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Acids: The word ‘Acid’ came from Latin word ‘Acidus or Acere’ which means sour.
Sour taste is the most common characteristic of acid. Acid turns blue litmus paper red. There are many substances which contain acid and hence taste sour, such as curd, tamarind, lemon, etc.

Types of Acids: Acids are divided into two types on the basis of their occurrence – Natural acids and Mineral acids.

Natural Acid: Acids which are obtained from natural sources are called natural acid or organic acid.

Mineral Acids: Acids that prepare from mineral are known as mineral acids, inorganic acids, man-made acids or synthetic acid, such as hydrochloric acid, sulphuric acid, nitric acid, etc.
Organic Acids & Source
Acid
Source
Acetic acid
Vinegar
Ascorbic acid
Guava, amla
Citric acid
Lemon, orange and other citrus fruits
Lactic acid
Sour milk, curd
Methanoic acid
Ant sting, nettle sting
Oxalic acid
Tomato
Tartaric acid
Tamarind

Chemical Properties of Acid

Reaction of acids with metal:
Acids give hydrogen gas along with respective salt when they react with a metal.
Metal + Acid  Salt + Hydrogen
Example: Hydrogen gas and zinc chloride are formed when hydrochloric acid reacts with zinc metal.
Zn + 2HCl  ZnCl2 + H2
Hydrogen gas and sodium chloride are formed when hydrochloric acid reacts with sodium metal.
2Na + 2HCl  2NaCl + H2
Hydrogen gas and iron chloride are formed when hydrochloric acid reacts with iron.
Fe + 2HCl  FeCl2 + H2
Hydrogen gas and zinc sulphate are formed when zinc metal reacts with sulphuric acid
Zn + H2SO4  ZnSO4 + H2
Test for hydrogen gas: The gas evolved after reaction of acid with metal can be tested by bringing a lighted candle near it. If the gas burns with pop sound, then it confirms the evolution of hydrogen gas. Burning with pop sound is the characteristic test for hydrogen gas.

Reaction of acids with metal carbonate:
Acids give carbon dioxide gas and respective salts along with water when they react with metal carbonates.
Metal carbonate + Acid  Salt + Carbon dioxide + Water
Examples: Hydrochloric acid gives carbon dioxide gas, sodium chloride along with water when reacts with sodium carbonate.
Na2CO3 + 2HCl  2NaCl + CO2 + H2O
Hydrochloric acid gives carbon dioxide, magnesium chloride and water when it reacts with magnesium carbonate.
MgCO3 + 2HCl  MgCl2 + CO2 + H2O
Hydrochloric acid gives carbon dioxide gas, calcium chloride and water when it reacts with calcium carbonate.
CaCO3 + 2HCl  CaCl2 + CO2 + H2O
Sulphuric acid gives calcium sulphate, carbon dioxide gas, calcium sulphate and water when it reacts with calcium carbonate.
CaCO3 + H2SO4  CaSO4 + CO2 + H2O

Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium carbonate.
Na2CO3 + H2SO4  Na2SO4 + CO2 + H2O
Nitric acid gives sodium nitrate, water and carbon dioxide gas when it reacts with sodium carbonate.
2HNO3 + Na2CO3  NaNO3 + 2H2O + CO2

Reaction of acid with hydrogen carbonates (bicarbonates):
Acids give carbon dioxide gas, respective salt and water when they react with metal hydrogen carbonate.
Acid + Metal hydrogen carbonate  Salt + Carbon dioxide + Water
Examples: Hydrochloric acid gives carbon dioxide, sodium chloride and water when it reacts with sodium bicarbonate.
NaHCO3 + HCl  NaCl + CO2 + H2O
Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium bicarbonate.
2NaHCO3 + H2SO4  Na2SO4 + 2CO2 + 2H2O
Sodium bicarbonate is also known as sodium hydrogen carbonate, baking soda, baking powder, bread soda and bicarbonate of soda.
Test for evolution of carbon dioxide gas: Carbon dioxide turns lime water milky when passed through it. This is the characteristic test for carbon dioxide gas.
The gas evolved because of reaction of acid with metal carbonate or metal hydrogen carbonate turns lime water milky. This shows that the gas is carbon dioxide gas. This happens because of formation of white precipitate of calcium carbonate.
Ca(OH)2 + CO2  CaCO3 + H2O
CaCO3 + CO2 + H2O  Ca(HCO3 )2
But when excess of carbon dioxide is passed through lime water, it makes milky colour of lime water disappear. This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is soluble in water, thus the milky colour of solution mixture disappears.

