Acids and bases and Salts
IGCSE Chemistry
Acids and Bases
Acid is a substance which gives out hydrogen ions (H+) when mixed in water. Base is a
substance which gives out hydroxide ions (OH-) when mixed in water.
Hydrogen ion is same as a Proton. So Acid is a proton donor. Base accept the proton (Base is a
proton acceptor)
Reactions of acids


Acids react with metals to form salt and hydrogen gas
Magnesium + Hydrochloric acid  Magnesium chloride + Hydrogen
(magnesium
chloride is the salt)
Mg + 2HCl  MgCl2 + H2
Name of the salt depends on the acid. First part of the salt is the metal or ammonium
(NH4+)
Hydrochloric acid  chloride ClSulphuric acid  sulphate
SO42Nitric acid  nitrate
NO3Carbonic acid  carbonate
CO32Acids react with bases to form salt and water. This reaction is called Neutralisation.
Most metal oxides and Metal hydroxides are bases. Ammonia solution (NH4OH,
Ammonium hydroxide) is also a base.
Sodium hydroxide + hydrochloric acid  sodium chloride + water
NaOH + HCl  NaCl + H2O
Calcium oxide + nitric acid  Calcium nitrate + water
CaO + 2HNO3  Ca(NO3)2 + H2O
 Acids react with carbonates to form salt, carbon dioxide and water.
Sulphuric acid + Zinc carbonate  Zinc sulphate + Carbon dioxide + water
H2SO4 + ZnCO3  ZnSO4 + CO2 + H2O
Acids give red colour to litmus (red colour to universal indicator too)
Bases give blue colour to litmus (purple colour to universal indicator)
Bases give salt and water when reacting with acids (Neutralisation). Metal oxides and hydroxides
are bases.
Alkalis are bases which can dissolve in water.
Test for ammonium compounds: (important)
Alkalis (like NaOH) when warmed with ammonium salts, ammonia gas will be given off.
NH4Cl
+
Ammonium chloride
NaOH

Sodium hydroxide
NaCl
+
Sodium chloride
NH3
+
Ammonia
Ammonia gas is an alkaline gas which turns litmus colour to blue.
H2O
Water
pH
is a measurement to show how strong an acid or a base is.
Lower the pH, stronger the acid and higher the pH, stronger the base.
pH below 7  acids, pH 7  neutral and pH above 7  alkaline
pH 0,1,2 are for strong acid and 3-7 is weak acids. pH 14,13 are for strong bases and 7-12
weaker bases.
pH 7 is neutral.
Universal indicator gives different colours for different pH
(strong acid) Red orange yellow green(neutral)  light blue  deep blue  purple
(strong base)
As pH indicator or universal indicator gives different colours at different pH, we can predict
the solution is strongly acidic, weak acidic, weak basic or strong alkaline. So Universal
indicator is a better indicator than many other indicators.
A pH meter can be also used in the place of pH indicator which shows the pH of a solution
digitally.
Strong and weak acids and bases
Strong acids and bases ionise completely when mixed with water.
Strong acids give out maximum hydrogen ions (H+) and strong base give out maximum hydroxide
ions (OH-)
HCl, H2SO4 and HNO3 are strong acids
HCl  H+ +
Cl-
H2SO4  2H+ + SO42NaOH, KOH and Ca(OH)2 are strong bases
NaOH  Na+ + OHWeak acids and bases ionise partially with water. So weak acids contain smaller number of
Hydrogen ions (H+) and weak bases contain smaller number of hydroxide ions (OH-).
Ethanoic acid, citric acid, carbonic acid etc., are weak acids
CH3-COOH
H2CO3
CH3-COO- + H+ (ethanoic acid)
CO32- +
2H+ (Carbonic acid )
Ammonia solution, copper hydroxide, Iron (II) oxide etc., are weak bases
NH4OH
NH4+
+ OH- (ammonia solution)




