Acids, Bases and Salts

Introduction

Acids, Bases and Salts are the three forms in which we can categorise almost all the chemicals. Earlier acids and bases were identified by their taste and some traditional indicators For example, acides taste sour and bases taste bitter. Litmus paper is an indicator which turns red from blue by an acid and blue from red by base. Turmeric is another natural indicator which changes reddish brown from yellow in basic medium.

Now a days synthetic indicators such as methyl orange, phenolphthalein, etc. are used to identify the nature of given sample whether acidic or basic.

Note: Litmus is obtained from a fungus “Roccella” (lichenized genus).

Properties of Acids and Bases

Properties of acids and bases were studied by Lavoisier.

general properties of acids and bases  --

Acids are

1. Sour to taste.
2. Turn blue litmus to red.
3. Neutralize bases.
4. Decompose carbonates to carbon dioxide.
5. Liberate hydrogen has with metals.

Bases are

1. Bitter to taste
2. Turns red litmus to blue.
3. Neutralize acids.
4. Soapy to touch.

Various Theories of Acids and Bases

According to Lavosier all acids should contain oxygen but acids like HCl, HCN are devoid of oxygen, later it is modified by Sir Humpry Davy and proposed that all acids must contains ‘H⁺’. Various theories to explain acids and bases are

1. Arrhenius theory
2. Bronsted Lowery concept
3. Lewis theory

Arrhenius Theory

It is based on the electrolytic dissociation of substances. According to this theory:

Acid - It is a substance that dissociates in water to give ‘H⁺’ ion.

                HCl → H⁺ + Cl^-

                H₂SO4 → 2H⁺ + SO₄^-2

Base - It is substance that dissociates in aqueous solution and liberate OH^- ions

                NaOH → Na⁺ + OH^-

                Ca(OH)₂ → Ca⁺² + 2OH^-

Strong Acid - It dissociates completely, liberates more number of H⁺ ions.

E.g. – HCI, H₂SO₄, HCIO₄ etc.

Weak Acid - It will not dissociate completely, liberates less number of H⁺ ions.

E.g.CH₃COOH, H₂CO3, H₂SO₃, HCN, HCOOH

Strong Bases - It dissociates completely, liberates more number of OH^- ions.

E.g.NaOH, KOH etc.

Weak Bases - It will not dissociate completely, liberated less number of OH^- ions.

E.g. NH₄OH.

Illustration 1:

Classify the following acids and bases in the categories of weak and strong

(i) $$ HNO_{3} $$

(ii) $$ H_{2}SO_{4} $$

(iii) $$ HCIO_{4} $$

(iv) $$ KOH $$

(v) $$ CH_{3}COOH $$

(vi) $$ NH_{4}OH

Solution:                  Weak acids          (v)

                                Strong acids        (i), (ii), (iii),

                                Weak base          (vi)

                                Strong base        (iv)

Neutralization

$$ \underset{Acid}{HCI}+\underset{base}{NaOH}\rightarrow \underset{salt}{NaCI}+\underset{water}{H_{2}O}+Heat $$

The process in which acid & base will combine to give salt & water is called “neutralization”. Or the process in which H⁺ & OH^- ions combine to give water is called “neutralization”.

Exercise 1:

(i) Classify the following as acid, base or salt: $$ NH_{4}OH,K_{2}SO_{4},H_{3}PO_{4},AgCI $$

Ans:  $$ Acid:H_{3}PO_{4};Base:NH_{4}OH;Salt:K_{2}SO_{4},AgCI $$.

(ii) Classify the following as strong or weak acid: $$ HCI, H_{2}SO_{4}, HCN, CH_{3}COOH $$

Ans:  $$ Strong acids:HCI, H_{2}SO_{4};Weak acids: HCN, CH_{3}COOH $$

(iii) Classify the following as strong and weak base: $$ NaOH, AI(OH)_{3}, KOH, NH_{4}OH $$

Ans:  $$ Strong base:NaOH, KOH;Weak base:AI(OH)_{3}, NH_{4}OH $$

Bronsted and Lowry Concept

According to this concept, acids are protogenic, i.e., proton donors and bases are protophilic, i.e., proton acceptors.