Reaction of acid with marble and egg shell: Since, marble and egg shell are made of calcium carbonate, hence when acid is poured over marble or egg shell, bubbles of carbon dioxide are formed.

Historical Monuments and Acid Rain:
Burning of fossil fuels releases oxides of sulphur and nitrogen. Nitrogen oxide and sulphur dioxide form nitric acid and sulphuric acid on reaction with water. When rain droplets mix with these gases; present in atmosphere because of pollution; they form acid rain.
Acid rain causes damage to the historical monuments and other buildings. For example Taj Mahal, which is made of marble, is getting damaged because of reaction with acid rain. Marble is calcium carbonate which reacts with the acid and thus gets corroded.

Acid & Base and Fire Extinguisher
Metal carbonate or metal hydrogen carbonate and acid are used in fire extinguisher to produce carbon dioxide gas. Acid and metal carbonate or bicarbonate are kept in separate chambers in a fire extinguisher. On emergency they are allowed to react with one another. The carbon dioxide gas so produce is poured over fire. As carbon dioxide does not support burning, it puts off the fire.

Base: Base is bitter in taste and feels soapy on touch. A base turns red litmus paper blue.
Sodium hydroxide (caustic soda), calcium hydroxide, sodium carbonate (washing soda), lime (Calcium oxide), potassium hydroxide (caustic potash), etc. are examples of base.

Types of base: Base can be divided in two types – Water soluble and water insoluble.
The ionic salt of alkali and alkaline earth metals are soluble in water. These are also known as alkali. For example – sodium hydroxide, magnesium hydroxide, calcium hydroxide, etc. Alkali is considered as strong base.

Reaction of Base with Metals:
When alkali (base) reacts with metal, it produces salt and hydrogen gas.
Alkali + Metal  Salt + Hydrogen
Example: Sodium hydroxide gives hydrogen gas and sodium zincate when reacts with zinc metal.
2NaOH + Zn  Na2ZnO2 + H2
Sodium aluminate and hydrogen gas are formed when sodium hydroxide reacts with aluminium metal.
2NaOH + 2Al + 2H2O  2NaAlO2 + 2H2

Reaction of base with oxides of non-metals:
Non-metal oxides are acidic in nature. For example; carbon dioxide is a non-metal oxide. When carbon dioxide is dissolved in water it produces carbonic acid.
Therefore, when a base reacts with non-metal oxide both neutralize each other resulting respective salt and water are produced.
Base + Non-metal oxide  Salt + Water
Example: Sodium hydroxide gives sodium carbonate and water when it reacts with carbon dioxide.
2NaOH + CO2  Na2CO3 + H2O
Calcium hydroxide gives calcium carbonate and water when it reacts with carbon dioxide.
Ca(OH)2 + CO2  CaCO3 + H2O

Neutralisation Reaction:
An acid neutralizes a base when they react with each other and respective salt and water are formed.
Acid + Base  Salt + Water
Since in the reaction between acid and base both neutralize each other, hence it is also known as neutralization reaction.
Example: Sodium chloride and water are formed when hydrochloric acid reacts with sodium hydroxide (a strong base).
HCl + NaOH  NaCl + H2O

