or Bases Makes Ionic Compounds

An acidic solution can be neutralized by reacting it with a base, a metal or a carbonate. The products of these neutralization reactions are called salts.

Salt, also known as sodium chloride, is a dietary mineral. Salts, on the other hand, are ionic compounds consisting of a metal cation (positively-charged ion) bonded to a simple or polyatomic anion (negatively-charged ion). High school chemistry courses typically require students to prepare metal salts in the laboratory by mixing various dilute acids with bases or certain metals.

An acid may be defined as any substance that dissociates and donates a hydrogen ion (H+) to a base. A base may be defined as any substance that contains a hydroxide ion (OH-) or produces it in solution (by reacting with water, for example). Not all bases are soluble in water; those that are soluble are called alkalis.

Laboratory Methods for Preparing Soluble Metal Salts

In high school laboratory settings, dilute acids should be used to ensure student safety. There are 4 standard methods for the preparation of metal salts:

Mixing an acid with a base
Mixing an acid with a metal
Mixing an acid with a metal oxide
Mixing an acid with a carbonate

Because the salts are soluble, it is necessary to gently heat the resultant solution to drive off the water. Eventually, the solution will become saturated and the metal salt will crystallize.

Reaction of an Acid with a Base

Sodium hydroxide (NaOH) is an example of an alkali, that is, a soluble base. Combining an aqueous solution of sodium hydroxide with dilute hydrochloric acid neutralizes the acid:

HCl(aq) + NaOH(aq) --> NaCl(aq) + H2O(l)

The hydrogen ion from the acid combines with the hydroxide ion from the base to produce water, and the cation from the base combines with the anion from the acid to produce the salt. The reaction seems unremarkable as the production of gas bubbles observed in some of the following reactions does not occur here. Students may observe some small degree of immiscibility, apparent as “squiggly” lines in the solution. The product of the reaction of this strong acid and strong base is simple salt water.

Reaction of an Acid with a Metal

In school laboratories it is necessary to use less reactive metals such as magnesium (Mg), aluminum (Al), zinc (Zn), iron (Fe) and tin (Sn) as a safety precaution. As an example, consider the addition of iron filings to a test tube containing sulfuric acid. The reaction equation can be written as follows:

H2SO4(aq) + Fe(s) --> FeSO4(aq) + H2(g)

Bubbles of hydrogen gas will be observed as the reaction proceeds. The test to confirm that hydrogen is produced is to place a lit splint near the mouth of the test tube. A popping sound will be heard.

Reaction of an Acid with a Metal Oxide

Metal oxides also are bases. They may be soluble or insoluble, depending on the cation species. Water-soluble oxides (oxides of alkali and alkaline earth metals) form metal hydroxides in solution. Oxides of transition metals generally are insoluble. Consider the following reaction between insoluble copper oxide and sulfuric acid:

H2SO4(aq) + CuO(s) --> CuSO4(aq) + H2O(l)

The reactant copper oxide will slowly dissolve in the acid. Because the product copper sulphate is dissolved, the solution will have a characteristic blue colour.

Reaction of an Acid with a Carbonate

Carbonates are also considered to be bases, even though they do not contain OH- groups. In any case, the reaction between an acid and a carbonate produces an obvious reaction in which carbon dioxide (CO2) gas is produced. For example:

2HCl(aq) + CaCO3(s) --> CaCl2(aq) + CO2(g) + H2O(l)

The test for CO2 involves bubbling the unknown gas through limewater (a saturated solution of calcium hydroxide). If the solution turns milky, the gas is carbon dioxide.

Summary of Acid-Neutralizing Reactions

To summarize, there are 4 standard reactions that neutralize acids and produce metal salts:

Acid + base --> a salt + water
Acid + metal --> a salt + hydrogen gas
Acid + metal oxide --> a salt + water
Acid + carbonate --> a salt + carbon dioxide gas + water

The common factor amongst these reactions is that they all produce a salt dissolved in water. Two of the reactions also produce gas which bubbles out of the solution.

Adding iron filings to test tube with acid