Example:

$$ HCL+H_{2}O\rightleftharpoons H_{3}O^{+}+CI^{-} $$

In this reaction HCI acts as an acid because it donates a proton to the water molecule. Water, on the other hand, behaves as a base by accepting a proton from the acid.

$$ NH_{3} + H_{2}O \rightleftharpoons NH^{+}_{4} + OH^{-} $$

In this reaction, H2O acts as an acid as it donates protons to NH3 molecule andNH3 molecule behaves as a base as it accepts a proton.

$$ HF + CH_{3}COOH \rightleftharpoons CH_{3}COOH^{+}_{2} + F^{-} $$

$$ CH_{3}COOH + NH_{3} \rightleftharpoons NH^{+}_{4} + CH_{3}COO^{-} $$

Note:

• After loosing a proton, anion formed from the acid will act as conjugate base and similarly after gaining a proton cation formed from the base will act as conjugate acid
• If acid is strong than its conjugate base is weak and similarly for bases.                                $$ NH_{3} + H^{+} \rightleftharpoons NH^{+}_{4} $$
• They are collectively known as conjugate acid-base pair.
• For example:     $$ HCI \rightleftharpoons H^{+} + CI^{-} $$

Illustration 2:

State the formula and name the conjugate base of each of the following:

(i) $$ H_{3}O^{+} $$

(ii) $$ HSO_{4}^{-} $$

(iii) $$ NH_{4}^{+} $$

(iv) HF

(v) $$ CH_{3}COOH $$

(vi) $$ H_{3}PO_{4} $$

Solution:            

Species	Conjugate Base
	Formula	Name
(i) $$ H_{3}O^{+} $$	$$ H_{2}O $$	Water
(ii) $$ HSO_{4}^{-} $$	$$ SO_{4}^{2-} $$	Sulphate ion
(iii) $$ NH_{4}^{+} $$	$$ NH_{3} $$	Ammonia
(iv) HF	$$ F^{-} $$	Fluoride ion
(v) $$ CH_{3}COOH $$	$$ CH_{3}COO^{-} $$	Acetate ion
(vi) $$ H_{3}PO_{4} $$	$$ H_{2}PO_{4}^{-} $$	Dihydrogen phosphate ion

 

Exercise 2:

(i)           State the formula and name of the conjugate acid of each of the following species:

                (a) $$ OH^{-} $$

                (b) $$ HPO_{4}^{2-} $$

                (c) $$ H_{2}PO_{4}^{-} $$

                (d) $$ CO_{3}^{2-} $$

                (e) $$ NH_{3} $$

                (f) $$ CH_{3}COO^{-} $$

Ans:

               

Species	Conjugate Base
	Formula	Name
(a) $$ OH^{-} $$	$$ H_{2}O $$	Water
(b) $$ HPO_{4}^{2-} $$	$$ HPO_{4}^{-} $$	Dihydrogen phosphate ion
(c) $$ H_{2}PO_{4}^{-} $$	$$ H_{3}PO_{4} $$	Phosphoric acid
(d) $$ CO_{3}^{2-} $$	$$ HCO_{3}^{-}	Bicarbonate ion
(e) $$ NH_{3} $$	$$ NH_{4} $$	Ammonium ion
(f) $$ CH_{3}COO^{-} $$	$$ CH_{3}COOH $$	Acetic acid

 

(ii)          The species $$ H_{2}O, HCO_{3}^{-}, HSO_{4}^{-} $$ and $$ NH_{3} $$ can act both as Bronsted acid and base. For each case, give the corresponding conjugate acid and base.

