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Definition: An acid is substance which in aqueous solution produces hydroxonium ion (H3O+) or hydrogen ion (H+) as the only positive ion. Also, acids can be referred to as proton donor.



There are two classes of acids:

(1). Organic acids occur as natural products in plants and animal material.

Organic acids Source
Ethanoic acids Vinegar
Lactic acids Milk
Citric acids Lime, Lemon
Amino acids Proteins
Fatty acids Fats and oils
Ascorbic acids (Vitamin C). Oranges


(2). Inorganic acid: Inorganic acid can be prepared from mineral elements or inorganic matter.

Inorganic acid Formula Constituents
Hydrochloric acid HCl Hydrogen, Chlorine
Tetraoxosulphate(VI) acid H2SO4 Hydrogen, Sulphur & Oxygen
Trioxonitrate(VI) acid HNO3 Hydrogen, Nitrogen & Oxygen

An acid is also defined as a substance which produces hydroxonium ion as the only positive ion when dissolved in water.

H+(aq)    +    H2O(l)       H3O+(aq)

Acid can be dilute or concentrated depending on the amount of water added. A dilute acid is acid produced when a large amount of water is added to a small amount of acid. A concentrated acid is acid produced when only a little amount of water is added to a relatively large amount of acid.



The strength of an acid can either be weak or strong.

(1) Strong acids: are acids which ionize completely in aqueous solution and such acid solution having a high concentration of H+. Examples are HCl, H2SO4 and HNO3.

H2SO4       2H+ + SO42-

HNO3                          H+ + NO3

HCl            H+ + Cl


(2) Weak acids: are acids which ionize or dissociate slightly or partially in aqueous solution and such acid solution have a low concentration of hydrogen ions. Examples are ethanoic acid (CH3COOH), H2CO3, H3PO4, H2SO3.


H2CO3                       2H+        +   CO32-

H3PO4                       3H+        +   PO43-

CH3COOH                     H+         +   CH3COO

H2SO3                     2H+         +   SO32-



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

Acid Basicity
Hydrochloric   acid Monobasic
Tetraoxosulphate (vi) acid Dibasic
Tetraoxophosphate(v) acid Tribasic
Ethanoic acid Monobasic



  1. Define the term acid
  2. Differentiate between strong acid and concentrated acid
  3. What is the basicity of the following acids: HCl, HNO3, H2SO4



  1. They have a sour taste.
  2. They turn blue litmus paper to red.
  3. They are corrosive in nature especially the strong acid.
  4. In aqueous solution, they conduct electricity.



Reaction with metals: They react with metals to liberate hydrogen gas and salt of metal i.e

Acid       +         Metal                         Salt           + Hydrogen gas.

E.g.  2HCl(aq)   +   Zn(s)                      ZnCl2(aq)     +    H2(g)

H2SO4(aq)  +   Mg(s)                     MgSO4(aq)    +    H2(g)


They react with soluble bases to form salt and water only. This reaction is known as neutralization.

Acid     +         Base              salt        +      water

E.g  H2SO4(aq)     +      2KOH(aq)           K2SO4(aq)   +    2H2O(l)

2HCl(aq)     +       CaO(s)            CaCl2(aq)      +      H2O(l)


  1. They react with trioxocarbonates (iv) salts to liberate carbon (iv) oxide, salt and water  e. Acid   +  trioxocarbonate (iv)            Salt  +     Water   +    CO2

E.g  2HCl (aq) +    Na2CO3(aq)                   2NaCl(aq)  +      H2O(l)  +  CO2(g)



Acid can be prepared by using the following methods:

  1. Dissolving an acid anhydride in water: Acid anhydride is oxides of non-metal that dissolve in water to produce the corresponding acids e.g SO2, CO2, CO, NO2, SO3.

SO2(g)      +        H2O(l)              H2SO3(aq)

CO2(g)    +        H2O(l)              H2CO3(aq)

SO3(g)    +        H2O(l)              H2SO4(aq)


  1. Combination of constituent elements.

(a).Burning hydrogen in chlorine, in the presence of activated charcoal as the catalyst, yields HCl gas which dissolves readily in water to give HCl acid.