In similar way, calcium chloride is formed along with water when hydrochloric acid reacts with calcium hydroxide (a base).
2HCl + Ca(OH)2  CaCl2 + 2H2O
Sodium sulphate and water are formed when sulphuric acid reacts with sodium hydroxide (a base).
H2SO4 + 2NaOH  Na2SO4 + 2H2O
In similar way, when nitric acid reacts with sodium hydroxide, sodium nitrate and water are formed.
HNO3 + NaOH  NaNO3 + H2O

Reaction of Acid with Metal Oxides:
Metal oxides are basic in nature. Thus, when an acid reacts with a metal oxide both neutralize each other. In this reaction, respective salt and water are formed.
Acid + Metal Oxide  Salt + Water
Example: Calcium is a metal, thus calcium oxide is a metallic oxide which is basic in nature. When an acid; such as hydrochloric acid; reacts with calcium oxide, neutralization reaction takes place and calcium chloride; along with water; is formed.
2HCl + CaO  CaCl2 + H2O
Similarly, when sulphuric acid reacts with zinc oxide, zinc sulphate and water are formed.
H2SO4 + ZnO  ZnCl2 + H2O
When hydrochloric acid reacts with aluminium oxide, aluminium chloride and water are formed.
Al2O3 + 6HCl  2AlCl3 + 3H2O

Common in Acid and Base
Acids give hydrogen gas when they react with metal. This shows that all acids contain hydrogen. For example: Hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3), etc.
When an acid is dissolved in water, it dissociates hydrogen. The dissociation of hydrogen ion in aqueous solution is the common property in all acids. Because of dissociation of hydrogen ion in aqueous solution, an acid shows acidic behavior.
Example: Hydrochloric acid (HCl) gives hydrogen ion (H+) and chloride ion (Cl−) when it is dissolved in water.
HCl (aq)  H+ + Cl−
Sulphuric acid (H2SO4) gives hydrogen ion (H+) and sulphate ion (SO4− −) in water.
H2SO4 (aq)  H+ + SO4− −
Nitric acid (HNO3) gives hydrogen ion (H+) and nitrate ion (NO3−) in water.
HNO3 (aq)  H+ + NO3−
Acetic acid (CH3COOH) gives acetate ion (CH3COO−) and hydrogen ion (H+).
CH3COOH (aq)  CH3COO− H+
Hydrogen ion which is produced by acid (when acid is combined with water molecule), exists in the form of hydronium ion (H3O−) in aqueous solution. That’s why hydrogen ion is always written with suffix (aq), such as H+ (aq).
HCl + H2O  H3O− + Cl−
H2SO4 + H2O  H3O− + SO4− −
Thus, because of dissociation of hydrogen ions; acid shows its acidic behavior.
Acids conduct electricity in their aqueous solution because of dissociation of hydrogen ion. Hydrogen ion in aqueous solution conducts electricity.
A dry acid, such as dry hydrochloric acid does not change the colour of blue litmus paper to red because a dry acid does not dissociate hydrogen ion. This is the cause that a moist litmus paper is used to check the acidic or basic character of a gas.

Acidic behavior of carbon dioxide gas: Carbon dioxide gas produces carbonic acid when dissolved in water. This carbonic acid dissociates hydrogen ion and carbonate ion in the aqueous solution.
CO2 + H2O  H2CO3  H+ + CO3− −
Are all compounds which contain hydrogen, necessarily acids?
No, all compounds which contain hydrogen are not acid. For example; glucose (C6H12O6), methyl alcohol (CH3OH), etc. are not acid in spite of the fact that they contain hydrogen. This is because these compounds do not dissociate hydrogen ion in their aqueous solution.

Common in all Base:
A base dissociates hydroxide ion in water, which is responsible for the basic behavior of a compound. Example:
When sodium hydroxide is dissolved in water, it dissociates hydroxide ion and sodium ion.
NaOH (aq)  Na+ + OH−
Similarly, when potassium hydroxide is dissolved in water, it dissociates hydroxide ion and potassium ion.
KOH (aq)  K+ + OH−
Thus, base shows its basic character because of dissociation of hydroxide ion.

