Acids, Bases and Salts
Class 10 ScienceAcids are sour in taste and change the colour of blue litmus to red, whereas, bases are bitter and change the colour of the red litmus to blue.
Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases. Acidic nature of a substance is due to the formation of H+ (aq) ions in solution. Formation of OH– (aq) ions in solution is responsible for the basic nature of a substance.
Reaction of Acids with Metals
When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed.
Acid + Metal → Salt + Hydrogen gas
The metal displaces hydrogen atoms from the acids as hydrogen gas and forms a compound called a salt.
(i) Reaction of dilute sulphuric acid and zinc
$$ \ce{Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)} $$
Reaction of Bases with Metals
When a base reacts with a metal, along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen.
Hydrogen is formed in the reaction. However, such reactions are not possible with all metals.
$$ \ce{2NaOH(aq) + Zn(s) -> Na2ZnO2(s) + H2(g)} $$
Metal Carbonates and Metal Hydrogencarbonates
When an acid reacts with a metal carbonate or metal hydrogen carbonate, it gives the corresponding salt, carbon dioxide gas and water.
Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
$$ \ce{Na2CO3(s) + 2HCl(aq) -> 2NaCl(aq) + H2O(l) + CO2(g)} $$
$$ \ce{NaHCO3(s) + HCl(aq) -> NaCl(aq) + H2O(l) + CO2(g)} $$
On passing the carbon dioxide gas evolved through lime water,
$$ \ce{Ca(OH)2(aq) + CO2(g) -> CaCO3(s) + H2O(l)} $$
On passing excess carbon dioxide,
$$ \ce{CaCO3(s) + H2O(l) + CO2(g) -> Ca(HCO3)2(aq)} $$
Limestone, chalk and marble are different forms of calcium carbonate. All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water.
Reaction of Acids and Bases
The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.
Base + Acid → Salt + Water
$$ \ce{NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l)} $$
Reaction of Metallic Oxides with Acids
The general reaction between a metal oxide and an acid can be written as:
Metal oxide + Acid → Salt + Water
(i) copper oxide and dilute hydrochloric acid
$$ \ce{CuO(s) + 2HCl(aq) -> CuCl2(aq) + H2O(l)} $$
The colour of the solution becomes blue-green and the copper oxide dissolves. The blue-green colour of the solution is due to the formation of copper(II) chloride in the reaction.
Since metallic oxides react with acids to give salts and water, similar to the reaction of a base with an acid, metallic oxides are said to be basic oxides.
Reaction of a Non-metallic Oxide with Base
(i) Reaction between carbon dioxide and calcium hydroxide
$$ \ce{CO2(g) + Ca(OH)2(aq) -> CaCO3(s) + H2O(l)} $$
Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt and water. Since this is similar to the reaction between a base and an acid, non-metallic oxides are acidic in nature.
Acids and Bases in Water
Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively.
Acids
$$ \ce{HCl + H2O -> H3O+ + Cl-} $$
Hydrogen ions in HCl are produced in the presence of water. Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H+(aq) or hydronium ion (H3O+).
Bases
Bases generate hydroxide (OH–) ions in water. Bases which are soluble in water are called alkalis.
$$ \ce{NaOH(s) ->[\mathrm{H_2O}] Na^{+}(aq) + OH^{-}(aq)} $$
$$ \ce{KOH(s) ->[\mathrm{H_2O}] K^{+}(aq) + OH^{-}(aq)} $$
$$ \ce{Mg(OH)2(s) ->[\mathrm{H_2O}] Mg^{2+}(aq) + 2OH^{-}(aq)} $$
pH Scale
The strength of an acid or an alkali can be tested by using a scale called the pH scale (0-14) which gives the measure of hydrogen ion concentration in a solution. A neutral solution has a pH of exactly 7, while an acidic solution has a pH less than 7 and a basic solution a pH more than 7.
Living beings carry out their metabolic activities within an optimal pH range. Mixing concentrated acids or bases with water is a highly exothermic process. Acids and bases neutralise each other to form corresponding salts and water.
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Salts have various uses in everyday life and in industries.