Ans:               

Conjugate Acid	Species	Conjugate Base
$$ H_{3}O^{+} $$	$$ H_{2}O $$	$$ OH^{-} $$
$$ H_{2}CO_{3} $$	$$ H_{2}CO_{3}^{-} $$	$$ CO_{3}^{2-} $$
$$ H_{2}SO_{4} $$	$$ HSO_{4}^{-} $$	$$ SO_{4}^{2-} $$
$$ \overset{+}{N}H_{4} $$	$$ NH_{3} $$	$$ NH_{2}^{-} $$

(iii)         Name a species which can act both as conjugate acid and conjugate base.

Ans:       $$ H_{2}O $$

Lewis Concept

Acid is electron pair acceptor, base is electron pair donor. So, all electron deficient species and cations are considered to be Lewis acids where all electron rich species and anions are considered to be Lewis bases.

Example:             $$ H_{3}N: + BF_{3} = H_{3}N \rightarrow BF_{3} $$

                                $$ H^{+} + : NH_{3} = [H\leftarrow NH_{3}]^{+} $$

                                $$ BF_{3} + [F^{-}] = [F\rightarrow BF_{3}]^{+} $$

Neutralization of Lewis acids with a Lewis bases results in the formation of co-ordinate bond (represented by and arrow → in the above products)

Other examples of Lewis acids are BCI₃, AICI₃, MgCI₂, BeCI₂etc and those of Lewis bases are H₂O, CN^-, OH^- etc. Note that all Bronsted bases are Lewis bases but all Lewis acides are not Bronsted acids.

Exercise 3:

(i)           Anhydrous $$ AICI_{3} $$ is an acid according to which concept?

Ans:       Lewis concept

(ii)          Select the Lewis acids from the following compounds:

                $$ BF_{3},AICI_{3}HCI,H_{2}SO_{4},H_{3}PO_{4}CO_{2} $$

Ans:       Lewis acids: $$ BF_{3}, AICI_{3} and CO_{2} $$

(iii)         Select the Lewis bases form the following: $$ BF_{3},NH_{3},CN^{-},HCN,CH_{3}COO^{-} $$

Ans:       $$ NH_{3}, CN^{-},CH_{3}COO^{-} $$

Salts

They are formed by neutralization reaction of acid with base.

Acid + Base → Salt + Water

$$ NaOH + HCI \rightarrowNaCI + H_{2}O $$

This reaction is called neutralization reaction.

Salts may be of following types on the basis of nature of acid and base combined

1. Acid salts contains replaceable hydrogen. Example-NaHSO₄.
2. Basic Salts contains replaceable hydroxyl group Example – Pb(OH)NO₃
3. Normal salts does not contain replaceable hydrogen or hydroxyl ion. Example NaCI.
4. Mixed salts contain two distinct cations or anion. Example – CaOCI₂
5. Double salts are stoichiometric combination of two simple salts Example- $$ K_{2}SO_{4}.Cr_{2}(S0_{4})_{3}.24H_{2}O. $$

Exercise 4:

(i)           Which of the following is an acid salt and why?

                (a) $$ K_{2}SO_{4} $$

                (b) $$ KHSO_{4} $$

Ans:       $$ KHSO_{4}, $$ it contains replacable hydrogen.

(ii)          Write the conjugate base of the following: (a) $$ H_{2}O $$, (b) $$ H_{2}SO_{4} $$, (c) $$ NH_{3} $$

Ans:       (a) $$ OH^{-} $$; (b) $$ HSO_{4}^{-1} $$; (c) $$ NH{_{2}}^{-} $$

(iii)         Identify the type of salt of the following:

                (a) $$ NaHCO_{3} $$

                (b) $$ CaCO_{3}.MgCO_{3} $$

                (c) $$ NaKSO_{4} $$

Ans:      

(a)          Acid salt

(b)          Double salt

(c)          Mixed salt

Reaction of Metallic Oxide with Acids and Bases

Most of the metal oxides are basic in nature and reacts with acid to show neutralization and do not react with base.

