Acid, Base and Salt

In this chapter, we discuss about the introduction, properties, uses in daily life of Acid , Base, and Salt in Grade 8 Science. Reference Notes for Grade 8. 

Acid:

Acid is a substance which gives hydrogen ions (H+) when dissolved in water. For examples, Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3), carbonic acid (H2CO3).

Acid is also defined as a substance which dissociates in water to give positively charged hydrogen (H+). 

Disaccociation of HCl is given as: HCl :- H(+)  + Cl(-)

Disaccociation of acetic acid in aqueous solution is: CH3COOH :- H(plus) + CH3COO(minus)

Inorganic compounds can be classified into three main classes namely acids, bases, and salt on the basis of their own properties.

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

f) They react with metallic oxides to give salt and water.

Na2O + 2HNO3 → 2NaNO3 + H2O

Sodium Oxide + Nitric Acid → Sodium nitrate + Water

g) Dilute acids decompose bicarbonate and carbonates and liberate carbon dioxide.

NaHCO3 + HCl → NaCl + H2O + CO2

Characteristics of acid:

They all contain hydrogen
Many acids are corrosive.
They turn blue to red litmous paper.
They turn methylorange into yellow color.

Uses of acid:

• 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.
• Tanic acid is used to tan leather.
• Sodium bicarbonate is used in the manufacture of ammonium sulphate.

Bases: 

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

Sodium hydroxide and ammonia are bases because they give hydroxyl ions in water.

NaOH → Na+ + OH-

Sodium hydroxide

NH3 + H2O → NH4+ + OH-

Ammonia (Ammonium ion)

Bases which are soluble in water are called alkalies. Some bases are not soluble in water. They are not alkalies. Thus, all alkalies are bases but all bases are not alkalies. Sodium oxide is called soda and potassium hydroxide is called caustic potash.

Bases	Formula
Sodium hydroxide	NaOH
Potassium hydroxide	KOH
Aluminum hydroxide	Al(OH)3
Ammonium hydroxide	NH4OH
Sodium oxide	Na2O
Potassium oxide	K2O
Magnesium oxide	MgO

Properties of base:

Physical properties:

• They are soapy in touch.
• They have a bitter test.
• They turn red litmus paper into blue.
• They turn turmeric paper brown.
• They turn methyl orange yellow and phenolphthalein pink.

Chemical properties:

• They produce hydroxyl ions (OH-) in aqueous solution. For E.g.: NaOH, KOH and Ca(OH)2.
• They neutralize acids to form salt and water.
  KOH + HCl → KCl + H2O
  Potassium hydroxide + Hydrochloric acid → Potassium chloride + Water
• Bases react with carbon dioxide to form carbonate.
  K2O + CO2 → K2CO3
• They react with metals of the type zinc and aluminum and produce hydrogen.
  Zn + 2NaOH → NaZnO2 + H2
• When a base is heated with an ammonium salt, ammonia gas is given off. It can recognized by its extremely pungent smell.
  NaOH + NH4Cl → NaCl + H2O + NH3
• Sodium hydroxide + Ammonium chloride → Sodium chloride + Water + Ammonia

Some bases used in daily life:

 

Bases	Uses
Sodium hydroxide	In the manufacture of soap.
Potassium hydroxide	Firewood ash-filtered wood ash is used for washing clothes in rural areas.
Aluminum hydroxide	As medicine (antacid) for gastric patients.

Differene between acids and bases:

S.N	Acids	Bases
1.	Turn blue litmus into the red.	Turn red litmus into blue.
2.	React with metals to give a salt and hydrogen gas.	Normally do not react with metals. Only some metals like zinc, aluminum and tin react to give hydrogen.
3.	Are corrosive to skin.	Are slippery to touch.
4.	Are sour in taste.	Are bitter in taste.
5.	React with bases to give salts and water.	React with acids to give salt and water.
6.	Aqueous solutions of acids contain replaceable hydrogen ions.	Aqueous solution of the base contains replicable hydroxyl ions.
7.	Give no color with phenolphthalein.	Give red color with phenolphthalein.

Salts: 

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. Example; CuSO4

The salt formed by weak acid and strong base are called basic salt. Example; Ca(HCO3)2

Properties of salts

• Some salts are salty in taste, but most of the salts are bitter in taste.
• Salts normally do not cause any change in the color of indicators like litmus paper, methyl orange, and phenolphthalein.
• Some salts are white while some salts are colored.
• Most of them are soluble in water.

Preparation of salts

Salts can be prepared in many ways. Some methods of preparing salts are given below:

1. The reaction between an acid and a base: Common salt (NaCl) can be prepared by the reaction of sodium hydroxide with hydrochloric acid.
   NaOH + HCl → NaCl + H2O
2. The reaction between an acid and a metal: A metal displaces hydrogen from an acid to form a salt.
   Zn + 2HCl → ZnCl2 + H2
   Mg + 2HNO3 → Mg (NO3)2 + H2
3. Reaction between a metal oxide or another salt with and acid
   ZnO + H2SO4 → ZnSO4 + H2O
4. By a direct combination of metals and non-metals
   2Na + Cl2 → 2NaCl

Some salts and their uses are given below:

Salts	Uses
Sodium chloride	As edible salts (table salt, rock salt)
Copper sulphate	As insecticides
Magnesium sulphate	Used in constipation
Calcium sulphate	Plastering fractured bones, in the manufacture of chalk.