Introduction: Acid, Base and Salts

Acids

• Acids is defined as the one which produces hydrogen ions in water. For Example, Sulphuric Acid, Hydrochloric Acid etc.
• They give sour taste.
• Acids turn blue litmus to red. This is used as confirmation test for the presence of acid.
• When acids react with metals, gases are evolved.

Reactions with Acids-

1. Reaction of Acid with MetalAcid + Metal → Salt + Hydrogen gas
Mg + H2 SO4 → H2 + Mg SO4

2. Reaction of Acid with Carbonates
Na2 CO3 (s) + 2 HCl (aq) → 2NaCl (aq) +H2O(l) + CO2(g)

3. Reaction of Acid with Bicarbonates
NaHCO3(s) + HCl (aq) → NaCl(aq) + H2O(l) + CO2(g)

Acid + metal carbonate or
bicarbonate → salt + water + carbon dioxide

Similarity between Acids and Bases

1. Both acids and base react with water. They produce ions in water
2. Both acids and bases acts as electrolytes, so are good conductors of electricity

Classification of Acids

1. Acids are classified as Organic Acids and Mineral Acids. Acids which are derived from plants and animals, they are known as Organic Acids. For Example, Citric Acid from fruit. Mineral acids are inorganic acids such as Sulphuric Acid. They are dangerous to be used, so need more precautions.

2. Acids are also classified as Strong Acids or Weak Acids. Strong acid is an acid, that completely dissociates into ions in aqueous solutions. For Example, Sulphuric Acid, Hydrochloric Acid.

HCI acid + H₂O = H₃O⁺ + CI^–

Weak acid is the one which does not dissociate completely into ions in aqueous solutions. For Example, Acetic Acid.

CH³COOH + H₂O ⇌ CH₃CHOO^– + H⁺

1. Acids can also be as Dilute Acid and Concentrated Acids. The one which has low concentration of acids in aqueous solution, they are known as Dilute Acids whereas the one which has high concentration of acids in aqueous solution, are known as Concentrated Acids.

2. It is advisable to add acid to water and not vice versa because large amount of heat is released if water is added to acid. This released heat is large enough to cause harm.

3. Acids can also be classified based on number of hydrogen ions. Monoprotic acid is the one which gives one mole of hydrogen ions per mole of acid, such as HCl. Diprotic Acid is the one which produces two mole of hydrogen ions per mole of acid. For Example, H2SO4.

Bases

Bases are the one which produces hydroxide ions in aqueous solutions. Bases which are water soluble they are known as Alkalis.

They turn red litmus to blue.

They have a bitter taste.

They also produced carbon-dioxide when reacted with carbonates.

They also evolved hydrogen gas when bases react with metals.

Reactions of Bases

1. Reaction with Metals-Base reacts with metals and produce hydrogen gas.
   2NaOH + Zn → Na2 → Na2ZnO2 +H2
   
2. Reaction with Acids-Base reacts with acids to form salts. For Example,
   KOH + HCl → KCl + H2O

Classification of Bases

• Bases are classified as Strong Base and Weak Base. Strong base is the one which dissociates completely into its ions in aqueous solution. For Example, NaOH.
• Weak base is the one which does not dissociate completely into its ions in aqueous solutions. For
  Example, Ammonium Hydroxide, NH4OH
• Bases are also classified as Dilute Base and Concentrated Base. The solution which has low concentration of base in aqueous solution is defined as Dilute Base whereas the one which has high
  concentration of base in aqueous solution is known as Concentrated Base

Effect on indicators by acids and bases-

• An indicator is a chemical substance which shows a change in its physical properties, mainly colour or odour when brought in contact with an acid or a base. Below mentioned are commonly used indicators and the different colours they exhibit:

a) Litmus

In a neutral solution – purple
In acidic solution – red
In basic solution – blue
Litmus is also available as strips of paper in two variants – red litmus and blue litmus. An acid turns a moist blue litmus paper to red.
A base turns a moist red litmus paper to blue.

b) Methyl orange

In a neutral solution – orange
In acidic solution – red
In basic solution – yellow

c) Phenolphthalein

In a neutral solution – colourless
In acidic solution – remains colourless
In basic solution – pink
Litmus is also available as strips of paper in two variants – red litmus and blue litmus.
An acid turns a moist blue litmus paper to red.
A base turns a moist red litmus paper to blue.