Metal oxide + Acide $$ \rightarrow $$ Salt + Water

$$ MgO + 2HCI \rightarrowMgCI_{2} + H_{2}O $$

Few of the metal oxides are amphoteric in nature and reacts with acids as well as bases.

Example-

$$ AI_{2}O_{3} + 6HCI \rightarrow 2AICI_{3} + 3H_{2}O $$

$$ AI_{2}O_{3} + 2NaOH \rightarrow 2NaAIO_{2} + H_{2}O $$

Reaction of Non-Metallic Oxide with Acids and Bases

Most of the non metal oxides are acidic n nature and reacts with base and opposite of it doesn’t show any significant reaction with acids.

Non metal oxide + Base $$ \rightarrow $$ Salt + Water

$$ CO_{2} + 2NaOH \rightarrow Na_{2}CO_{3} + H_{2}O $$

Note:    $$ CO_{2} $$ is an acidic non-metallic oxide.

                CO is a neutral non-metallic oxide.

Strength of Acid-Base

Quantitatively it can be measured in terms of pH scale

pH – Scale

In 1909, Sorenson, defined a new scale for measurement of nature of solution. It is known as pH – scale. As per Sorenson, pH is define as the magnitude of negative power to which 10 must be raised to express the hydrogen ion concentration in molar. pH of a solution can also be defined as negative logarithm of hydrogen ion concentration i.e.

$$ pH = -log_{10}[H^{+}] $$

Where $$ [H^{+}] $$ = concentration of $$ H^{+} $$.

Since $$ H^{+} $$ is associated with $$ H_{2}O $$ therefore we normally write, $$ pH = -log[H_{3}O^+] $$

pH of a neutral solution is 7. If the pH of solution is less than 7 or greater than 7 solution is acidic or basic respectively

…………………….. PENDING ………………………

pH range of some common substances are given below

Human blood                                                     7.36 – 7.42

Human urine                                                      4.8 – 8.4

Human saliva                                                     6.5 – 7.5

Gastric juice                                                       1.0 – 3.0

Tears                                                                     7.4

Soft drinks                                                           2.0 – 4.0

Vinegar                                                                2.4 – 3.4

Illustration 3:

Calculate pH of following solution

(i)           $$ 10^{-5}M HCI $$

(ii)          $$ 5\times 10^{-4} M H_{2}SO_{4} $$

Solution:

                (i) $$ pH = -log[H^{+}]\Rightarrow -log[10^{-5}]=5 $$

                (ii) Concentration of $$ H_{2}SO_{4} $$ = $$ 5\times 10^{-4}, H_{2}SO_{4}\rightarrow 2H^{+}+SO_{4}^{2-},so[H^{+}]=2\times 5\times 10^{-4} $$

                Ph = $$ -log[2\times 5\times10^{-4}] = 3 $$

Illustration 4:

Calculate H⁺ concentration in the HCI solution having pH = 2?

Solution:             $$ pH = -log[H^{+}] $$

                                $$ 2=-log[H^{+}] $$

                                $$ [H^{+}] = antilog (-2) = 10^{-2}M $$

Exercise 5:

Calculate pH of the following solutions.

(i)           $$ 0.002 M H_{2}SO_{4} $$

Ans:       pH = 2.398

(ii)          $$ 0.0001 M HNO_{3} $$

Ans:       pH = 4

(iii)         0.01 M NaOH

Ans:       pH = 12

pH of Salts

Salts show different pH in aqueous solution which is based on the nature of combining acid and base

Example-

1. Salt of weak acid-strong base shows pH > 7.
2. Salts of strong acid-weak base shows pH < 7.

Washing Soda $$ (Na_{2}CO_{3}.10H_{2}O) $$

Sodium carbonate decahydrate $$ Na_{2}CO_{3}.10H_{2}O $$ IS called washing soda.