Neutralisation Reaction:
When an acid reacts with a base, the hydrogen ion of acid combines with the hydroxide ion of base and forms water. As these ions combine together and form water; instead of remaining free, thus both neutralize each other.
OH− + H+  H2O
Example: When sodium hydroxide (a base) reacts with hydrochloric acid, sodium hydroxide breaks into sodium ion and hydroxide ion and hydrochloric acid breaks into hydrogen ion and chloride ion. Hydrogen ion and hydroxide ion combine together and form water, while sodium ion and chloride ion combine together and form sodium chloride.
NaOH + HCl  OH− + Na+ + H+ + Cl−  NaCl + H2O

Dilution of Acid and Base:
The concentration of hydrogen ion in an acid and hydroxide ion in a base; per unit volume; shows the concentration of acid or base.
By mixing of acid to water, the concentration of hydrogen ion per unit volume decreases. Similarly, by addition of base to water the concentration of hydroxide ion per unit volume decreases. This process of addition of acid or base to water is called dilution and the acid or base is called diluted.
The dilution of acid or base is exothermic. Thus, acid or base is always added to water and water is never added to acid or base. If water is added to a concentrated acid or base a lot of heat is generated, which may cause splashing out of acid or base and may cause severe damage as concentrated acid and base are highly corrosive.

Strength of Acid and Base
Acids in which complete dissociation of hydrogen ion takes place are called strong acid. Similarly, bases in which complete dissociation of hydroxide ion takes place are called strong base.
In mineral acids, such as hydrochloric acid, sulphuric acid, nitric acid, etc. hydrogen ion dissociates completely and hence they are considered as strong acid. Since, in organic acids hydrogen ions do not dissociate completely, so they are weak acid.
Alkalis are water soluble base, thus in alkali; complete dissociation of hydroxide ions takes place and they are considered as strong base.
The complete dissociation of hydrogen ions or hydroxide ions is shown by a single arrow. The incomplete dissociation of hydrogen ions or hydroxide ions is denoted by double arrow.

Example of complete dissociation:
NaOH (aq)  Na+ + OH−
HCl  H+ + Cl−

Example of incomplete dissociation:
Mg(OH)2  Mg+ + + OH− −
CH3COOH  CH3COO− + H+
Although acetic acid being an organic acid is a weak acid, but concentrated acetic acid is corrosive and can damage the skin if poured over it.

pH – Measurement of strength of Acid and Base
The strength of acid or base depends upon the hydrogen ion concentration. If the concentration of hydrogen ion is greater than hydroxide ion, the solution is called acidic. If the concentration of hydrogen ion is smaller than the hydroxide ion, the solution is called basic. If the concentration of hydrogen ion is equal to the concentration of hydroxide ion, the solution is called neutral solution.
pH is a scale which quantifies the concentration of hydrogen ion in a solution. The range of pH scale is between 0 to 14.

The pH value decreases with increase in hydrogen ion concentration. If the value of pH is 0, this shows maximum hydrogen ion concentration. pH value equal to 14 shows lowest hydrogen ion concentration. pH value equal to 7 shows the hydrogen ion concentration is equal to hydroxide ion concentration.

A neutral solution, such as distilled water has value of hydrogen ion concentration equal to 7 on pH scale. The acidic solution has value of hydrogen ion concentration less than 7 on pH scale. The basic solution has value of hydrogen ion concentration greater than 7 on pH scale.
In pH scale ‘p’ stands for ‘potenz’. Potenz is a German word which means ‘power’ or ‘potential’. Here; ‘H’ stands for hydrogen ion. Thus, pH means the potential of hydrogen or power of hydrogen.
pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity (a(H+ )), in a solution.

Indicator:
Substances which show the acidic or basic behavior of other substance by change in colour are known as indicator.

Type of Indicator: There are many types of indicators. Some common types of indicators are
1.Natural
2.Olfactory Indicator
3.Synthetic Indicator
4.Universal Indicator

Natural Indicator: Indicators obtained from natural sources are called natural indicators. Litmus, turmeric, red cabbage, China rose, etc. are some common natural indicators used widely to show the acidic or basic character of substances.