Salts
In a reaction acids give out hydrogen ions, H+ (Hydrogen ion is formed after hydrogen atom
loses an electron. It is left with only a proton). So acid is a proton donor.
HCl  Cl- + H+
In a reaction, a base accepts the hydrogen ion (proton).
NaOH + H+  Na+ + H2O
Weak bases such as Calcium oxide (Lime) or calcium hydroxide (slaked lime) are used to
remove soil and water acidity.
Magnesium hydroxide (weak base) is used to remove stomach acidity.
IGCSE Chemistry
A salt has two parts. First part is normally a metal ion or ammonium ion which is positive. Second
part comes from the acid. This part depends on the acid and is negative ion.
Hydrochloric acid HCl
makes chloride ion Cl-
Sulphuric acid H2SO4
makes sulphate ion SO42-
Nitric acid HNO3
makes nitrate ion NO3-
Carbonic acid H2CO3
makes carbonate ion CO32-
Phosphoric acid H3PO4
makes phosphate ion PO43-
Example:
Making salts
You can make salts by reacting metals, insoluble bases, or soluble bases (alkalis) with acids.
A fourth method is precipitation (Double Decomposition)
1. Acid with metals:
Example: To make Zinc sulphate salt
1. Take sulphuric acid in a beaker. Add excess zinc to the sulphuric acid. Warm the mixture.
Zn(s) + H2SO4(aq)  ZnSO4(aq) + H2(g)
Hydrogen bubbles are given off. Bubbling stops when all the acid is used up. Reaction ends.
2. Excess zinc is removed by filtration. This leaves an aqueous solution of zinc sulphate.
Excess zinc will help for the complete reaction of the acid so that the filtrate at the end is
without acid.
3. The solution is heated in an evaporating dish to evaporate water until crystallisation point.
Then it is left to cool in room temperature and crystals of zinc sulphate start to form. Filter out
the salt crystals, wash and dry using filter paper or in an oven.
2. Acid with soluble base (alkali):
Example: To make Potassium chloride salt(soluble salt) by using Potassium hydroxide soluble
in water) and Hydrochloric acid
KOH(aq) + HCl(aq)  KCl(aq) + H2O(l)
1. Conduct a titration of alkali (Potassium hydroxide) with acid (Hydrochloric acid) using an
indicator. Phenolphthalein or Methyl orange indicator is used in titration)
2. Add the acid from a burette to the alkali containing the indicator in the conical flask, just a little
at a time. Mix the contents of acid and alkali.
3. When the indicator shows a colour change, stop adding acid.
(Phenolphthalein pink to colourless; Methyl orange  orange to red)
4. Find how much acid was used.
5. Now mix the acid and alkali in the same ratio by volume without the indicator for complete
neutralisation.. This is because the indicator will make the salt impure.
6. Heat and evaporate the solution in an evaporating dish until crystallisation point. Leave it in
room conditions for the complete crystallisation. Filter out the the salt crystals, wash and dry using
filter paper or in an oven.
3. Acid with insoluble base
Example: Making copper(II) sulphate crystals
CuO(s) + H2SO4(aq)  CuSO4(aq) + H2O(l)
1. Take sulphuric acid in a beaker.
2. Add excess copper (II) oxide to the acid. (Copper(II) oxide is a black powder). Warm the
mixture. Excess copper(II)oxide will help for the complete reaction of the acid so that the
filtrate at the end is pure without acid. During the reaction, we see the solution turns the
colour to blue.
3. Continue adding the base until black copper(II) oxide no more dissolves. The reaction
finishes.
4. Remove the unreacted copper(II) oxide by filtration.
5. Heat and evaporate the filtrate until crystallisation point.
6. Leave the content at room temperature to form crystals. Filter out the crystals, wash and
dry using filter paper or in an oven.
4. Precipitation (Double Decomposition)
Insoluble salts can be made by precipitation. This method includes mixing two soluble salt
solutions to form the insoluble salt.
Example: Preparing barium sulphate
K2SO4(aq) + BaCl2(aq)  BaSO4(s) + 2KCl(aq)
Barium sulphate is an insoluble salt. You can make Barium sulphate, mix solutions of barium
chloride (soluble) and Potassium sulphate (soluble).
1. Make solutions of barium chloride and potassium sulphate separately in two different beakers.
2. Mix them. A white precipitate of barium sulphate forms at once.
3. Filter the mixture. The precipitate is trapped in the filter paper.
5. Wash the residue and dry it in the filter paper in room temperature or in an oven.
Soluble and insoluble compounds
How do I know which substances dissolve in water????






All compounds start with sodium, potassium and ammonium will dissolve in water
All compounds contain nitrate will dissolve in water.
All sulphate compounds dissolve except Barium sulphate and Lead sulphate.
All chloride compounds dissolve except Silver chloride and Lead chloride.
All carbonates are insoluble except sodium carbonate, Potassium carbonate and
Ammonium carbonate.
Oxides and hydroxides of Sodium, Potassium and Calcium are soluble. Others are
insoluble.