Acid , Base and Salt characteristics

ACIDS, BASES AND SALTS

ACIDS

The word acid comes from the Latin word “acere” which means sour.
The substances which are sour in taste contain acids and the chemical nature of such substances is acidic.
Examples: Curd, lemon juice, orange juice and vinegar etc. (all are natural acids)

BASES

Substances which are bitter in taste and feel soapy on touching are known as bases.
The nature of such substances is said to be basic.
Examples: Baking soda, lime water, soap etc.

INDICATORS USED FOR DETECTING BASES AND SALTS

Special type of substances are used to test whether a substance is acidic or basic. These substances are known as indicators.
The indicators change their color when added to a solution containing an acidic or a basic substance.
Naturally occurring indicators: Turmeric, litmus, China rose petals (Gudhal), etc.
There are some substances whose odor changes in acidic or basic media. These are called “olfactory indicators”. Such as vanilla, onion and clove. (Acidic: retains smell, basic: loses smell.)

natural

1.Litmus: A natural dye

It is extracted from lichens
When added to an acidic solution, it turns red and when added to a basic solution, it turns blue.
It is available as red and blue litmus paper

2.turmeric

This indicator turns basic solutions to red brown or red orange and does not change color in acidic solution.

3.China rose petals (Gudhal)

This indicator turns acidic solutions to dark pink (magenta) and basic solutions to green.

SYNTHETIC INDICATORS

1.Phenolphthalein

when the solution is basic, it gives a pink colour. On the other hand, when the solution is acidic, it remains colourless.

2.Methyl orange

when the solution is basic, it gives a yellow colour. On the other hand, when the solution is acidic, it becomes red.

Table depicting solution  with respect to red and blue litmus and their inference

Solution	Red litmus	Blue litmus	Inference
Tap water	No change	No change	Neutral
Detergent solution	Changes to blue	No change	Basic
Aerated drinks	No change	Changes to red	Acidic
Soap solution	Changes to blue	No change	Basic
shampoo	No change	Changes to red	Acidic
Common salt solution	No change	No change	neutral
Sugar solution	No change	No change	Neutral
Vinegar	No change	Changes to red	Acidic
Baking soda solution	Changes to blue	No change	Basic
Milk of magnesia	Changes to blue	No change	Basic
Washing soda solution	Changes to blue	No change	Basic
Lime water	Changes to blue	No change	Basic

 

Strong acid-acids which dissociate completely when dissolved in water. E.g. Sulphuric acid (H₂SO₄), HCl, HNO₃, H₃SO₄ etc.

Weak acid-the acids which dissociate partially when dissolved in water. E.g. CH₃COOH, Oxalic acid, H₂CO₃ (Carbonic Acid) etc.

BASES

Strong base: bases which dissociate completely when dissolved in water. E.g. Calcium Hydroxide [Ca(OH)₂) etc.
Weak base: bases which dissociate partially when dissolved in water. E.g. iron hydroxide (Fe(OH)₂) etc.

Properties of Acids

Acid reacts with metal to liberate hydrogen gas.
Acid + Metal Salt + Hydrogen gas
Acid reacts with all metal carbonates and metal hydrogen carbonates to form corresponding salt, water and evolve carbon dioxide.
Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water
Acid reacts with base to give salt and water.
Acid+Base- Salt+Water
Acid reacts with metallic oxides (basic in nature) to give salt and water.
Acids give hydrogen ions (H⁺) or Hydronium ions (H₃O⁺) in water
HCl + H₂O H₃O⁺ + Cl⁻
Acid Reaction with Metals
H₂SO₄ (sulphuric acid) + Zn ZnSO₄ (zinc sulphate) + H₂

Uses of Acids

 Nitric acid: used for making fertilizers, dyes, medicines, explosives etc.
 Citric acid (C6H8O7): it is used in the processing and preserving of food stuffs, for washing metals and in cloth industries.
 Hydrochloric acid: used for making plastics (PVC), medicines, cosmetic, dyes, aqua regia, in leather industry etc.
 Sulphuric acid: manufacturing of fertilizers, plastics, paints, explosives, dyes etc.
 Oxalic acid: photography, in coloration and printing of of clothes,
bleaching leather, removing ink or rust spots from the clothes.
 Acetic acid: it is used as vinegar, in the preparation of acetone and in the processing of food stuffs.
 Benzoic acid and Formic acids: preservative for food stuff. FA is also used for making insecticides, processing of rubber etc.