Litmus: Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colour – blue and red.
1.An acid turns blue litmus paper red.
2.A base turns red litmus paper blue.

Turmeric: Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.

Red cabbage: The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.

Olfactory Indicators: Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example onion, vanilla, clove, etc.

Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.

Vanilla: The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid.
Olfactory indicators are used to ensure the participation of visually impaired students in laboratory.

Synthetic Indicator: Indicators that are synthesized in laboratory are known as synthetic indicators. For example; phenolphthalein, methyl orange, etc.

1.Phenolphthalein is a colourless liquid. It remains colourless with acid but turns into pink with a base.
2.Methyl orange is originally orange in colour. It turns into red with acid and turns into yellow with base.

Indicator
Original colour
Acid
Base
Red litmus
Red
No change
Blue
Blue litmus
Blue
Red
No change
Turmeric
Yellow
No change
Reddish brown
Red cabbage juice
Purple
Reddish
Greenish yellow
Phenolphthalein
Colourless
Colourless
Pink
Methyl orange
Orange
Red
Yellow
Onion
n/a
No change
Smell vanishes
Vanilla
n/a
No change
Smell vanishes

Universal Indicator:
Using a litmus paper, phelophthalein, methyl orange, etc. only the acidic or basic character of a solution can be determined, but use of these indicators does not give the idea about the strength of acid or base. So, to get the strength as well as acidic and basic nature of a given solution universal indicator is used.
Universal indicator shows different colour over the range of pH value from 1 to 14 for a given solution. Universal indicator is available both in the form of strips and solution. Universal indicator is the combination of many indicators, such as water, propanol, phelophthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt. The colour matching chart is supplied with universal indicator which shows the different colours for different values of pH.

Characteristics of salt:
1. Most of the salts are crystalline solid
2. Salts may be transparent or opaque
3. Most of the salts are soluble in water
4. Solution of salts conducts electricity. Salts conduct electricity in their molten state also
5.The salt may be salty, sour, sweet, bitter and umami (savoury)
6.Neutral salts are odourless
7.Salts can be colourless or coloured

Salt
Salts are the ionic compounds which are produced after the neutralization reaction between acid and base. Salts are electrically neutral. There are number of salts but sodium chloride is the most common among them. Sodium chloride is also known as table salt or common salt. Sodium chloride is used to enhance the taste of food.

Family of Salt:
Salts having common acidic or basic radicals are said to belong to same family.
Example: Sodium chloride (NaCl) and Calcium chloride (CaCl2) belong to chloride family.
Calcium chloride (CaCl2) and calcium sulphate (CaSO4) belong to calcium family.
Zinc chloride (ZnCl2) and Zinc sulphate (ZnSO4) belong to zinc family.
Acidic, Basic and Neutral Salts

Neutral Salt: Salts produced because of reaction between strong acid and strong base are neutral in nature. The pH of value of such salts is equal to 7, i.e. neutral. Example; Sodium chloride, sodium sulphate, potassium chloride, etc.
Sodium chloride (NaCl) is formed after the reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).
NaOH + HCl  NaCl + H2O

Sodium sulphate (Na2SO4): It is formed after the reaction between sodium hydroxide (a strong 
base) and sulphuric acid (a strong acid).
2NaOH + H2SO4  Na2SO4 + 2H2O

Potassium chloride (KCl): It is formed after the reaction between potassium hydroxide (a strong base) and hydrochloric acid (a strong acid).
KOH + HCl  KCl + H2O

Acidic salt: Salts which are formed after the reaction between a strong acid and weak base are called acidic salt. The pH value of acidic salt is lower than 7. Example: ammonium sulphate, ammonium chloride, etc.
Ammonium chloride is formed after reaction between hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base).
NH4OH + HCl  NH4Cl + H2O
Ammonium sulphate is formed after reaction between ammonium hydroxide (weak base) and sulphuric acid (a strong acid).
2NH4OH + H2SO4  (NH4 )2SO4 + 2H2O

