Chapter 8: ACIDS, BASES AND SALTS

Chapter 8: Acid , Bases and Salts Class 10 Science and Technology (212) NIOS Textbook Solutions

Chapter 8. ACIDS, BASES AND SALTS

INTEXT QUESTION 8.1

1. Put the following substances in acid or base bottle.

Photo

(a) Milk of magnesia (b) gastric juice in humans (c) soft drinks (d) lime water (e) vinegar (f) soap

Answer: The classification is :

Substance	Acid / Base
(a) Milk of magnesia	Base
(b) Gastric juice in humans	Acid
(c) Soft drinks	Acid
(d) Lime water	Base
(e) Vinegar	Acid
(f) Soap	Base

2. What will happen if you add a drop of the following on a cut unripe apple, curd, causting soda solution and soap solution.

(i) phenolphthalein (ii) litmus

Answer: The effect of indicators on the given substances is:

Substance	Phenolphthalein	Litmus
Cut unripe apple (acidic)	Remains colourless	Turns blue litmus red
Curd (acidic)	Remains colourless	Turns blue litmus red
Caustic soda solution (basic)	Turns pink	Turns red litmus blue
Soap solution (basic)	Turns pink	Turns red litmus blue

INTEXT QUESTIONS 8.2

1. Name the substances in which the following acids are present:

(a) Ethanoic acid (b) Tartaric acid

Answer: (a) Ethanoic acid is present in vinegar, which gives it a sour taste.
(b) Tartaric acid is present in tamarind and grapes.

2. Which of these acids would be partially dissociated in their aqueous solution?

(a) HBr (b) HCN (c) HNO3 (d) C2H5COOH

Answer: The acids that would be partially dissociated in aqueous solution are HCN and C₂H₅COOH because they are weak acids.

[ HBr → Strong acid → Completely dissociates .

HCN → Weak acid → Partially dissociates .

HNO₃ → Strong acid → Completely dissociates.

C₂H₅COOH (Propanoic acid) → Weak acid → Partially dissociates . ]

3. An acid reacts with a substance X with liberation of a gas which burns with a ‘pop’ sound when a burning match stick is brought near it. What is the nature of X?

Answer: The gas that burns with a ‘pop’ sound is hydrogen gas. When an acid reacts with a metal, hydrogen gas is released. So, X is metallic in nature (a metal).

4. An acid reacts with a substance Z with the liberation of CO2 gas. What can be the nature of Z?

Answer: When an acid reacts with substance Z and releases CO₂ gas, Z is basic in nature .

[ The reaction: Na2CO3+2HCl→2NaCl+H2O+CO2

Na₂CO₃ (sodium carbonate) is a carbonate. Therefore, Z is basic in nature and can be a carbonate or bicarbonate. ]

5. Which of the following oxides will react with a base?

(a) CaO (b) SO2

Answer: A base reacts with an acidic oxide.

[ CaO → Basic oxide → Does not react with a base

SO₂ → Acidic oxide → Reacts with a base. ]

INTEXT QUESTIONS 8.3

1. Why does the colour of dry blue litmus paper remains unchanged even when it is brought in contact with HCl gas?

Answer: The colour of dry blue litmus paper remains unchanged in contact with HCl gas because HCl shows acidic properties only in the presence of water. Without water, HCl does not produce hydrogen ions (H⁺), so the litmus paper does not change colour.

2. How does water help in dissociation of acids and bases?

Answer: Water helps in the dissociation of acids and bases by separating them into ions. Acids release H⁺ ions in water, while bases release OH⁻ ions. These ions are responsible for showing acidic and basic properties. Without water, acids and bases cannot dissociate properly.

3. Identify the nature of the following aqueous solutions (whether acidic, basic or neutral)

(a) Solution A: [H+]<[OH-]

(b) Solution B: H+>[OH-]

Answer: (a) A is basic because the concentration of OH⁻ ions is greater than that of H⁺ ions

([H+]<[OH-]).

(b) Solution B is acidic because the concentration of H⁺ ions is greater than that of OH⁻ ions.

( H+>[OH-])

INTEXT QUESTIONS 8.4

1. pOH of a solution is 5.2. What is its pH. Comment on the nature (acidic, basic or neutral) of this solution.

Answer: Given, pOH=5.2

We know that, pH+pOH=14

pH = 14 − 5.2 = 8.8

Since the pH is greater than 7, the solution is basic (alkaline) in nature.

Therefore, pH = 8.8 and the solution is basic.

2. pH of a solution is 9. What is the concentration of H+ ions in it.

Answer: Given, pH = 9

We know that, pH=-log[H+]

⇒9=-logH+

⇒-9log10=logH+

⇒10-9=H+

⇒H+=10-9

3. What is the nature (whether acidic, basic or neutral) of the following solutions?

(a) Solution A: pH = pOH

(b) Solution B: pH > pOH

Answer: (a) Solution A: pH = pOH → The solution is neutral.

(b) Solution B: pH > pOH → The solution is basic (alkaline).

INTEX QUESTIONS 8.5

1. Identify acid radical and basic radical in CaSO4 .

Answer: In CaSO₄ (Calcium sulphate), SO₄²⁻ is the acid radical and Ca²⁺ is the basic radical.

2. CuSO4 was prepared by reacting an acid and a base. Identify the acid and the base that must have been used in this reaction.

Answer: Copper sulphate (CuSO₄) can be prepared by a neutralisation reaction between sulphuric acid (H₂SO₄) and copper(II) oxide (CuO), which acts as a base.