H2(g)     +         Cl2(g)            activated charcoal       2HCl(g)

(b)Heating hydrogen gas and bromine vapour, in the presence of platinum as the catalyst, produces hydrogen bromide which dissolves readily in water to form hydrobromic acid.

H2(g)         +          Br2(g)                  Platinum         2HBr(g)

(3)By displacement of a weak or more volatile acid from it salt by a stronger or less

volatile acid. For example

(a)Displacement of the more volatile hydrogen chloride from metallic chloride by the less volatile concentrated tetraoxosulphate (vi) acid.

NaCl(s)   +  H2SO4(aq)     NaHSO4(aq)        +         HCl(aq)


(b)Displacement of weaker trioxoborate (iii) acid from ‘borax’ by tetraoxosulphate (vi) acid.

Na2B4O7(s) +  H2SO4(aq)    +   5H2O(l)      Na2SO4(aq)    +    4H3BO3(aq)

Borax                                            Trioxoborate (iii) acid

(4)By precipitating an insoluble sulphide from a metallic salt by hydrogen sulphide

Pb (CH3COO)2(aq)    +  H2S(g)         PbS(s)    + CH3COOH(aq)


Uses of acid

(1) Acids are useful chemicals which are used in many industries to make other consumer chemicals such as fertilizers, detergent and drugs.

(2) They are used in industrial process as drying agents, oxidizing agents and catalysts.



Name Uses
HCl Needed by industries to make chemicals used to remove rust.

Used to clean the surface of metals before electroplating.

H2SO4 Needed by industries to make chemicals used as a drying and dehydrating agent.

Used as an electrolyte in lead-acid accumulators

Required in oil refineries.

HNO3 Needed by industries for making fertilizers, explosives etc.
Boric acid Used as mild antiseptic or germicide.
Tartaric acid Used in making baking soda, soft drinks and health salts
Acetic acid (ethanoic acid) Used in preserving food.

Used in dyeing silk and other textiles.

Citric acid Used in making fruits juice.
Fatty acid (palmitic and stearic acid) Used in the manufacture of soap. This process is known as saponification.

Fatty acid + Caustic soda     Soap   +     H2O.


  1. Mention three physical properties of acids
  2. Using balanced equations, state the chemical properties of acids
  3. State two methods of preparing acids
  4. Outline the uses of acids



All acidic solution contains H+ and all alkaline solution contains OH ions. The PH scale measure the concentration of H+ ions present in a solution and start from 0 to 14



pH is defined as the negative logarithms of the hydrogen ion [H+] concentration to the base of 10.

i.e. pH = -log [H+].

Thus: If [H+] = 0.00001 or 10-5.

log [H+] = log10-5 = -5

pH= -log [H+] = – (-5) = 5.

If [H+] =10-x

Therefore, pH= -log10-x = – (-x) = x

If [H+] = 10-2, PH = 2



POH is defined as the negative logarithms of the hydroxide ion [OH] concentration to the base of 10.

i.e. pOH= -log [OH].

PH is the degree of acidity. A solution with PH 7 is neutral. A solution with PH less than 7, i.e. PH 6,5,4, e.t.c, indicate acidity increasing as the numbers decreases. A solution with PH greater than 7, i.e. PH 8,9,10, e.t.c, indicate alkalinity increasing as the numbers increase.

PH 1    2          3          4          5          6          7          8          9          10        11        12        13        14

Increasing acidity       Neutral     Increasing alkalinity


A solution with PH 1 is very acidic [with high concentration of H+]. A solution with pH 13 is very alkaline [with low concentration of H+, but high concentration of OH].

Note that: If pH is 1, it has concentration of H+ 10 times greater than pH 2 and 100 times greater than PH 3 e.t.c.

pH 1 > pH 2 > pH 3.

Concentration of H+    10-1      10-2      10-3.

0.1       0.01     0.001.


Relationship between pH and pOH

H2O          H+    +     OH

From conductivity measurement, [H+]=10-7moldm-3, [OH]=10-7moldm-3.