Anhydrous $$ Na_{2}CO_{3} $$ is called soda ash.

Sodium carbonate occurs in crude form as dry deposits in the lakes of many dry regions of Est Africa, Egypt, USA etc. In India it is found in Dehradun, Mathura, Varanasi etc in the form of reh&sajji.

Manufacture

It is prepared on large scales by Solvay process (ammonia soda process).

The raw materials used in the process are

Sodium chloride (NaCI), lime stone $$ (CaCO_{3}) $$ AND ammonia $$ (NH_{3}) $$

Principle

The following principle in involved in the process.

                $$ NH_{3} + H_{2}O + CO_{2} \rightarrow NH_{4}HCO_{3} $$

                $$ NH_{4}HCO_{3} + NaCI \rightarrowNaHCO_{3} + NH_{4}CI $$

                $$ 2NaHCO_{3} \overset{\Delta}{\rightarrow} Na_{2}CO_{3} + CO_{2} + H_{2}O $$

The process consists of saturation of cold, concentrated solution of sodium chloride (brine) with $$ NH_{3} $$.

The ammoniacal brine is dropped from the top of the tower known as carbonating tower and carbon dioxide gas obtained by heating lime stone is passed up to the tower from near its base. The carbonating tower is packed with partitions, having a horizontal iron plate with a hole in the center and covered with a perforated plate. These partitions are to slow down the down coming ammoniacal brine solution so that the upcoming carbon dioxide has enough time to react with it completely and produces sodium hydrogen carbonate by the following reaction.

$$ NaCI + NH_{3} + H_{2}O + CO_{2} \rightarrow NH_{4}CI + NaHCO_{3} $$

The carbon dioxide used in the above reaction is obtained by heating lime stone in a lime kiln.

                $$ CaCO_{3} \overset{\bigtriangleup }{\rightarrow} C

aO + CO_{2} $$

Quick lime obtained in the above reaction changed into slaked lime by dissolving in water.

                $$ CaO + H_{2}O \rightarrowCa(OH_{2}) $$

Slaked lime is then boiled with $$ NH_{4}CL $$ TO liberate ammonia, which is used to recycle for further use in reaction

                $$ Ca(OH)_{2} + 2NH_{4}CI \rightarrowCaCI_{2} + 2NH_{3} + 2H_{2}O $$

Most of the ammonia used in Solvay process is reconverted and utilized. $$ NaHCO_{3} $$ IS sparingly soluble in water. It’s heated to obtain $$ Na_{2}CO_{3} $$.

                $$ 2NaHCO_{3} \overset{\Delta }{\rightarrow}Na_{2}CO_{3} + H_{2}O + CO_{2} $$

The carbon dioxide obtained by heating $$ NaHCO_{3} $$ is also reused. $$ Na_2CO_{3} $$ is recrystallized by dissolving in water to get washing soda. In this way NaCI and lime stone are only the two materials which are actually consumed in Solvay process. $$ CaCI_{2} $$ is obtained as a by product (which is not reused).

Properties

1. It is a transparent crystalline solid.
2. It is one of the few metal carbonates which is soluble in water.
3. The solution of washing soda in water is alkaline it turns red litmus to blue.
4. When crystals of washing soda are left open in air, they lose nine molecules of water of crystallization and form a monohydrate. The process is known as efflorescence.

$$ Na_{2}CO_{3}.10H_{2}O \rightarrow Na_{2}CO_{3}.H_{2}O + 9H_{2}O $$

On heating washing soda does not decompose, but loses all its water crystallization to form the anhydrous salt.

$$ Na_{2}CO_{3}.10H_{2}O \rightarrow Na_{2}CO_{3} + 10H_{2}O $$

Uses

1. It is used in laundry as washing soda.
2. It is used as a cleaning agent for domestic purpose. It is a component of many dry soap powders.
3. It is used for softening hard water (removing hardness).
4. It is used in the manufacture of many useful sodium compounds like caustic soda, borax, glass and soap. It is also used in the manufactures of paper.
5. It is used as laboratory regent.

