This article is written for the Tnpsc exam for the topic “Acids, Bases, Salts“.
This is prepared from Samacheer Kalvi’s books.
Let’s study acid, bases, salts, and how are acids, bases, and salts distinguished from one another?
Acid and base difference
Acids, Bases, and Salts are used in daily life. Ex: Soap, Juice, Detergent, etc. Our body metabolism is carried out by stomach juice called Hydrochloric acid (HCl).
Acid is a compound that forms Hydrogen Ion(H+) in an aqueous solution. Based forms Hydroxyl ions (OH-) in solution.
Acid and Base react with each other to form a neutral product known as Salt.
What is Acid?
All foods such as Curd, Fruit Juices, Fruit, and Vinegar tastes similar i.e Sour Taste due to the presence of acids.
Acid got its name from the Latin word “Acidus” which means sour taste. A substance with a sour taste is called Acids.
|Ant, Bee||Formic acid|
|Stomach juice||Hydrochloric acid|
Svante Arrhenius proposed, Arrhenius theory of acids, a substance which furnishes H+ or H3O+ ions in an aqueous solution, contains one or more replaceable hydrogen atoms.
Ex: Hcl when dissolved in water give H+ and Cl – ions in water.
Thus, hydrogen ions must always be H+ (or) Hydronium (H3O+)
H+ + H2O → H3O+
All acid contains hydrogen and all hydrogen-containing compounds are not acids. Ex: Methane (CH4) and Ammonia (NH3) as they do not produce H+ ions in an aqueous solution.
Classification of Acids
Acids are classified based on their source as
- Organic Acids
- Inorganic Acids
Present in Plants and Animal Sources.Ex: HCOOH, CH3COOH.
Prepared from rocks and minerals. Ex: HCl, HNO3, H2SO4
Acids are classified based on their Basicity as Monobasic, Tribasic, and Dibasic Acids.
Only one replaceable hydrogen atom/hydrogen ion.
Example: HCl, HNO3
Two replaceable hydrogen ions per molecule.
Example: H2SO4, H2CO3
Three replaceable hydrogen ions per molecule.
Acidic acid(CH3COOH) has only four hydrogen atoms, but only one can be replaced hence it is a monobasic acid.
Classification of Acids based on Ionization
- Strong Acids:Ionize completely in water. Eg:HCl.
2. Weak Acid: Ionize partially in the water. Eg: CH3COOH.
Ionisation is the condition of being dissociated into ions by heat or radiation or chemical reactions or electrical discharge.
Classification of Acids based on concentration
Concentrated Acid: Acid in its pure form, a relatively large amount of acid dissolved in a solvent.
Dilute Acid: Acid mixed with other substances/solvents such as water. It has a relatively smaller concentration of pure acid.
Properties of Acids
a) They have a sour taste
b) conducts electricity due to the presence of ions in an aqueous state
c) Acids turns blue litmus red
d) Acids react with active metals to give hydrogen gas.
Mg + H2SO4 → MgSO4 + H2 ↑
Zn + 2HCl → ZnCl2 + H2 ↑
Some metals do not react with acids. Ex:Gold(Ag),Cu(Copper).
e) Acids react with metal carbonate and metal hydrogen carbonate to give carbon dioxide.
Na2CO3 + 2HCl → 2NaCl + H2O + CO2 ↑
NaHCO3 + HCl → NaCl + H2O + CO2 ↑
f) Acids react with metallic oxides to give salt and water.
CaO + H2SO4 → CaSO4 + H2O
g) Acids react with bases to give salt and water.
HCl + NaOH → NaCl + H2O
Role of water in acid solution
- Acids property showed only when dissolved in acid.
- Based on their ionisation in water it is classified as strong and weak acids.
- Acids do not ionise in organic solvents. Ex: Hcl dissolved in water give H+ and Cl- ions and remains as a molecule when dissolved in ethanol.
Uses of Acids
Sulphuric acid (King of Chemicals), is used in the preparation of many chemicals and compounds and is also used in Car batteries.
Hydrochloric acid is used for cleaning toilets. Citric acid is used as a food preservative and effervescent salt. Nitric is used in fertilizers, dyes, drugs, and paints.
Oxalic acid is used to clean iron and manganese deposits from quartz crystals and to bleach wood and remove black strains.
Carbonic acids are used in aerated drinks. Tartaric acid is used in making baking soda.
Aquaregia(Latin Word also known as “Kings Water”)- Gold and Siver do not react with HCl and HNO3, but when these two acids are mixed together in the ratio of 3:1, three parts of HCl and One part of HNO3 can dissolve gold. This mixture is known as Aqua Regia. This compound also dissolves Platinum and Palladium.
Chemical formula : 3 HCl + HNO3
Solubility in Water: Miscible in water
Melting point : – 42°C (- 44°F, 231K)
Boiling point: 108°C (226°F, 381K)
Uses of Aquaregia:
Used to dissolve gold and palladium etc, Cleaning and refining of gold.
What are the Bases?
According to the Arrhenius theory, the substances that ionize in water to form hydroxyl ions (OH-) are basic.
Bases mixed with water from Alkali. A base reacts with an acid to give Salt and water.
Base + Acid → Salt + Water.
Zinc oxide (ZnO) reacts with an acid to salt Zinc chloride and water.
ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l)
Similarly, Sodium Hydroxide ionizes in water to give hydroxyl ios and dissolved water and it is an Alkali.
NaOH(aq) → Na+(aq) + OH- (aq)
All alkalis are bases but not all bases are alkalis. For example, NaOH and KOH are alkalis whereas Al(OH)3 and Zn(OH)2 bases.