Basic Salt: Salts which are formed after the reaction between weak acid and strong base are called basic salt. For example; sodium carbonate, sodium acetate, etc.
Sodium carbonate is formed after the reaction between sodium hydroxide (a strong base) and carbonic acid (a weak base).
H2CO3 + 2NaOH  Na2CO3 + 2H2O
Sodium acetate is formed after the reaction between a strong base, sodium hydroxide and a weak acid, acetic acid.
CH3COOH + NaOH  CH3COONa + H2O
Cause of formation of acidic, basic and neutral salt:
When a strong acid reacts with a weak base, the base is unable to fully neutralize the acid. Due to this an acidic salt is formed in this case.
When a strong base reacts with a weak acid, the acid is unable to fully neutralize the base. Due to this a basic salt is formed in this case.
When equally strong acid and base react they fully neutralize each other. Due to this a neutral salt is formed in this case.

pH value of salt:
1.Neutral salt: The pH value of a neutral salt is almost equal to 7.
2. Acidic salt: The pH value of an acidic salt is less than 7.
3.Basic salt: The pH value of a basic salt is more than 7.

Common Salt (Sodium Chloride)
Sodium chloride (NaCl) is also known as common or table salt. It is formed after the reaction between sodium hydroxide and hydrochloric acid. It is a neutral salt. The pH value of sodium chloride is about 7. Sodium chloride is used to enhance the taste of food. Sodium chloride is used in manufacturing of many chemicals.
Important chemicals from sodium chloride:

Sodium Hydroxide (NaOH): Sodium hydroxide is a strong base. It is also known as caustic soda or Iye. It is obtained by the electrolytic decomposition of solution of sodium chloride (brine). In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide. In this process, chlorine is obtained at anode and hydrogen gas is obtained at cathode as byproducts. This whole process is known as Chlor-Alkali process.
2NaCl + 2H2O  2NaOH + Cl2 + H2
Use of products after the electrolysis of brine:
1.Hydrogen gas is used as fuel, margarine, in making of ammonia for fertilizer, etc.
2.Chlorine gas is used in water treatment, manufacturing of PVC, disinfectants, CFC, pesticides. It is also used in manufacturing of bleaching powder and hydrochloric acid.
3.Sodium hydroxide is used for de-greasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc.

Bleaching Powder (CaOCl2 ):
Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine.
When calcium hydroxide (slaked lime) reacts with chlorine, it gives calcium oxychloride (bleaching powder) and water is formed.
Ca(OH)2 + Cl2  CaOCl2 + H2O
Aqueous solution of bleaching powder is basic in nature. The term bleach means removal of colour. Bleaching powder is often used as bleaching agent. It works because of oxidation. Chlorine in the bleaching powder is responsible for bleaching effect.

Use of bleaching powder:
1.Bleaching powder is used as disinfectant to clean water, moss remover, weed killers, etc.
2.Bleaching powder is used for bleaching of cotton in textile industry, bleaching of wood pulp in paper industry.
3.Bleaching powder is used as oxidizing agent in many industries, such as textiles industry, paper industry, etc.

Baking Soda (NaHCO3 )
Baking soda is another important product which can be obtained using byproducts of chlor-alkali process. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO3 ) or sodium bicarbonate. Bread soda, cooking soda, bicarbonate of soda, sodium bicarb, bicarb of soda or simply bicarb, etc. are some other names of baking soda.
Baking soda is obtained by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process.
NaCl + CO2 + NH3 + H2O  NH4Cl + NaHCO3
In this process, calcium carbonate is used as the source of CO2 and the resultant calcium oxide is used to recover ammonia from ammonium chloride.

