Lesson 2 - Acids, Bases and Salts-Class 10 Science Notes

 Acids, Bases and Salts

Indicator

Indicators are substances which indicate the acidic or basic nature of the solution by the colour change.

Types of Indicator

There are many types of indicators-

1. Natural Indicators

Indicators obtained from natural sources are called natural indicators.

Example- Litmus, turmeric, red cabbage, china rose, etc. 

Litmus:- Litmus is obtained from lichens. The solution of litmus is purple in color. Litmus paper comes in two colors- blue and red.

* An acid turns blue litmus paper to red.

* A base turns red litmus paper to blue.

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

2. Olfactory Indicators

Substances which change their smell when mixed with acid or base are known as Olfactory Indicators. 

Example- Onion, vanilla 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 its smell does not vanish with an acid.

(*** Olfactory Indicators are used to ensure the participation of visually impaired students in the laboratory)

3. Synthetic Indicators

Indicators that are synthesized in the laboratory are known as Synthetic Indicators. 

Example- Phenolphthalein, methyl orange, etc.

Phenolphthalein is a colorless liquid. It remains colorless with acid but turns into pink with a base.

Methyl orange is originally orange in color. It turns into the red with acid and turns into yellow with base.

4. Universal Indicators

Universal indicator is the combination of many indicators, such as water, propanol, phenolphthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt. Universal indicator show different color 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.

Colour of different indicators

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
Flower of hydrangea plant	Blue/Pink	Blue	pink
Phenolphthalein	Colourless	Colourless	Pink
Methyl orange	Orange	Red	Yellow
Onion	N/A	No change	Smell vanishes
Vanilla	N/A	No change	Smell vanishes
Clove oil	Pale yellow	No change	Smell vanishes

Acids

Acids are sour in taste, turn blue litmus red, and dissolve in water to release H+ ions.

Example- Sulphuric acid (H2SO4), Acetic acid (CH3COOH), Nitric acid (HNO3) etc.

Properties of acids

1. Acids are sour in taste.

2. All acids give H+ hydrogen ion with water.

3. It turns color of blue litmus to red.

4. Acids conduct electricity current.

5. It reacts with metals produces salt and hydrogen gas.

Types of acids

Acids are divided into two types on the basis of their occurence-

1. Natural Acids:- Acids which are obtained from natural sources are called Natural acids or Organic acids.

Example- Methanoic acid (HCOOH)

Acetic acid (CH3COOH)

Oxalic acid (C2H2O4) etc.

2. Mineral Acids:- Acids that are prepared from minerals are known as Mineral acids. 

Example- Hydrochloric acid (HCl)

Sulphuric acid (H2SO4)

Nitric acid (HNO3)

Carbonyl acid (H2CO3)

Phosphoric acid (H3PO4) etc. 

Some common acids and their sources

Sources	Acids	Chemical formula
Vinegar	Acetic acid	CH3COOH
Guava, Amla	Ascorbic acid	C6O8H6
Lemon, Orange and other citrus fruits	Citric acid	C6O8H7
Sour milk, curd	Lactic acid	C3H6O3
Ant sting, Nettle sting	Formic acid	HCOOH
Wasp sting	Formic acid, Hydrochloric acid, Orthophosphoric acid	HCOOH, HCl, H3PO4
Tomato	Oxalic acid	C2H2O4
Tamarind	Tartaric acid	C4H6O6

Chemical Properties of Acids and Bases

1. Reaction of acids with metals

Acids react with metals give corresponding salt and hydrogen gas.

Acid + Metal → Salt + Hydrogen gas

Example:- (i) Hydrochloric acid reacts with zinc and produces zinc chloride and hydrogen gas.

2HCl + Zn → ZnCl2 + H2

(ii) Hydrochloric acid reacts with sodium and gives sodium chloride and hydrogen gas.

2HCl + 2Na → 2NaCl + H2

(iii) Sulphuric acid reacts with zinc and produces zinc sulphate and hydrogen gas.

H2SO4 + Zn → ZnSO4 + H2

Testing of Hydrogen gas

When we react any metal with an acid the reaction gives corresponding salt and produces hydrogen gas. During this period of reaction, when we place a burnt candle near by gas it produces a pop sound. The pop sound indicates that the produced gas is hydrogen.

2. 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, the respective salt and water are formed.

Acid + Metal oxide → Salt + Water

Example:- 1. Calcium oxide is a metallic oxide which is basic in nature. When hydrochloric acid, reacts with calcium oxide, neutralization reaction takes place nd calcium chloride along with water is formed.

