Acids, bases and salts (1)
Francine Taylor-Campbell, Contributor

How are acids, bases and salts formed? 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.

How is acidity or alkalinity tested?

pH scale

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 seven described as acidic, and a pH greater than seven 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.

What determines the strength of an acid?

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 ionises or breaks up into ions when in solution. For a strong acid, the molecules form ions while only a small proportion of ions is formed from a weak acid.

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) (no undissociated molecules).

Undissociated

For weak acids, their molecules remain undissociated in solution, hence ionisation 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.

How are salts prepared in the lab?

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.

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 a part of the replaceable hydrogen ions is removed, then an acid salt is formed, for example NaHCO3. Dibasic and tribasic acids can form both normal and acid salts.

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.

Students at William Knibb High School watch keenly as the school's science team explains how to create power from water, during the Jamaica Public Service Science and Technology Expo held in association with the Scientific Research Council and the Association of Science Teachers of Jamaica. - Contributed

Francine Taylor-Campbell is an independent contributor.