Properties of sodium bicarbonate:
1.Sodium bicarbonate is white crystalline solid, but it appears as fine powder.
2.Sodium hydrogen carbonate is amphoteric in nature.
3.Sodium hydrogen carbonate is sparingly soluble in water.
4.Thermal decomposition of sodium hydrogen carbonate (baking soda).
5.When baking soda is heated, it decomposes into sodium carbonate, carbon dioxide and water.
2NaHCO3 + heat  Na2CO3 + CO2 + H2O
6.Sodium carbonate formed after thermal decomposition of sodium hydrogen carbonate; decomposes into sodium oxide and carbon dioxide on further heating.
Na2CO3  Na2O + CO2
This reaction is known as dehydration reaction.

Use of Baking Soda:
1.Baking soda is used in making of baking powder, which is used in cooking as it produces carbon dioxide which makes the batter soft and spongy.
2.Baking soda is used as antacid.
3.Baking soda is used in toothpaste which makes the teeth white and plaque free.
Baking soda is used in cleansing of ornaments made of sliver.
4.Since, sodium hydrogen carbonate gives carbon dioxide and sodium oxide on strong heating, thus it is used as fire extinguisher.

Baking powder:
Baking powder produces carbon dioxide on heating, so it is used in cooking to make the batter spongy. Although baking soda also produces carbon dioxide on heating, but it is not used in cooking because on heating; baking soda produces sodium carbonate along with carbon dioxide. The sodium carbonate; thus produced; makes the taste bitter.
2NaHCO3 + heat  Na2CO3 + CO2 + H2O
Baking powder is the mixture of baking soda and a mild edible acid. Generally, tartaric acid is mixed with baking soda to make baking powder.
NaHCO3 + C4H6O6  CO2 + H2O + Na2C4H4O6
When baking powder (mixture of baking soda and an edible acid) is heated, the sodium carbonate formed because of heating of baking soda neutralizes after reacting with tartaric acid and sodium tartarate salt is formed. The smell of sodium tartarate is pleasant and taste is good. This makes the cake or any other food tasty.

Washing Soda (Sodium carbonate)
Sodium carbonate is manufactured by the thermal decomposition of sodium hydrogen carbonate obtained by Solvay process.
NaCl + CO2 + NH3 + H2O  NH4Cl + NaHCO3
NaHCO3 + C4H6O6  CO2 + H2O + Na2C4H4O6
The sodium carbonate obtained in this process is dry. It is called soda ash or anhydrous sodium carbonate. Washing soda is obtained by rehydration of anhydrous sodium carbonate.
Na2CO3 + 10H2O  Na2CO3.10H2O
Since there are 10 water molecules in washing soda, hence it is known as Sodium bicarbonate decahydrate.
Sodium carbonate is a crystalline solid and it is soluble in water when most of the carbonates are insoluble in water.

Use of sodium carbonate:
1.It is used in cleaning of cloths; especially in rural areas.
2.In making of detergent cake and powder.
3.In removing permanent hardness of water.
4.It is used in glass and paper industries.

Water of crystallization: Many salts contain water molecule and are known as hydrated salts. The water molecule present in salt is known as water of crystallization.
Examples:

Copper sulphate pentahydrate (CuSO4.5H2O):Blue colour of copper sulphate is due to presence of 5 molecules of water. When copper sulphate is heated, it loses water molecules and turns into grey-white colour, which is known as anhydrous copper sulphate. After adding water; anhydrous copper sulphate becomes blue again.
CuSO4.5H2O + heat  CuSO4

Ferrous Sulphate heptahydrate (FeSO4.7H2O): The green colour of Ferrous sulphate heptahydrate; commonly known as ferrous sulphate; is due to the presence of 7 molecules of water in it.

Plaster of Paris
Plaster of Paris is obtained by heating of gypsum, a hydrated salt of calcium.
CaSO4.2H2O + Heat  CaSO4.(0.5)H2O + (1.5)H2O
After addition of water Plaster of Paris is again converted into gypsum.
CaSO4.(0.5)H2O + (1.5)H2O  CaSO4.2H2O

Plaster of Paris is used in making of toys, designer false ceiling, etc. Doctors use Plaster of Paris to set the fractured bone.

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