Importance of pH in everyday life

1.pH sensitivity of plants and animals-Plants and animals are sensitive to pH. Crucial life processes such as digestion of food, functions of enzymes and hormones happen at a certain pH value.

2. pH of a soil-The pH of a soil optimal for the growth of plants or crops is 6.5 to 7.0.

3. pH in the digestive system-The process of digestion happens at a specific pH in our stomach which is 1.5 to 4.The pH of the interaction of enzymes, while food is being digested, is influenced by HCl in our stomach.

4. pH in tooth decay-Tooth decay happens when the teeth are exposed to an acidic environment of pH 5.5 and below.

5. pH of self-defence by animals and plants-Acidic substances are used by animals and plants as a self-defenses mechanism. For example, bee and plants like nettle secrete a highly acidic substance for self-defenses. These secreted acidic substances have a specific ph.

Salts

When acid and base neutralize, salts are formed. Strong acid and strong base combines to form neutral salt.

NaOH + HCl → NaCl + H₂O
Strong acid and weak base combine to form acidic salt. For Example, Hydrochloric Acid and ammonium hydroxide combine to form ammonium chloride. Other examples, sodium hydrogen carbonate, sodium hydrogen sulphate etc.

HCl + NH₄OH → NH₄Cl + H₂O
Similarly, weak acid and strong base combine to form basic salt. For Example, Acetic Acid and sodium hydroxide combine to form sodium acetate. Other examples are calcium carbonate, potassium cyanide etc.

CH₃COOH + NaOH → CH₃COONa + H₂O
The most common salt is table salt or sodium chloride (NaCl).

Preparation of Sodium hydroxide

Chemical formula – NaOH
Also known as – caustic soda

Preparation (Chlor-alkali process)

Electrolysis of brine (solution of common salt, NaCl) is carried out.
At anode: Cl₂ is released
At cathode: H₂ is released
Sodium hydroxide remains in the solution.

Bleaching powder

Chemical formula – Ca(OCl)Cl or CaOCl₂

Preparation – Ca(OH)₂(aq)+Cl₂(g)→CaOCl₂(aq)+H₂O(l)
On interaction with water – bleaching powder releases chlorine which is responsible for bleaching action.

Baking soda

Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO₃

Preparation (Solvay process):

a. Limestone is heated: CaCO₃→CaO+CO₂
b. CO₂ is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq)+NH₃(g)+CO₂(g)+H₂O(l)→NaHCO₃(aq)+NH₄Cl(aq)

Uses:
1. Textile industry
2. Paper industry
3. Disinfectant

Washing soda

Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO₃

Preparation (Solvay process)

a. Limestone is heated: CaCO₃ → CaO + CO₂
b. CO₂ is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq) + NH₃(g) + CO₂(g) + H₂O(l) → NaHCO₃(aq) + NH₄Cl(aq)

Uses
1. In glass, soap and paper industries
2. Softening of water
3. Domestic cleaner

Crystals of salts-

Certain salts form crystals by combining with a definite proportion of water. The water that combines with the salt is called water of crystallisation.

Plaster of Paris

Gypsum, CaSO₄.2H₂O (s) on heating at 100°C (373K) gives CaSO₄. ½ H₂O and 3/2 H₂O
CaSO₄. ½ H₂O is plaster of paris.
CaSO₄. ½ H₂O means two formula units of CaSO₄ share one molecule of water.

Uses 
cast for healing fractures.