9.0     DEFINITION OF ACID

An acid is a substance which produces hydrogen ions (or protons), H⁺ as the only positive ion when dissolved in water. Also simply defined as a H₃O⁺ proton donor. An acid has at least one ionizable hydrogen atom in its molecule. Strong acids completely ionize in water (e.g HCl, HNO₃, H₂SO₄₎, while weak acids partially ionize in water (e.g H₃PO₄, H₂CO₃, CH₃COOH, H₃SO₄ etc.)

There are two classes of acids – organic and mineral/inorganic acids. Organic acids are produced from plants or animal materials while the latter is produced from mineral elements or inorganic matter.

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EXAMPLES OF ORGANIC ACIDS

• Ethanoic acid – from vinegar.
• Lactic acid – from milk.
• Citric acid – from lime, lemon.
• Amino acid – from proteins.
• Fatty acid – from fats and oils.
• Ascorbic acid (vitamin C) – from oranges etc.

EXAMPLES OF INORGANIC ACIDS

• Hydrochloric acid, HCl.
• Tetraoxosulphate (VI) acid, H₂SO_4.
• Trioxonitrate (V) acid, HNO_3.

         9.1     BASICITY OF AN ACID

The basicity of an acid is the number of replaceable hydrogen ions, H⁺, in one molecule of an acid.

ACID	IONS PRODUCED	BASICITY
HCl	H+, Cl –	1 (monobasic)
H2SO4	2H+, SO4 2–	2 (dibasic)
H3PO4	3H+, PO4 3–	3 (tribasic)
CH3COOH	CH3COO –, H+	1 (monobasic)

         9.2     PHYSICAL PROPERTIES OF ACIDS

• It turns blue litmus paper red.
• Dilute acids have sour taste.
• Concentrated forms of acids are corrosive.

         9.3     CHEMICAL PROPERTIES OF ACIDS

• Reacts with metals to liberate hydrogen gas.
• Reacts with insoluble bases and alkalis to yield salt and water only.
• Reacts with trioxocarbonates(IV) to liberate carbon (iv) oxide.

         9.4     METHODS USED FOR THE PREPARATION OF ACIDS

• Dissolution of acid anhydrides in water e.g. carbon (IV) oxide, sulphur (IV) oxide and sulphur (VI) oxides are acid anhydrides for trioxocarbonates (IV) acid, trioxosulphate (IV) acid and tetraoxosulphate (VI) acid respectively.
• Combination of constituent elements.
• Using of strong acids to displace weak/volatile acid from its salt.

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         9.5     USES OF ACIDS

• Used in the removal of rust e.g. HCl.
• Needed by industries for the manufacturing of chemicals, fertilizers, explosives… e.g. HCl, H₂SO₄, HNO₃.
• Used as oxidizing agents.
• Used as various catalyst for chemical reactions.
• Used in the manufacturing of soap e.g. fatty acids.
• Used as an electrolyte in lead-acid accumulators e.g. H₂SO_4.
• Used in preservation of food e.g. tartaric acid. Etc.

         9.6     BASES AND ALKALIS

Base is a substance which will neutralize an acid to yield salt and water only. A soluble base is known as an alkali. They ionize to give a negatively charged hydroxyl ions, OH^–. Like acids, alkalis may be strong or weak. NaOH and KOH are strong alkalis while Ca(OH)₂ and aqueous ammonia are weak alkalis.

         9.7     PHYSICAL PROPERTIES OF ALKALIS

• It turns red litmus blue.
• Alkalis have a bitter taste.
• Alkalis are soapy to the touch.
• Concentrated forms of the two caustic alkalis (i.e. NaOH and KOH) are corrosive.

         9.8     CHEMICAL PROPERTIES OF BASES/ALKALIS

• All bases react with acids to form salts and water only.
• Alkalis reacts with ammonium salts in the presence of heat, to generate ammonia gas. This reaction can be used as a test for alkalis.

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         9.9     USES OF ALKALIS

• They are used in the manufacturing of soap e.g. NaOH, KOH.
• They are used as a bleaching agent and detergent e.g. aqueous ammonia.
• They are used in neutralizing acidic soils e.g Ca(OH)₂.
• Some alkalis are used to soften hard water.
• Alkalis are also used in industries concerned with the manufacturing of glass, mortar, cement, plaster, plastic, toothpaste, rayon etc.

         9.10     SALTS

A salt is a compound formed when all or part of the ionizable hydrogen of an acid is replaced by metallic or ammonium ions.

TYPES OF SALTS

• NORMAL SALTS – Formed when all ionizable hydrogen of an acid have been completely replaced by metallic ions. Examples are NaCl, ZnSO₄, KNO₃ etc.
• ACID SALTS – Formed when the ionizable hydrogen of an acid are only partially replaced by metallic ions. This type of salt cannot be formed by a monobasic acid. Acid salts result when there is an insufficient supply of metallic ions to completely replace the hydrogen ions in the acid. In the presence of excess metallic ions, the remaining hydrogen ions become replaced, converting the acid salt to a normal salt. H₂SO_4­ + KOH = KHSO₄ (acid salt) … + KOH = K₂SO₄ (normal salt) … Acid salt has acidic properties.
• BASIC SALTS – Contains hydroxyl ion, OH ^–. They occur when there is an insufficient supply of acid needed to completely neutralize the base. Basic salts have the properties of a base. It will also react with excess acid to form a normal salt and water:

Ca(OH)₂ + HCl = Ca(OH)Cl (basic salt) … + HCl = CaCl₂ (normal salt) … Other examples of basic salt include Zn(OH)Cl, Mg(OH)NO₃ etc.

