Acids, Bases and Salts- NCERT Notes UPSC

Chemical Properties of Acids and Bases

• Reaction with indicators: Indicator such as litmus, and phenolphthalein helps to recognize the nature of a substance (acidic or basic) by the change in colour.
• Reaction with Metals:
• An acid reacts with a metal, hydrogen gas is evolved, and a corresponding salt is formed.

Acid + Metal → Salt + Hydrogen gas

• A base reacts with a metal, and along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen.
• Reaction with Metal Carbonates and Metal Hydrogen carbonates: All metal carbonates and hydrogen carbonates react with acids to give a corresponding salt, carbon dioxide and water.

Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water

• Reaction with each other:  The reaction between an acid and a base to give salt and water is known as a neutralisation reaction. 

Base + Acid → Salt + Water

• The reaction of Metallic Oxides with Acids: Metallic oxides react with acids to give salts and water, similar to the reaction of a base with an acid, metallic oxides are said to be basic oxides.

Metal oxide + Acid → Salt + Water

• The reaction of a Non-metallic Oxide with Base: It is similar to the reaction between a base and an acid since non-metallic oxides are acidic in nature.
• Acids and Bases in a Water Solution: Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively.

• Bases that are soluble in water are called alkalis. 
• The process of dissolving an acid or a base in water is a highly exothermic one.
• Mixing an acid or base with water results in a decrease in the concentration of ions per unit volume. Such a process is called dilution, and the acid or the base is said to be diluted.

Strength of Acid or Base Solutions

• The strength of acids and bases depends on the number of H⁺ ions and OH^– ions produced, respectively.

• Strong Acids: Acids that give rise to more H⁺ ions.
• Weak acids: Acids that give less H⁺ ions.

• The strength of an acid or an alkali can be tested by using a scale called the pH scale (0-14) which gives the measure of hydrogen ion concentration in a solution.
• The p in pH stands for ‘potenz’ in German, meaning power.
• The higher the hydronium ion concentration, the lower is the pH value.
• A neutral solution has a pH of exactly 7, while an acidic solution has a pH of less than 7 and a basic solution has a pH of more than 7.

Variation of pH with the change in concentration of H⁺(aq) and OH^–(aq) ions

pH of some common substances shown on a pH paper (colours are only a rough guide)

Importance of pH in Everyday Life

• pH sensitivity of Plants and Animals:

• Living organisms can survive only in a narrow range of pH changes.
• The human body works within the pH range of 7.0 to 7.8.
• When the pH of rainwater is less than 5.6, it is called acid rain.

• When acid rain flows into the rivers, it lowers the pH of the river water.
• The survival of aquatic life in such rivers becomes difficult.

• pH of the soil: Plants require a specific pH range for their healthy growth.
• pH in our digestive system:

• The stomach produces hydrochloric acid which helps in the digestion of food.
• During indigestion, the stomach produces too much acid, and this causes pain and irritation. To get rid of this pain, people use bases called antacids.
• Antacids such as Magnesium hydroxide (base) are used to neutralise the excess acid.
• pH as the cause of Tooth decay:

• Tooth decay starts when the pH of the mouth is lower than 5.5.
• Toothpastes are generally basic and neutralise the excess acid and prevent tooth decay.
• Self-defence by animals and plants through chemical warfare:

• Bee-sting leaves an acid that causes pain and irritation.
• Stinging hair of nettle leaves injects methanoic acid causing burning pain.
• The use of a mild base like baking soda on the stung area gives relief.

Some Naturally Occurring Acids

Salts

Salts are the results of the neutralization reaction between acid and base. For example, Sodium chloride.

Salt Family: Salts having the same positive or negative radicals are said to belong to a family. For example,

• NaCl and Na₂SO₄ belong to the family of sodium salts.
• NaCl and KCl belong to the family of chloride salts.

pH of Salts

• Salts of a strong acid and a strong base are neutral with pH value of 7.
• Salts of a strong acid and weak base are acidic with pH value less than 7.
• Salts of a strong base and weak acid are basic in nature, with pH value more than 7.

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Chemicals from Common Salt

The salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride.

• It is used in food and hence called Common Salt.
• It is an important raw material for various materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder, etc.

Sodium hydroxide: When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process.

Bleaching powder:  During the electrolysis of aqueous sodium chloride, chlorine is produced. Bleaching powder is produced by the action of this chlorine on dry slaked lime. Its major uses are:

• To make drinking water free from germs.
• For bleaching cotton and linen in the textile industry, bleaching wood pulp in paper factories and bleaching washed clothes in the laundry.
• As an oxidising agent in many chemical industries.

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Baking Soda: It is a by-product of Chlor-alkali process. 

• Other names: Bread soda, cooking soda, bicarbonate of soda, sodium bicarb, bicarb of soda or simply bicarb, etc.
• Uses: Following are some of its uses:

• It is used in baking powder, which is used in cooking as it produces carbon dioxide which makes the batter soft and spongy.
• It is used as an antacid.
• It is used in toothpaste which makes the teeth white and plaque-free.
• It is used in cleansing ornaments made of silver.
• Since sodium hydrogen carbonate gives carbon dioxide and sodium oxide on strong heating, thus, it is used as a fire extinguisher.

Washing soda: Sodium carbonate is obtained by heating baking soda, recrystallisation of sodium carbonate gives washing soda. Its uses are:

• It is used in the glass, soap and paper industries.
• It is used in the manufacture of sodium compounds such as borax.
• It is used as a cleaning agent for domestic purposes.
• It is used for removing the permanent hardness of the water.

The Water of Crystallisation

It is the fixed number of water molecules present in one formula unit of salt. For example,

• Hydrated copper sulphate (CuSO₄. 5H₂O) contains five molecules of water in one formula unit of copper sulphate.
• Gypsum (CaSO₄.2H₂O) has two water molecules as water of crystallisation. 

Plaster of Paris:  On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate. This is called Plaster of Paris.

• It is a white powder and on mixing with water it changes to gypsum once again giving a hard solid mass.
• It is used by doctors in plaster for supporting fractured bones in the right position. 

Interesting Points

• The atmosphere of Venus is made up of thick white and yellowish clouds of sulphuric acid.
• Tooth enamel made up of calcium hydroxyapatite (a crystalline form of calcium phosphate) is the hardest substance in the body.
• Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal.

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