2HCl (aq) + CaO → CaCl2 (aq) + H2O (l)

2. When sulphuric acid reacts with zinc oxide, zinc sulphate and water are formed.

H2SO4 (aq) + ZnO (s) → ZnCl2 (aq) + H2O (l)

3. Reaction of acid with metal carbonate

When acids react with metal carbonates, they give carbon dioxide gas and respective salts along with water when they react with metal carbonates.

Acid + Metal carbonate → Salt + carbon dioxide + water

Example:- (i) Hydrochloric acid reacts with sodium carbonate produces carbon dioxide gas, sodium chloride along with water.

Na2CO3(aq) + HCl (aq) → 2NaCl (aq) + CO2 (g) + H2O (l)

(ii) Sulphuric acid gives Calcium sulphate, Carbon dioxide gas with water when it reacts with Calcium carbonate.

CaCO3 (s) + H2SO4 (aq) → CaSO4 (aq) + CO2 (g) + H2O (l)

(iii) Nitric acid gives Sodium nitrate, carbon dioxide gas with water when it reacts with Sodium carbonate.

2HNO3 (aq) + Na2CO3 (aq) → Na2NO3 (aq) + H2O (g) + CO2 (g)

4. 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

Example:- (i) Sulphuric acid gives Sodium sulphate, carbon dioxide gas and water when it reacts with Sodium bicarbonate.

2NaHCO3 (aq) + H2SO4 (aq) → NaCl (aq) + CO2(g) + H2O (l)

(ii) Sodium carbonate reacts with Hydrochloric acid produces Sodium chloride 

NaHCO3 (aq) + HCl → NaCl (aq) + CO2 (g) + H2O (l)

Use of Reaction between Acid and Metal Carbonate or Metal Bicarbonate in Fire Extinguisher:-

Metal carbonate or metal bicarbonate 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.

Test for Evolution of Carbon Dioxide gas

Carbon dioxide turns lime water milky when passed through it. This is because of reaction of acid with metal carbonate or metal hydrogen carbonate. This happens because of formation of white precipitate of calcium carbonate..

Ca(OH)2 + CO2 → CaCO3 + H2O

CaCO3 (s) + CO2 (g) + H2O (l) → Ca(HCO3)2 (aq) 

    But when excess of carbon dioxide is passed through lime water, it makes milky color of lime water disappear. This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is soluble in water. Thus the milky color of solution mixture disappears.

Common property of Acids

Acids give hydrogen gas when they react with metal. This shows that all acids contains hydrogen.

Example: Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3), etc.

When an acid is dissolved in water, it dissociates hydrogen. Because of dissociation of hydrogen ion (H+) in aqueous solution, an acid shows acidic behavior.

Example- 1. Hydrochloric acid (HCl) gives hydrogen ion (H+) and chloride ion (Cl-) when it is dissolved in water.

HCl (aq) → H+ (aq) + Cl- (aq)

2. Acetic acid (CH3COOH) gives acetate ion (CH3COOH-) and hydrogen ion (H+).

CH3COOH (aq) → CH3COO- (aq) + H+ (aq)

As hydrogen seems to be common to all acids, but all compounds having hydrogen are not acids.

Example- we take three substances-

1. Vinegar - CH3COOH - Acidic

2. Glucose - C6H12O6 - Neither acidic nor basic

3. Alcohal - CH3CH2OH - Neither acidic nor basic

Acids on the basis of ionization capacity

Strong Acids:-

An acid which is completely ionised in water and produces (H+) is called Strong acid.

Example- Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3) etc. 

Weak Acids:-

An acid which is partially ionised in water and thus produces a small amount of hydrogen ions (H+) is called a Weak acid.

Example- Acetic acid (CH3COOH), Carbonyl acid (H2CO3)

When a concentrated solution of acid is diluted by mixing water, then the concentration of Hydrogen ions (H+) or hydronium ion (H3O-) per unit volume decreases.

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.

NO2 + H2O → HNO3

SO2 + H2O → H2SO4

When rain droplets mix with these gases, present in atmosphere because of pollution, they form acid rain.

Causes:- Acid rain causes damage to the historical monuments and other buildings. 

Example- Taj Mahal, which is made of marble, is getting damaged because of reaction with acid rain. Marble is calcium carbonate (CaCO3) which reacts with acid and thus gets corroded.

Bases

Bases are bitter in taste, have soapy touch, turn red litmus blue and give hydroxide ions (OH-) in aqueous solution.