[H+] [OH] = Kw=10-7 x 10-7=10-14mol2dm-6.

Taking logarithm of both sides

log ([H+] [OH]) = logKw

log [H+] + log[OH] =logKw

Subtracting both sides

-(log[H+]  +  [OH]) = -logKw

-log [H+] – log[OH] = -logKw

-log [H+] + (-log [OH]) = -logKw

pH + pOH = PKw

pKw = -log10-14 = -(-14) = 14

Therefore, pH + pOH = 14.


Worked examples

  1. Find the hydrogen and hydroxide ion concentrations in

(a) 0.01moldm-3 tetraoxosulphate (vi) acid solution.

(b) 0.001moldm-3 potassium hydroxide solution.



(a).  H2SO4(aq)             2H+(aq)  +  SO42-(aq)


From the equation,  1 moldm-3 H2SO4 ionizes to give 2moldm-3 H+

Therefore, 0.01moldm-3 H2SO4 would ionize to give (2×0.01) moldm-3 H+

[H+] = 2×10-2moldm-3

[H+] [OH] = 10-14

(2×10-2) [OH] = 10-14

[OH] =    10-14

2x 10-2

[OH] = 0.5x (10-14- -2)

[OH] =0.5 x10-14+2

[OH] =0.5×10-12moldm-3.

(b).     KOH(aq)                              K+(aq)  +     OH(aq)

From the equation,

1moldm-3 of KOH ionizes to give 1moldm-3 of OH

10-3moldm-3 of KOH would ionize to give 10-3moldm-3 of OH


[H+] [OH]=10-14

[H+] (10-3) = 10-14.

[H+] = 10-14


[H+] = 10-14+3

[H+] = 10-11moldm-3


  1. A glass cup of orange juice is found to have a POH of 11.40. Calculate the concentration of the hydrogen ions in the juice.


pH + pOH = 14.

pH = 14 – 11.4.

pH = 2.6.

pH = -log [H+]

2.6 =-log [H+].

[H+] = Antilog (-2.6)

[H+] = 0.0025moldm-3

[H+] = 2.5×10-3moldm-3.


Measuring pH of a solution

We use pH meter and a universal indicator to detect PH of a solution.

Universal indicator is a mixture of indicator and can change to several colours corresponding to a particular PH and compared with the standard colour provided by the manufacturer of the universal indicator. Universal indicator measures PH between 3 and 11.


Put 10cm3 of test solution in a test tube, add 2 drops of universal indicator and compare with the colour chart or place 2 drops of test solution on universal indicator paper and compare the colour with the chart.



  1. Define the term pH.
  2. What is the pH of a solution having hydrogen ion concentration of 6x 10-9 mol/dm3



  1. Give the chemical formula of the following acids (a) Tetraoxosulphate (vi) acid

(b) Trioxonitrate (v) acid (c) Oxochlorate (i) acid

  1. What is the IUPAC nomenclature of the following (a) HNO2 (b) HOBr (c) H3PO4 (d) H2S
  2. Determine the oxidation number of Cl and C in each of the following (a) KClO3

(b) HOCl (c) H2CO3 (d) CO2

  1. Mention the laboratory apparatus that are used in for an acid-base titration

What can be used to determine the acidity or alkalinity of a solution?



  1. The following acids are monobasic except (a) HNO2 (b) HBr (c) HOCl (d) H2SO3
  2. Which of the following ions is acidic? (a) K+ (b) NO3 (c) S2-   (d) H3O+.
  3. The number of hydroxonium ions produced by one molecule of an acid in aqueous solution is it (a) acidity (b) basicity (c) concentration (d) pH.
  4. The basicity of ethanoic acid CH3COOH is: (a) 0 (b) 1 (c) 2 (d) 3
  5. A solution with pH 7 is (a) Acidic (b) dilute (c) neutral   (d) saturated



  1. 1. What is (i) an acid (ii) basicity of an acid?
  2. What is the basicity of tetraoxophosphate (V) acid.
  3. The concentrations of H+ in two solutions are (a) 1 x10-4moldm-3and (b) 5 x 10-9moldm-3 . What is the PH of each solution?


See also






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