Classification of Bases Based on their Acidity
a) Mono-acidic Base: Ionizes in water to give one hydroxide ion per molecule. Ex: NaOH, KOH.
b) Diacidic Base: a base that ionizes in water to give two hydroxide ions per molecule.
Example: Ca(OH)2. Mg(OH)2
c) Tri-acidic Base: based that ionizing three hydroxide ions per molecule.
Example: Al(OH)3, Fe(OH)3
Classification Bases based on Concentration
a) Concentrated Alkali: relatively high percentage of alkali in its aqueous solution.
b) Dilute Alkali: relatively low percentage of alkali in its aqueous solution
Classification Bases based on Ionisation
a) Strong Bases: bases such as NaOH, KOH which ionize completely in
b) Weak Bases: bases such as that ionize partially in aqueous solution.
The acidity in the base refers to the number of replaceable hydroxyl groups present in one molecule of a base.
Properties of Bases:
It has a bitter taste, soapy to touch and it turns red litmus blue. Bases aqueous solution conducts electricity.
Bases react with metals to give salt and liberation of hydroxide gas. Zn + 2 NaOH → Na2ZnO2 + H2↑
Based gives salt and water when reacted with non-metallic oxides, similar to base and acid reaction. Here non-metallic oxides act as acid. Ca(OH)2 + CO2 → CaCO3+ H2O.
Based form salt and water when reacted with acid. KOH + HCl → KCl + H2O. This reaction is called a Neutralisation reaction.
Ammonia gas is liberated when bases are heated with ammonium salts. NaOH + NH4Cl → NaCl + H2O + NH3↑
Some metals such as Cu, Ag, Cr etc do not react with sodium hydroxide.
Application and Uses of Bases
Sodium Hydroxide is used in the preparation of soap.
Calcium Hydroxide is used for the white washing of buildings.
For stomach problems, Magnesium hydroxide is used.
To remove grease stains from clothes Ammonium Hydroxide is used.
Tests for Acids and Bases
Acid turns blue litmus red and base turns red litmus paper into blue.
Phenolphthalein is colorless in acid medium and phenolphthalein is pink in basic medium.
Methyl orange is pink is color in acid medium and methyl orange is yellow is color in basic medium.
Test of Strength for Acid and Bases
pH Scale– It is a measure of hydrogen ion concentration in a solution. Where pH (Potenz in German), means power.
pH scale measures from 0 to 14 which indicates acidic, basic, or neutral.
- The pH of less than 7 is Acid.
- pH is greater than 7 is Base.
- pH equal to 7 is neutral.
pH measurement is done by pH solution or pH paper.
Our body pH is 7 to 7.8.
A living organism can survive only at a very narrow range to pH.
The pH of stomach fluid is 2.0.
The white enamel coating of teeth is made of calcium phosphate, which is the hardest substance in our body. A change in pH in the mouth can cause tooth decay.
For neutralizing the acid in the mouth to prevent tooth decay toothpaste is used which is Basic in nature.
The pH of soil differs for different plants, Alkaline soil for Citrus fruits, acidic soil for rice, and neutral soil for sugarcane.
The pH of rainwater is 7 which is neutral, it is purest. But due air pollution of sulphur and nitrogen oxides makes it acidic as the pH of acid rainfall is below 7.
Salt is produced when acid and bases react together. It produces and positive and negative ions when dissolved with water.
Acid + Base → Salt + Water
Types of Salts
(i) Normal Salts-obtained by complete neutralization of an acid by based.
NaOH + HCl → NaCl + H2O
(ii) Acid Salts-obtain by partial replacement of hydrogen ions of acid by metal. Acid salt is derived when polybasic acid is mixed with a calculated amount of base. NaOH + H2SO4 → NaHSO4 + H2O
(iii) Basic Salts-formed by the partial replacement of hydroxide ions of a di-acidic or tri-acidic base with an acid radical. Pb(OH)2 + HCl → Pb(OH)Cl + H2O
(iv) Double Salts-formed by the combination of the saturated solution of two simple salts in equimolar ratio followed by crystallization.
Ex: Potash alum is a mixture of potassium sulfate and aluminum
Properties of Salts
Melt and boil at a high temperature.
Salt is generally solids, odorless, mostly white, cubic crystals or crystalline powder with a salty taste.
Soluble in water, most of them. Ex: chloride salts of potassium are soluble in water but silver chloride is insoluble in water. It is hygroscopic in nature.
Identification of Salts
Physical examination of salt- study based on color, odor, and density. This test is not reliable.
The dry Heating test is performed by heating a small amount of salt in a dry test tube, the water gets evaporated, the dissolved salts are sedimented in the container.
Flame Test-some salts react with concentrated hydrochloric acid(HCL) form chlorides. The paste formed out of this mixture is put into the flame with the help of a platinum wire.
HCl with a carbonate salt gives off CO2 gas with brisk effervescence.
Uses of Salts
Nacl is used in food and as a food preservative.
Sodium Carbonate(Washing soda) for softening hard water, also used in glass, soap, and paper industries.
Baking Soda (Sodium bicarbonate -NaHCO3) is used as a baking powder which is a mixture of baking soda and tartaric acid, used in fire extinguishers, preparation of cake, bread and also used to neutralize excess acid in the stomach and provides relief.
Bleaching powder(Calcium Oxychloride-CaOCl2) is used as a disinfectant and bleaching agent in textile industries.
Plaster of Paris(Calcium Sulphate Hemihydrate – CaSO4 .½ H2O)-for plastering bones and making casts for statues.
Some Extra thing to Know about Acids
Q1. Why formic acid is more acidic than acetic acid?
A1. Formic acid is rated acidic than Acetic acid because the CH3 in acetic acid is electron-donating and provides electron density towards the O-H bond and making it hard to separate the H and consequently making acetic acid a weaker one than the formic acid.