Common Properties of Acids and Bases
You have read that acids give hydrogen gas when they react with metal. This shows that all acids contain hydrogen. For example: Hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3), etc.
Water is necessary for acid to show acidic behaviour
When an acid is dissolved in water, it dissociates hydrogen. The dissociation of hydrogen ion in aqueous solution is the common property in all acids.
The acidic behaviour of acid is because of its ability to dissociate hydrogen ion in aqueous solution. An acid does not dissociate hydrogen ion in absence of water.
That is why, if you will bring blue litmus in contact with dry hydrochloric acid there will be no change in colour of litmus paper. But if a wet blue litmus paper is brought in contact with dry hydrochloric acid then blue litmus paper turns red due to dissociation of hydrogen ion in presence of moisture. This simple experiment shows the necessity of water for an acid to show acidic behaviour.
Example 1: Hydrochloric acid (HCl) gives hydrogen ion (H+) and chloride ion (Cl−) when it is dissolved in water.
HCl (aq) ⇨ H+ + Cl−
Example 2: Sulphuric acid (H2SO4) gives hydrogen ion (H+) and sulphate ion (SO4− −) in water.
H2SO4 (aq) ⇨ H+ + SO4− −
Example 3: Nitric acid (HNO3) gives hydrogen ion (H+) and nitrate ion (NO3−) in water.
HNO3 (aq) ⇨ H+ + NO3−
Example 4: Acetic acid (CH3COOH) gives acetate ion (CH3COO−) and hydrogen ion (H+).
CH3COOH (aq) ⇨ CH3COO− H+
Hydrogen ion which is produced by acid (when acid is combined with water molecule), exists in the form of hydronium ion (H3O−) in aqueous solution. That’s why hydrogen ion is always written with suffix (aq), such as H+ (aq).
HCl + H2O ⇨ H3O− + Cl−
H2SO4 + H2O ⇨ H3O− + SO4− −
Thus, because of dissociation of hydrogen ions, acid shows its acidic behavior.
Acids conduct electricity in their aqueous solution because of dissociation of hydrogen ion. Hydrogen ion in aqueous solution conducts electricity.
Acidic behavior of carbon dioxide gas: Carbon dioxide gas produces carbonic acid when dissolved in water. This carbonic acid dissociates hydrogen ion and carbonate ion in the aqueous solution.
CO2 + H2O ⇨ H2CO3 ⇨ H+ + CO3− −
Are all compounds which contain hydrogen, necessarily acids?
No, all compounds which contain hydrogen are not acid. For example, glucose (C6H12O6), methyl alcohol (CH3OH), etc. are not acid in spite of the fact that they contain hydrogen. This is because these compounds do not dissociate hydrogen ion in their aqueous solution.
Common in all Base:
Let us see what happens when a base is dissolved in water. A base dissociates hydroxide ion in water, which is responsible for the basic behavior of a compound. A base is not going to show basic properties in absence of water. It needs to be in aqueous solution to show the bhaviour of a base. Example:
Example 1: When sodium hydroxide is dissolved in water, it dissociates hydroxide ion and sodium ion.
NaOH (aq) ⇨ Na+ + OH−
Example 2: When potassium hydroxide is dissolved in water, it dissociates hydroxide ion and potassium ion.
KOH (aq) ⇨ K+ + OH−
Thus, base shows its basic character because of dissociation of hydroxide ion.
In previous section, you read about a general definition of neutralisation reaction. In thise section, you will learn about neutralisation reaction in terms of dissociation and recombination of different ions of acid and base when they react with each other.
When an acid reacts with a base, the hydrogen ion of acid combines with the hydroxide ion of base and forms water. As these ions combine together and form water instead of remaining free, both neutralize each other. Here, neutralisation means that the acid is erasing the basic property of base, and bases is erasing the acidic property of acid.
OH− + H+ ⇨ H2O
Example: When sodium hydroxide (a base) reacts with hydrochloric acid, sodium hydroxide breaks into sodium ion and hydroxide ion and hydrochloric acid breaks into hydrogen ion and chloride ion. Hydrogen ion and hydroxide ion combine together and form water, while sodium ion and chloride ion combine together and form sodium chloride.
NaOH + HCl ⇨ OH− + Na+ + H+ + Cl− ⇨ NaCl + H2O
Dilution of Acid and Base:
The concentration of hydrogen ion in an acid and hydroxide ion in a base (per unit volume) shows the concentration of acid or base.
By mixing of acid to water, the concentration of hydrogen ion per unit volume decreases. Similarly, by addition of base to water the concentration of hydroxide ion per unit volume decreases. This process of addition of acid or base to water is called dilution and the acid or base is called diluted.
The dilution of acid or base is exothermic. Thus, acid or base is always added to water and water is never added to acid or base. If water is added to a concentrated acid or base a lot of heat is generated, which may cause splashing out of acid or base and may cause severe damage as concentrated acid and base are highly corrosive.