Acids, bases and salts
Francine Taylor-Campbell, Contributor

YOU SHOULD BE ABLE TO:

• Define acid, acid anhydride, base, alkali, salt, acidic, basic, amphoteric and neutral oxides
• Relate acidity and alkalinity to the pH scale
• Discuss the strengths of acids and alkalis on the basis of their completeness of ionization
• Investigate the reactions of non oxidizing acids with metals, carbonates, hydrogen carbonates, bases
• Investigate the reaction of bases with ammonium salts
• Identify an appropriate method of salt preparation based on the solubility of the salt
• Distinguish between acid salts and normal salts

POINTS TO NOTE

• Non-metals form acidic oxides while metals form basic oxides.
• Amphoteric oxides show both basic and acidic properties, while neutral oxides show neither
• Non-metallic oxides dissolve in water to form acids
• Metallic oxides are basic oxides and dissolve in water to form alkalis
• Acids combine with bases to form salt and water only
• Salts can be formed from the reaction of acids with carbonates, hydrogen-carbonates, alkalis and metals
• ? The pH of a substance indicates how acidic or basic (alkaline) its aqueous solution is. The pH scale goes from 0 - 14 with a pH less than 7 described as acidic and a pH greater than 7 described as basic or alkaline. One can use pH paper (litmus) or indicator solutions, such as methyl orange and phenolphthalein, to determine whether substances are alkaline or acidic
• Strong acids and weak acids differ in the number of molecules that react or dissociate to form ions. For a strong acid, a vast majority of the molecules forms ions, while only a small proportion of ions is formed from a weak acid
• Dibasic and tribasic acids can form both normal and acid salts
• Soluble salts can be prepared from the action of an acid on a metal, carbonate and/or base. Insoluble salts are obtained from ionic precipitation reactions between two soluble salt solutions.
  Anhydrous salts must be prepared by direct combination

An acid is a substance which dissolves in water or aqueous solution to produce H+ ions. An acid is formed when an acid anhydride (acidic oxide) dissolves in water.

Some acids are stronger than others. The strength of an acid depends on how completely it ionizes or breaks up into ions when in solution. So the mineral acids such as hydrochloric, sulphuric and nitric acids completely dissociate in solution and are called strong acids.

HCl (aq) —————- H+ (aq) + Cl- (aq)
There are no undissociated molecules.

For weak acids, a large number of their molecules remain undissociated in solution hence ionization is incomplete. Organic acids are generally weak such as ethanoic (acetic) acid.

Alkalis can also be strong or weak. NaOH is a strong alkali, while aqueous ammonia is weak as it is only slightly dissociated in aqueous solution.

A salt is formed when all or part of the hydrogen in an acid is replaced by a metal ion or the ammonium ion. Acids generally consist of a negative ion from an acid and a positive ion from a base or metal. For example, NaCl Na+ is from a metal or base (NaOH) and Cl- is from the acid (HCl). If all the replaceable hydrogen ions are removed, a normal salt is formed, for example, NaCl. If only part of the replaceable hydrogen ions is removed, then an acid salt is formed, for example NaHCO3.

Neutralisation reactions in everyday life

1. Antacids - These are mild alkalis or basic salts used to neutralise the acidity on the stomach which causes indigestion. Examples include calcium and magnesium carbonate, magnesium oxide and hydroxide. These substances are able to neutralise excess stomach acidity by reacting with the acid.

2. During baking, the use of baking powder allows the dough to rise. This happens because baking powder is a carbonate which, when heated, produces carbon dioxide gas which is responsible for raising the dough.

Francine Taylor-Campbell teaches at Jamaica College. Send questions and comments to kerry-ann.hepburn@gleanerjm.com