Exercise 6:

(i)           Mention the chemical formulae of washing soda and soda ash.

Ans:       Washing soda    : $$ Na_{2}CO_{3}. 10H_{2}O $$

                Soda ash              : $$ Na_{2}CO_{3} (anhydrous) $$

(ii)          Give three uses of washing soda.

(iii)         Give the reaction which takes place in carbonating tower

Ans:       $$ NaCI+H_{2}O+CO_{2}+NH_{3}\rightarrow NaHCO_{3}+NH_{4}CI: $$

Baking Soda $$ (NaHCO_{3}) $$

Baking Soda is sodium hydrogen carbonate.

Manufacture of Sodium hydrogen carbonate

Commercial $$ NaHCO_{3} $$ IS manufactured from NaCI by the solvay process. In fact $$ NaHCO_{3} $$ is the primary product of the solvay process, it is obtained by the purification of $$ NaHCO_{3} $$ at commercial grade.

Properties of $$ NaHCO_{3} $$

1. $$ NaHCO_{3} $$ consists of white crystals which are sparingly soluble in water.
2. Its aqueous solution is mildly alkaline. It is due to hydrolyses.This solution gives yellow colour with methyl orange but no colour with phenolphthalein.
3. $$ NaHCO_{3} + H_{2}O \rightleftharpoonsNaOH + H_{2}CO_{3} $$
4. If a solution of $$ NaHCO_{3} $$ (solid) is boiled or heated $$ CO_{2} $$ gas is given off, hence it is used as a constituent of baking powder to ‘aerate’ the dough.
5. $$ 2NaHCO_{3} \overset{\Delta }{\rightarrow}Na_{2}CO_{3} + H_{2}O + CO_{2} $$

Uses

1. It is used as antacid in medicine to remove the acidity of stomach.
2. It is used in making baking powder used in preparing cakes, bakes etc. It is an additive in food and drinks. Baking powder contains $$ NaHCO_{3} $$ and an acid like tartaric acid. As it gives $$ CO_{2} $$ on heating, it causes bread and cakes to raise. Tartaric acid present in baking powder neutralizes $$ Na_{2}CO_{3} $$. If tartaric acid is not present in baking powder, the cake will taste bitter due to the formation of $$ Na_{2}CO_{3} $$.
3. Sodium hydrogen carbonate is used in fire extinguishers:    Fire extinguishers contains a solution of $$ NaHCO_{3} $$ & $$ H_{2}SO_{4} $$, these can be made to come into contact with each other either by pressing a knob or by inverting the extinguisher $$ CO_{2} $$ is liberated and it forces a stream of effervescent liquid on the fire. The $$ CO_{2} $$ surrounds the combustible substances and cuts off the supply of air. In this way it helps to put out the fire.

Exercise 7:

(i)           What is baking soda?

(ii)          Name the acid present in baking powder.

(iii)         Which compound is used as antacid in medicine $$ NaHCO_{3} $$ or $$ Na_{2}CO_{3} $$

(iv)         A baker found that the cake prepared by him is hard and small in size. Which ingredient has he                 forgotten to add that would have made the cake fluffy? Give reason.

Bleaching Powder $$ (CaOCI_{2}) $$

Bleaching powder is calcium oxy chloride. Its formula is $$ (CaOCI_{2}) $$. It is also called chloride of lime. It is manufactured by the action of chlorine of dry slaked lime $$ Ca(OH)_{2} $$.

                $$ Ca(OH)_{2} + CI_{2} \rightarrowCaOCI_{2} + H_{2}O $$

The manufacture of bleaching powder is carried out either in Hasenclever plant or Bachmann plant. The Hasenclever plant consists a number of hollow horizontal cylinders made of cast iron which are fitted with shafts and connected to one another. The shafts have a large number of blades fixed along their whole lengths. The shafts having blades can be rotated inside the cylinders by connecting their one end to motor. The upper most cylinder has a hopper at its top for putting is slaked lime. This hopper has also an outlet for the waste gases to go out. The lower most cylinder has inlet for passing in chlorine gas. It has also an outlet for the bleaching powder formed to come out.