Example- Sodium hydroxide (Caustic soda) - NaOH

Calcium hydroxide - Ca(OH)2

Potassium hydroxide (Caustic potash) - KOH

Properties of Bases

1. Have a bitter taste.

2. Soapy to touch.

3. Turns red litmus blue.

4. Conducts electricity in solution.

5. Release OH- ions in Aqueous solution.

Types of Bases

Bases can be divided in two types- Water soluble and Water insoluble.

The hydroxide of alkali and alkaline earth metals are soluble in water. These are also known as alkali.

Example- Sodium hydroxide (NaOH), Magnesium hydroxide (Mg(OH)2), Calcium hydroxide (Ca(OH)2), etc.

Alkali is considered as a strong base.

Chemical properties of bases

(i) Reaction of Base with Metal:- When alkali (base) reacts with metal, it produces salt and hydrogen gas.

Alkali + Metal → Salt + Hydrogen

Example-1. Sodium hydroxide gives hydrogen gas and Sodium zincate when reacts with zinc metal.

2NaOH (aq) + Zn (s) → Na2ZnO2 (aq) + H2 (g)

2. Sodium aluminate and hydrogen gas are formed when sodium hydroxide reacts with aluminum metal.

2NaOH (aq) + Al (s) + H2O (l) → 2NaAlO2 (aq) + 3H2 (g)

(ii) Reaction of Base with Oxides of  Non-metals:- Non-metal oxides are acidic in nature.

Example- Carbon dioxide is non-metal oxide. When carbon dioxide is dissolved in water it produces carbonyl acid.

Therefore, when a base is react with non-metal oxide, both neutralize each other resulting in water it produces salt and water.

Base + Non-metal oxide → Salt + Water

Example:- 1. Sodium hydroxide gives sodium carbonate and water when it reacts with carbon dioxide.

2NaOH (aq) + CO2 (g) → Na2CO3 (aq) + H2O (l)

2. Calcium hydroxide gives calcium carbonate and water when it reacts with carbon dioxide.

Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)

(iii) Neutralization reaction:- An acid neutralizes a base when they react with each other and respective salt and water are formed.

Acid + Base → Salt + Water

Since, the reaction between acid and base both neutralize each other. Hence it is known as Neutralization reaction.

Example:- Sodium chloride and water are formed when hydrochloric acid reacts with sodium hydroxide (a strong base).

2HCl (aq) + Ca(OH)2 (aq) → CaCl2 (aq) + 2H2O (l)

Common property of Bases

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+ (aq) + OH- (aq)

When Potassium hydroxide is dissolved in water, it dissociates hydroxide ion and potassium ion.

KOH (aq) → K+ (aq) + OH- (aq)

The base shows its basic character because of dissociation of hydroxide ion.

Concentration of Acid and Base

The concentration of hydrogen ion in acid and hydroxide ion in base per unit volume shows the concentration of acid or base.

Dilution of Acid and Base

By mixing of acid or base to water, the concentration of hydrogen ion or hydroxide ion per unit volume decreases respectively. 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.

 pH Scale

A scale for measuring hydrogen ion concentration in a solution. The pH of a solution is defined as the negative logarithm of hydrogen ion concentration in moles per litre.

pH = -log[H+]

pH = -log[H3O+]

where [H+] or [H3O+] represents concentration of hydrogen ions in a solution.

pH scale has different color range from 1 to 14 and each color represents specific pH value. Each pH value represents acidic or basic nature for a solution.

a. The pH of a neutral solution = 7

b. The pH of a acidic solution < 7

c. The pH of a basic solution > 7

pH in our every day life

(i) pH in our digestive system: Dilute HCl (Hydrochloric acid) helps in digestion of food (protein) in our stomach. Excess acid in stomach causes acidity (indigestion).

Antacids like magnesium hydroxide [Mg(OH)2] also known as milk of magnesia and sodium hydrogen carbonate (NaHCO3) known as baking soda are used to neutralize excess acid.

(ii) Tooth decay caused by acids:- The bacteria present in our mouth converts the sugar into acids. When the pH of acid formed in the mouth falls below 5.5, tooth decaying starts. The excess acid has to be removed by cleaning the teeth with a good quality toothpaste because these kinds of toothpaste ae alkaline in nature.

(iii) pH of blood:- Our blood must maintain a pH around 7.35 - 7.45. If the pH is above 7.45 you would have a condition called alkalosis. If the pH is below 7.35, then one would suffer form acidosis.

(iv) pH of soil and plant growth:- Most of the plants have a healthy growth when the soil has a specific pH (close to 7) range which should be neither alkaline nor highly acidic. 

