9.Acid base and salt

An acid is a substance which gives hydrogen ions or proton (H+) when dissolved in an aqueous solution.
Examples: - Hydrochloric acid (HCl), sulphuric acid (H2 SO4) , nitric acid (HNO3), carbonic acid ( H2CO3 )

HCl H+   + Cl-

Properties of acids

a) Acid is sour in taste.

b) Acid reacts with base to give salt and water.
For example:
HCl + NaOH → NaCl + H2O
(acid)   (base)     (salt)     (water)
c) It gives hydrogen ion (H+) when dissolved in water.
For example:
HCl → H+ + Cl-

d) Acid turns blue litmus paper to red, light yellow color of methyl orange to red and phenolphthalein to colorless.

e) When acid reacts with carbonates and bicarbonates, carbon dioxide is produced.

CaCO3 + 2HCl → CaCl2 + H2O + CO2

Uses

Carbonic acid is used in soft drinks.

Sulphuric acid is used in fertilizers

Hydrochloric acid is used for bleaching purpose.

Carbolic acid is used as germ killer.

Bases are the substance, which gives hydroxyl ion when dissolved in aqueous solution.
For example: Sodium oxide (NaO), Potassium hydroxide (KOH) etc.

Properties of base

Base turns red litmus paper into red, phenolphthalein into pink and methyl orange in to yellow.

Alkalis are water-soluble bases. They give hydroxyl ions when dissolved in water.

For E.g.: NaOH, KOH and Ca(OH)2.

Alkali reacts with carbon dioxide to produce in the presence of heat.

NaOH + CO2Na2CO3   + H2O

Acid reacts with base to give salt and water.
For example:
HCl + NaOH → NaCl + H2O
(acid)   (base)     (salt)     (water)

Salts are the neutral compounds, which are formed by the reaction between acid and base. Example: NaCl, CaCL2 etc.

 The salt formed by the complete displacement of hydrogen atoms by a metal or electropositive radical is called normal salt. Normal salt are neutral in nature.

For e.g.: NaCl, KNO3 etc

The salt formed by strong acids and weak base are strong salt.

E.g. CuSO4

The salt formed by weak acid and strong base are called basic salt.

E.g.Ca (HCO3)2

Methods of preparation of salts are as follows:
Neutralization of acid and base
NaOH + HCl → NaCl + H2O

Reaction of acid on metallic oxides
FeO + 2HCl → FeCl2 + H2O

Action of acids on metal
Zn + H2SO4 → ZnSO4 + H2

The characteristics of salt are:

Common salt (NaCl) is an essential constituent of our diet.

Most salts are insoluble in water but few are soluble in water.

The solution during the formation of salt the radical derives from an acid is called acid radical and the radical obtained from a base is called basic radical.

For E.g.:

NaOH + H2SO4   →    Na2SO4 + H2O

In the salt Na2SO4, sodium radical (Na+) is obtained from the base (NaOH) and SO4--radical is obtained from the acid H2SO4. So Na+ and SO4—are called basic radical and acid radical respectively.

Indicators are the chemical substance used to identify acidic or basic nature of the substance.

Indicators	Litmus	Phenolphthalein	Methyl Orange
Acid	Blue to Red	Colorless	Red color
Base	Red to Blue	Pink Color	Yellow color
Salt	No change	Neutral	No change

A universal indicator is a special kind of indicator which is used to measure the strength of acidity or alkalinity .It is prepared by several organic indicators of different of different colors. It is found in green blue solution or in the form of yellow litmus paper.

pH Scale is the scale which shows the quantity of hydrogen ions in acid or base.
If the concentration of hydrogen ion is less than 7, then it shows acidity of the substance and if the concentration is more than 7 then it shows the basicity of the substance.

When we add alkali on acid, the pH value of the acid goes from its acidic value to value 7 where it acts as a neutral compound as the pH value of neutral compound is 7.