Acids, bases and salts
Francine Taylor Campbell, Contributor

Students make their way through the gate at Charlie Smith Comprehensive High School in Trench Town, Kingston. - Norman Grindley Photo

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 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

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 form ions, while only a small proportion of ions are 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, e.g. NaCl. If only part of the replaceable hydrogen ions is removed then an acid salt is formed eg NaHCO3.

Question 1.

1. Describe in detail how you would prepare a sample of copper (II) nitrate (Cu(NO3)2. 3H2O) crystals from copper (II) carbonate and nitric acid.

Answer

1. Copper (II) nitrate (Cu(NO3)2 . 3H2O) are hydrated crystals of a soluble salt. (All nitrates are soluble). To prepare this salt, copper (II) carbonate and nitric acid would be reacted together in a reaction flask, where nitric acid would be added gradually from a funnel, on to the copper carbonate crystals. The copper nitrate formed is in solution as it is soluble. The solution would need to be filtered to remove any excess solid then evaporated to get the salt. The copper nitrate must contain water of crystallisation, hence, it will not be evaporated to dryness, but instead to a small bulk and then left to crystallise out of solution. The crystals can then be collected.

* Francine Taylor-Campbell is an independent contributor.