Salts

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 (NaCl) is the most common salt among them. Sodium chloride is also known as table salt or common salt. Sodium chloride is used to enhance the taste of food.

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 in their molten state also.
5. The salt may be salty, sour, sweet, bitter or umami (savoury).
6. Neutral salts are odourless.
7. Salts can be colorless or colored.

Family of salt

Salts having common acidic or basic radicals are said to belong to the same family.

Example:- (i) Sodium chloride (NaCl) and Calcium chloride (CaCl2) belong to chloride family.

(ii) Calcium chloride (CaCl2) and Calcium sulphate (CaSO4) belong to calcium family.

(iii) Zinc chloride (ZnCl2) and Zinc sulphate (ZnSO4) belongs to the zinc family.

Types of salts

(i) Neutral salt:- Salts produced because of reaction between a srong acid and strong base. The pH value of such salts is equal to 7.

Example:- Sodium chloride, Sodium sulphate, Potassium chloride etc.

(i) Sodium chloride (NaCl) is formed after the reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).

NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)

(ii) Sodium sulphate is formed after the reaction between sodium hydroxide (a strong base) and sulphuric acid (a strong acid).

2NaOH (aq) +H2SO4 (aq) → Na2SO4 (aq) + 2H2O (l)

(ii) Acidic salts:-  Salts which are formed after the reaction between a strong acid and a weak base. The pH value of acidic salts is lower than 7.

Example:- Ammonium sulphate, Ammonium chloride, etc.

(i) Ammonium chloride is formed after reaction between hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base).

NH4OH (aq) + HCl (aq) → NH4Cl (aq) + H2O (l)

(ii) Ammonium sulphate is formed after reaction between ammonium hydroxide (a weak base) and sulphuric acid (a strong acid).

2NH4OH (aq) + H2SO4 (aq) → (NH4)2SO4 (aq) + 2H2O (l)

(iii) Basic salts:- Salts which are formed after the reaction between a weak acid and strong base are called basic salts. The pH value of basic salts is more than 7.

Example:- Sodium carbonate, Sodium acetate, etc.

(i) Sodium carbonate is formed after the reaction between  sodium hydroxide (a strong base) and carbonic acid ( a weak acid).

H2CO3 (aq) + 2NaOH (aq) → Na2CO3 + H2O (l)

(ii) Sodium acetate is formed after the reaction between sodium hydroxide (a strong base) and acetic acid (a weak acid).

CH3COOH (aq) + NaOH (aq) → CH3COONa (aq) + H2O (l)

Cause of formation of acidic , basic and neutral salts

1. 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.
2. 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.
3. When equally strong acid and a base react, they fully neutralize each other. Due to this, a neutral salt is formed.

Some Important Chemical Compounds

1. Common Salt (Sodium Chloride) (NaCl):

Sodium chloride (NaCl) is also known as common or table salt. It is a neutral salt. The pH value of sodium chloride is 7. Sodium chloride is used to enhance the taste of food. It is used in the manufacturing of  many chemicals.

2. Sodium Hydroxide (NaOH):

Sodium hydroxide is a strong base. It is also known as caustic soda. It is obtained by chlor-alkali process.

Chlor-Alkali process: 

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

This process is taken place by electrolysis of aqueous sodium chloride.

2NaCl (aq) + 2H2O (l) → 2NaOH (aq) + Cl2 (g) + H2 (g)

When electricity is passed through brine, chlorine gas is given off at the anode, hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode.

Products of chlor-alkali process:

1. Sodium hydroxide (NaOH)

2. Chlorine gas (Cl2)

3. Hydrogen gas (H2)

Uses of Sodium hydroxide:

1. It is used for de-greasing metals.

2. In soaps and detergent.

3.In paper making.

4. For making artificial fibres.

Uses of Chlorine gas:

1. It is used in water treatment.

2. In swimming pool.

3. In manufacturing of PVC, CFCs and pesticides.

4. It is also used as disinfectants.

Uses of Hydrogen:

1. It is used as fuels.

2. It is used in Margarine.

3. In manufacturing of ammonia for fertilizers.

**Margarine is a processed food that is designed to taste and look similar to butter.

3. Hydrochloric Acid (HCl): 

Chlorine and Hydrogen are the main products of chlor-alkali process. These are used for production of Hydrochloric acid. 

Chlorine gas is combined with hydrogen gas to produce hydrogen chloride in the presence of UV light.

