INTRODUCTION TO ACIDS AND BASES

1. Introduction to acids and bases

ACIDS tend to have a sour taste (vinegar, for example, contains ethanoic acid, and lemon juice contains citric acid). By the way, tasting unknown chemicals in order to try and identify them is an extremely dangerous practice, and should NEVER be carried out!

Acids which are commonly found in the laboratory are hydrochloric acid, HCl (still sometimes called by its ancient name "spirits of salts"), nitric acid, HNO3, and sulphuric acid, H2SO4 (still sometimes commonly called "oil of vitriol" in some parts of the world. It is also called "hydrogen sulphate".)

Many ALKALIS (BASES) have a soapy feel. Beware of strong alkalis (which are water-soluble bases), such as sodium hydroxide, as these substances can cause serious damage to the skin. Bases which are commonly found in the laboratory are sodium hydroxide (caustic soda), NaOH, potassium hydroxide (caustic potash), KOH, and ammonium hydroxide (an aqueous solution of the gas ammonia, NH3). Metal carbonates are also bases.

Generally, oxides of metals react with acids, and are classified as BASIC OXIDES (examples: Na2O, CuO, Fe2O3). Many oxides of non-metals tend to react with bases, and these are called ACIDIC OXIDES (examples: CO2, SO2, SO3)

Concentrated acids such as hydrochloric acid, nitric acid and sulphuric acid, and alkalis such as sodium hydroxide, potassium hydroxide and ammonia solutions are dangeous chemicals and should be handled with caution.

2. Reactions of acids

1. Acids react with metal carbonates to form carbon dioxide gas, CO2:

Carbon dioxide, CO2, can be identified by the fact that it will cause a solution of calcium hydroxide ("lime water"), Ca(OH)2, to become cloudy. This is due to the formation of an insoluble precipitate of calcium carbonate, CaCO3:

2. Many acids will react with reactive metals such as magnesium, zinc and aluminium to form hydrogen gas and a salt of the metal:

3. Acids and bases react together to form SALTS and water only. Metallic oxides, such as copper(II) oxide, CuO, (the so-called BASIC OXIDES, which are actually bases) react with acids to form salts.


3. Reactions of bases

Water soluble bases are also called ALKALIS. Most metal oxides are also basic.

1. Bases react with acids to form salts and water. The process is called NEUTRALIZATION. Carbonates, which are also basic, also form carbon dioxide and salts when they react with acids.

2. Both acids and alkalis cause colour changes in certain substances called INDICATORS, such as litmus:

4. Acid-base reactions

Acid-base reactions are reactions in which an acid and a base react with one another. From a Grade 11 standpoint, we can include among these:

5. The Arrhenius theory

The reactions of acids and bases can be understood in terms of a theory proposed in 1887 by the Swedish chemist Svante Arrhenius. In terms of this theory, acids dissolve in water to produce hydrogen ions, H+ (more correctly, the hydronium ion H3O+), while bases produce hydroxide, OH- ions. Reactions between acids and bases then are essentially a reaction between these two ions:

This theory, while essentially correct, has been superseded by the Lowry-Brønsted theory.

6. Additional questions