Chemical changes Study Guide

When preparing soluble salts from an acid and an insoluble reactant (like a metal oxide, hydroxide, or carbonate), a specific method is employed to ensure purity. First, an excess of the insoluble reactant is added to the acid. This is crucial because it ensures that all the acid reacts completely, leaving no unreacted acid in the final solution. If acid were left over, it would contaminate the salt product. After the reaction, the excess insoluble reactant, which has not dissolved, is removed, typically by filtration. This step separates the solid unreacted material from the aqueous solution containing the dissolved salt and water. Finally, the remaining solution contains only the soluble salt dissolved in water, ready for crystallisation or further purification. If precise neutralisation were attempted using indicators, the indicator itself would contaminate the salt, making this method unsuitable for preparing pure salts.

Aqueous solutions of acids react with a variety of substances, typically yielding a salt and other products. The common feature in all these reactions is the replacement of hydrogen ions from the acid with a metal ion to form a salt. With metals, excluding those less reactive than hydrogen, acids produce a salt and hydrogen gas. For example, hydrochloric acid reacts with magnesium to form magnesium chloride and hydrogen. Metal oxides and metal hydroxides (bases) neutralise acids to form a salt and water; these are classic neutralisation reactions. Metal carbonates react with acids to produce a salt, water, and carbon dioxide gas, which is often observed as effervescence. The specific salt formed depends on the acid used and the metal involved in the base or metal reactant.