• DOUBLE SALTS – These are salts which ionizes to produce three different types of ions in solution; usually two positively charged ions and a negatively charged ion. This series of salt have a general formula: M⁺M³⁺(SO₄)₂. 12H₂O where M⁺may be Na⁺, K⁺or NH₄⁺ and M³⁺ may be a large ion such as Al³⁺. Examples of double salt include potash alum (KAl(SO₄)₂.12H₂O), chrome alum (KCr(SO₄)₂.12H₂O), (NH₄)₂Fe(SO₄)₂.6H₂O etc.
• COMPLEX SALTS – Formed by mixing two simple salts. A complex salt has a central atom, bounded with surrounding ions/molecules (ligands). An example is potassium hexacyanoferrate(II) K₄[Fe(CN₆)].

         9.11     USES OF SALTS

• It is used in the preservation of food and glazing of pottery e.g NaCl.
• It is used for making plaster of Paris (POP) e.g CaSO₄.
• It is used in making gun powder, matches and soil fertilizer e.g KNO₃.
• It is used as a drying agent e.g fused calcium chloride.
• It is used as an electrolyte in dry cells e.g ammonium chloride.
• It is used as an antifreeze e.g. CaCl₂.
• It is used as a laxative e.g MgSO₄.

         9.12     PREPARATION OF SALTS

Soluble salts may be prepared by:

• Action of dilute acid on metal.
• Neutralization of an acid by an alkali.
• Action of dilute acid on an insoluble base.
• Action of dilute acid on trioxocarbonates(IV).

Insoluble salts are formed by:

• Double decomposition.
• Direct combination of constituent elements.

NB – Salts are recovered from their solution by evaporation or crystallization.

         9.13     MEASUREMENT OF ACIDITY AND ALKALINITY

THE PH SCALE – This is a scale numbered from 0 to 14, used to measure the degree of acidity or alkalinity of a substance. A solution with:

• pH value of 7 is neutral i.e, neither acidic nor basic.
• pH value less than 7 is acidic.
• pH value more than 7 is alkaline.

Acidity increases with decreasing pH value, while alkalinity increases with increasing pH value. pH means power of hydrogen. Sorensen defined the pH of a solution as the negative logarithm of the hydrogen ion concentration to the base 10.

pH + pOH = 14                  [H⁺] [OH–] = 10^-14                        pH = -log₁₀[H⁺]

The pH of a solution can be measured using the universal indicator or the pH meter.

IMPORTANCE OF pH

• For our bodies to function properly, the body fluids must be maintained at the correct pH value.
• In our body, acidic medium is required for digestion in the stomach, while alkaline medium is required for digestion in the small intestine.
• In the farm, certain pH values of the soil are required for various plants to grow well.
• pH values are also important in medicine, pharmacy, water purification, sewage treatment and several other industrial processes.

         9.14   BUFFER SOLUTION

A buffer solution is one which resists changes in pH on dilution or addition of small amounts of acids or alkalis. They usually consist of weak acid or base in the presence of one of its salts. Examples include ethanoic acid and sodium ethanoate, aqueous ammonia and ammonium chloride etc.

Buffer solutions are widely used in hospitals, fermentation processes, enzyme reactions, processed food and drinks to prevent excess acidity etc.

         9.15    INDICATORS

Indicators are weak organic acids, bases or dyes which will produce different colours in solution depending on the level of acidity/alkalinity of that solution. Indicators ionize slightly in solutions. The colour change of some indicators are as follows:

INDICATOR	ACIDIC MEDIUM	ALKALINE MEDIUM	NEUTRAL MEDIUM
Methyl orange	Red/orange	Yellow	Any colour
Phenolphthalein	Colourless	Pale pink	Red
Litmus	Red	Blue	Purple

FOR ACID–BASE TITRATION:

Strong acid vs Strong base	Any indicator
Weak acid vs Strong base	Phenolphthalein
Strong acid vs Weak base	Methyl orange
Weak acid vs Weak base	No suitable indicator

NEUTRALIZATION – This is the reaction process between an acid and an appropriate amount of a base/alkalis to form salt and water only. It is also the reaction between the hydrogen ion of an acid, and the hydroxyl ion of a base/alkali to form water molecules (a salt is also formed).

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         9.16     SALT HYDROLYSIS

 Hydrolysis is the reaction of a salt with water to form a solution that is either acidic or basic.

• Normal salt (formed from strong acids and strong bases) dissolves in water to yield a neutral solution.
• Salts formed from strong acids and weak bases hydrolyses in water to form acidic solutions.
• Salts formed from weak acids and strong bases hydrolyses in water to form alkaline solutions.