Cl2 (g) + H2 (g)  → 2HCl (g)

The resulting hydrogen chloride gas is absorbed in deionized water to produce chemically pure hydrochloric acid.

It is used in manufacturing of medicines, cosmetics and ammonium chloride and also used for cleaning steel.

4. Bleaching Powder (CaOCl2):

Bleaching powder is also known as chloride of lime. It is solid and yellowish white in colour. It can easily be 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 (aq) + Cl2 (aq) → CaOCl2 (aq) + H2O (l)

Uses of Bleaching Powder:

1. It is used as disinfectant to clean water, moss(lichen) remover, weed killers etc.

2. It is used for bleaching of cotton in textile industry, bleaching of pulp in paper industry.

3. It is used as oxidizing agent in many industries, such as textiles industry, paper industry etc.

5. Baking Soda (NaHCO3):

The chemical name of  baking soda is Sodium Hydrogencarbonate (NaHCO3) or sodium bicarbonate.

Baking soda is obtained by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process.

NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3

In this process, calcium carbonate is used as the source of CO2 and the resultant calcium oxide (CaO) is used to recover ammonia (NH3) from ammonium chloride(NH4Cl).

Properties:

1. It 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).

    2NaHCO3 +heat → Na2CO3 + CO3 + H2O

    This reaction produces carbon dioxide which is responsible for making bread or cake risen, soft and spongy. The sodium carbonate, thus, produced makes the taste bitter.

5. Sodium carbonate formed after thermal decomposition of sodium hydrogen carbonate decomposes into sodium oxide and carbon dioxide on further heating.
            Na2CO3 +heat → Na2O + CO2

        This reaction is known as dehydration reaction.

Uses:

1. It 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. It is used as antacid.
3. It is used in toothpaste which makes the teeth white and plaque free.
4. It is used in cleansing of ornaments made of silver.
5. Since it gives carbon dioxide and sodium oxide on strong heating, thus it it used as a fire extinguisher.

6. Baking Powder (Baking Soda + Mild Edible Acid):

Baking powder is the mixture of baking soda (NaHCO3) and a mild edible acid (Tartaric acid, C4H6O6). Generally tartaric acid is mixed with baking soda to make baking powder.

NaHCO3 + C4H6O6 → CO2 + H2O + Na2C4H4O6

When baking powder is heated, it decomposes to give CO2 and sodium carbonate (Na2CO3). CO2 causes bread and cake fluffy. Tartaric acid helps to remove bitter taste due to formation of Na2CO3.

7. Washing Soda (Sodium Carbonate, Na2CO3):

Sodium carbonate is manufactured by the thermal decomposition of sodium hydrogen carbonate obtained by Solvay process.

NaCl + CO2 + NH3 + H2O → NH4Cl + NaHCO3

NaHCO3 → CO2 + H2O + Na2CO3

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 crystalline solid and it is soluble in water while most of the carbonates are insoluble in water.

Uses:-

i. It is used in the cleaning of clothes, especially in rural areas.

ii. In the making of detergent cake and powder.

iii. In removing the permanent hardness of water.

iv. It is used in glass and paper industries.

8. Water of Crystallization

The number of water molecules in a formula unit of a salt is called 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.

Example:- CaSO4.1/2H2O    -    Plaster of paris

CaSO4.2H2O       -    Gypsum

CuSO4.5H2O       -    Copper sulphate pentahydrate

i. Plaster of Paris/ POP (CaSO4.1/2H2O)

On heating gypsum (CaSO4.2H2O) at 373K(100C) it loses water molecules and become Plaster of Paris (POP).

CaSO4.2H2O + heating(100C) → CaSO4.1/2H2O + 3/2H2O

It is a white powder and on mixing with water it changes to gypsum.

CaSO4.1/2H2O + 3/2H2O → CaSO4.2H2O

Uses:-

i. It is used in hospital for setting fractured bones in the right position to ensure correct healing.

ii. It is used in making toys, decorative thing.

iii. It is used in making casts of statues.

iv. It is used as fire-proofing material.

v. It is used for making smooth surfaces, ornamental designs on ceiling of houses.

ii. Copper Sulphate Pentahydrate

Blue colour of copper sulphate is due to presence of 5 molecules of water. When copper sulphate is heated above 100C, it loses water molecules and turns into grey-white colour, which is known as anhydrous copper sulphate(CaSO4). This anhydrous copper sulphate(CaSO4) is also known as Dead Burnt Plaster. After adding water, anhydrous copper sulphate becomes blue again.

CaSO4.2H2O + heating(above 100C) → CaSO4

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