Properties of Base

 Base reacts with metal to form salt and liberates hydrogen gas.
Sodium hydroxide + Zinc sodium Zincate + Hydrogen gas
 Base reacts with acids to from salt and water.
Base + Acid Salt + Water
 Base reacts with non-metallic oxide to form salt and water.
Non-metal Oxide+ Base → Salt + Water
 Bases give hydroxide ions (OH⁻) in water.
 Bases which are soluble in water are called Alkalies. For e.g. NH₄OH, KOH etc.

Uses of Bases

 Sodium Hydroxide (NaOH): Manufacturing drugs,paper andtextile industry, petroleum refining etc.
 Calcium Oxide (CaO): Bleaching powder production.
 Calcium Hydroxide [ca (OH)₂]: Manufacturing bleaching powder, concrete and plaster, treatment of acidic soil, white washing etc.
 Magnesium Hydroxide [Mg (OH)₂]: Used as an antacid and in sugar industry.
 Potassium Hydroxide (KOH): Manufacturing of shampoos, shaving creams, soft soaps etc.

NEUTRALISATION

The reaction between an acid and a base is known as neutralization. Salt and water are produced in this process with the evolution of heat.

Examples

 Acid+Base → Salt+Water (Heat is evolved)
Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H2O)

pH Scale

 A scale for measuring hydrogen ion concentration in a solution which indicates the acidic or basic nature of a solution.
 The p in pH stands for ‘potenz’ in German, meaning power.
 The pH of a neutral solution is 7.
 Values < 7 on the pH scale represent an acidic solution.
 Value >7 on the pH scale represent an basic solution.
 Generally paper impregnated with the universal indicator is used for measuring pH.

Importance of pH in Everyday Life

1.Indigestion

To relieve indigestion (due to excessive hydrochloric acid), we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralises the effect of excessive acid.

2.Ant bite

it injects the acidic liquid (formic acid) into the skin. It can be neutralised by rubbing moist baking soda (sodium hydrogencarbonate) or calamine solution, which contains zinc carbonate.

3.Soil treatment

When the soil is too acidic, it is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is basic, organic matter (compost) is added to it.

4.Factory wastes

The factory wastes are neutralised by adding basic substances.

5.Tooth decay

starts when the pH of the mouth is lower than 5.5. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.

SALTS

Salts are the compounds obtained from the reaction of acid and base (Neutralisation). Cation of the salt comes from a base while its anion comes from an acid.

TYPES:

Neutral salt: Salts of a strong acid and a strong base are with pH value of 7.
Acidic salt: salts of a strong acid and weak base with pH value less than 7
Basic salt: salts of a strong base and weak acid with pH value more than 7

USES OF SALTS

 Sodium Chloride (NaCl): Common ordinary salt. Important raw material for various daily use materials such as baking soda, washing soda, bleaching powder etc.
 Sodium Bicarbonate (NaHCO₃): It is used in baking powder and antacid
 Epsom Salt (MgSO₄ . 7H₂O): Medicinal uses
 Washing Soda (NaCO₃ . 10 H₂O): A basic salt. Used in glass, soap and paper industry, dry cleaning of clothes etc.
 Potassium Nitrate(KNO₃): Production of gun powder, firecrackers, in glass industry, fertilizers .
 Copper Sulphate (CuSO₄ . 5H₂O): Used as an insecticide, in coloration and printing, purification of copper etc.
 Gypsum (CaSO₄ . 5H₂O): It loses water molecules of crystallization to form POP which is used as a plaster, in makin