The slaked lime is introduced into the uppermost, cylinder through the hopper and moved forward by the revolving blades from one cylinder to the next till if falls down. The lime comes out in contact with the ascending chlorine gas, which is introduced from the lowermost pipe, so that by the lime if reaches the bottom it gets thoroughly saturated with the gas and gets converted completely into bleaching powder.

Properties

1. It is a yellow white powder with strong smell of chlorine. It gradually loses its chlorine when left exposed to the air.
2. On long standing it loses chlorine by the action of CO₂ or dilute hydrochloric or dilute H₂SO₄.$$ CaOCI_{2} + 2HCI \rightarrowCaCI_{2} + H_{2}O + CI_{2} $$The amount of chlorine thus set free is called “available chlorine”. Greater the amount of available chlorine, more is the quality of bleaching powder.
3. $$ CaOCI_{2} + H_{2}SO_{4} \rightarrowCaSO_{4} + H_{2}O + CI_{2} $$
4. $$ CaOCI_{2} + CO_{2} \rightarrowCaCO_{3} + CI_{2} $$

Uses

1. It is used for bleaching cotton, linen in textile industry, wood pulp in paper industry. The bleaching action of it is due to the chlorine released by it.
2. It is used for disinfecting drinking water.
3. As a reagent in laboratory
4. Manufacture of chloroform
5. For making wool unshrinkable

Exercise 8:

(i)           Complete the following chemical equation: $$ Ca(OH)_{2}+CI_{2}\rightarrow ........... $$

(ii)          What type of salt is bleaching powder?

(iii)         Write two uses of bleaching powder.

Plaster of Paris $$ \left ( CaSO_{4}.\frac{1}{2}H_{2}O \right ) $$

Calciumsulphatehemihydrate $$ CaSO_{4}.\frac{1}{2}H_{2}O $$ is called plaster of paris. It is obtained by heating gypsum upto 373K in a kiln.

                $$ CaSO_{4}.2H_{2}O \overset{373K}{\rightarrow}CaSO_{4}.\frac{1}{2}H_{2}O + 1\frac{1}{2}H_{2}O $$

The heating of gypsum should be controlled carefully, this is because if gypsum is heated above $$ 100^0C $$ or (373K) the entire water is eliminated and anhydrous calcium sulphate $$ CaSO_{4} $$ dead burnt gypsum is formed. Anhydrous plaster of paris does not set like plaster of paris on adding water.

Properties

1. It is a white powder.
2. When it comes in contact with water crystals of gypsum are formed, this sets to hard mass.
3. $$ CaSO_{4}.1\frac{1}{2}H_{2}O\overset{373K}{\rightarrow}CaSO_{4}.2H_{2}O $$

Uses

1. In hospitals for setting fractured bones (surgical bandages)
2. In fire proof material.
3. In making toys, decorative materials, cosmetics casts and statues.
4. In laboratories for sealing the air gaps in apparatus to make it air tight.

Illustration 5:

What is the remaining content in the vessel during the electrolysis of aqueous NaCI?

Solution:     Sodium hydroxides.

Illustration 6:

What is dead plaster?

Solution:     At $$ 200^{\circ}C $$ Plaster of Paris loses all water molecules and this anhydrous form is called as dead plaster.

Exercise 9:

(i)                   Anhydrous calcium sulphate cannot be used as plaster of Paris. Why?

(ii)                  What happens when sodium carbonate is heated?

(iii)                 Why plaster of Pairs is used as a fire proof material?

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