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Acids, Bases and Salts- NCERT Notes UPSC
Apr 14, 2022
Acids are sour in taste and change the colour of blue litmus to red while Basesare bitter and change the colour of the red litmus to blue. Give an extra edge to your UPSC exam preparation with this detailed article on Acids, Bases, and Salts.
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 isformed 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 strengthof 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.