Acids are chemical compounds defined by their behavior in an aqueous environment. When dissolved in water, these substances release positively charged hydrogen ions (\(\text{H}^+\)) into the solution. This release of hydrogen ions makes a substance acidic and gives it a capacity for chemical reaction. Acids seek stability by reacting with other compounds that can accept the \(\text{H}^+\) ions or exchange them for other atoms. This reaction results in the formation of new, less reactive substances.
Substances That Neutralize Acids
One of the most common reactions involving an acid is neutralization, which occurs when an acid reacts with a base. A base is a substance that accepts the \(\text{H}^+\) ions donated by the acid. Strong bases often contain hydroxide ions (\(\text{OH}^-\)), or they may be metal oxides, which readily react with the hydrogen ions.
In a classic neutralization reaction, the acid and base react to form two primary products: a salt and water. The hydrogen ion from the acid combines with the hydroxide ion from the base to form a molecule of water (\(\text{H}_2\text{O}\)). The remaining ions, the cation from the base and the anion from the acid, combine to form the resulting salt.
This process effectively eliminates the acidic properties of the solution, which is why the reaction is termed neutralization. The \(\text{pH}\) of the final solution moves closer to a neutral value of seven, though the exact \(\text{pH}\) depends on the relative strengths of the initial acid and base. A practical application of this chemistry is the use of antacids, which contain weak bases like aluminum or magnesium hydroxide, to neutralize excess stomach acid.
Substances That Produce Hydrogen Gas
Acids also react vigorously with certain metals in a process known as a single displacement reaction. This reaction involves an active metal replacing the hydrogen within the acid compound. The products of this exchange are a salt and the release of hydrogen gas (\(\text{H}_2\)).
For this reaction to proceed, the metal must be more chemically reactive than hydrogen, a concept that is determined by its position on the reactivity series. Metals like zinc or magnesium are sufficiently active to donate electrons to the hydrogen ions in the acid. When magnesium metal is dropped into hydrochloric acid, for example, it yields magnesium chloride salt and bubbles of hydrogen gas.
The defining feature of this reaction is the evolution of flammable hydrogen gas, observed as effervescence. Metals such as gold or platinum, which are less reactive than hydrogen, do not participate in this type of reaction. This difference in reactivity explains why certain metals are more susceptible to corrosion from acids than others.
Substances That Produce Carbon Dioxide Gas
A distinct reaction occurs when acids contact substances containing carbonate (\(\text{CO}_3^{2-}\)) or bicarbonate (\(\text{HCO}_3^-\)) ions. In this case, the acid reacts to produce three products: a salt, water, and carbon dioxide gas (\(\text{CO}_2\)). The initial reaction forms a temporary, unstable carbonic acid, which immediately decomposes into water and carbon dioxide.
This decomposition is visible as rapid bubbling or fizzing, known as effervescence. This reaction is commonly seen when mixing vinegar (a weak acid) with baking soda (sodium bicarbonate). The resulting foam is the escaping carbon dioxide gas.
The reaction is the chemical basis for how acid rain erodes buildings and statues made of limestone, which is primarily calcium carbonate. In a laboratory setting, this reaction is used to test for the presence of carbonates by observing the release of carbon dioxide. The production of carbon dioxide gas makes this reaction distinct from a simple acid-base neutralization that forms only a salt and water.