Acid: Sulphuric acid (H₂SO₄)
Base: Copper(II) oxide (CuO)

Chemical equation: CuO+H2SO4→CuSO4+H2O

So, the acid used is sulphuric acid and the base used is copper(II) oxide.

3. Which one of the following is the correct formula of plaster of paris? CaSO4.H2O or 2CaSO4.H2O

Answer: The correct formula of Plaster of Paris is 2CaSO4.H2O .

It is called calcium sulphate hemihydrate.

TERMINAL EXERCISE

A. Objective Type Questions

I. Mark the correct choice

1. Lemon juice contains

(a) tartaric acid (b) ascorbic acid

Answer: (b) ascorbic acid

[ Lemon juice contains ascorbic acid, which is also known as Vitamin C. ]

2. Aqueous solutions of acids conduct electricity. This shows that

(a) They contain H+ ions

(b) They contain OH- ion

(d) They contain both H+ and OH- ions

Answer: (c) They contain cations and anions

[ Aqueous solutions of acids conduct electricity because they produce ions in water. Electric current is carried by both positive ions (cations) and negative ions (anions).]

3. Which of the following is not a strong acid?

(a) HCl (b) HBr (c) HI (d) HF

Answer: (d) HF

[ HF (Hydrofluoric acid) is not a strong acid because it does not completely ionise in water.]

4. Self dissociation of water produces

(a) a large number of H+ ions

(b) a large number of OH- ions

(d) H+ and OH- ions in unequal numbers

Answer: (c) H⁺ and OH⁻ ions in equal numbers

[ Water undergoes self-dissociation (self-ionisation) and forms hydrogen ions (H⁺) and hydroxide ions (OH⁻) in equal amounts.]

5. In any aqueous basic solution

(a) [ H+] > [OH-] (b) [ H+] < [OH-] (c) [ H+] = [OH-] (d) [ H+] = 0

Answer: (b) [H⁺] < [OH⁻]

[ In an aqueous basic (alkaline) solution, the concentration of hydrogen ions (H⁺) is less than the concentration of hydroxide ions (OH⁻). ]

6. In an aqueous solution of HCl which of the following species is not present?

(a) H+ (b) OH- (c) HCl (d) Cl- Cl–

Answer: (b) OH⁻

[ In an aqueous solution, HCl dissociates into: HCl→H++Cl-

So, the solution contains H⁺ ions and Cl⁻ ions. Since HCl is an acidic solution, OH⁻ ions are not present in significant amount. ]

7. Which of the following is not a raw material for manufacturing washing soda?

(a) Lime stone (b) Ammonia

Answer: (c) Slaked lime.

[ Slaked lime [Ca(OH)₂] is not a raw material; it is actually a byproduct formed in the process.]

II. Mark the following statements as true (T) or false (F):

1. Acids furnish H+ ions only in the presence of water.

2. Lime water turns blue litmus red.

3. HF is a strong acid.

4. H2 gas is produced when acids react with metal oxides.

5. Corrosive action of acids is due to H+ ions present in them.

6. When the pH of the rain water become more than 5.6 it is called acid rain.

7. Aqueous solutions of all the salts are neutral in nature i.e. neither acidic nor basic in nature.

Answer: 1. Acids furnish H+ ions only in the presence of water. → True

2. Lime water turns blue litmus red. → False

3. HF is a strong acid.→ False

4. H2 gas is produced when acids react with metal oxides. → False

5. Corrosive action of acids is due to H+ ions present in them.→ True .

6. When the pH of the rain water become more than 5.6 it is called acid rain. → False.

7. Aqueous solutions of all the salts are neutral in nature i.e. neither acidic nor basic in nature. → False.

III. Fill in the blanks

1. Acids taste ...................... while bases taste ......................

2. Milk of magnesia turns ...................... litmus ......................

3. One mole of sulphuric acid would furnish ...................... mole/s of H+ ions and ...................... moles of SO42- ions.

4. ...................... gas is produced when acids react with metal hydrogen carbonates.

5. Lime water turns milky on passing CO2 gas due to the formation of ......................

6. The reaction between an acid and a base is known as ......................

7. Bee sting injects ...................... acid which causes severe pain and burning sensation.

8. In NH4NO3 the acid radical is ...................... and the basic radical is ......................

9. Chemically baking soda is ......................

Answer: 1. Acids taste sour while bases taste bitter.

2. Milk of magnesia turns red litmus blue.

3. One mole of sulphuric acid would furnish 2 mole/s of H⁺ ions and 1 mole of SO₄²⁻ ions.

4. Carbon dioxide (CO₂) gas is produced when acids react with metal hydrogen carbonates.

5. Lime water turns milky on passing CO₂ gas due to the formation of calcium carbonate (CaCO₃).

6. The reaction between an acid and a base is known as neutralisation reaction.

7. Bee sting injects formic acid (methanoic acid) which causes severe pain and burning sensation.

8. In NH₄NO₃, the acid radical is NO₃⁻ (nitrate) and the basic radical is NH₄⁺ (ammonium).

9. Chemically baking soda is sodium hydrogen carbonate (NaHCO₃).

B. Descriptive Questions

1. What is an acid?

Answer: Acids are the substances which taste sour, change blue litmus red, are corrosive to metals and furnish H+ ions in their aqueous solutions.

2. Give two examples of acids found in food articles.

Answer: Two examples of acids found in food articles are:

(i) Citric acid – found in lemon and orange.

(ii) Acetic acid – found in vinegar.

3. What is a base?

Answer: Bases are the substances which taste bitter, change red litmus blue, feel slippery and furnish OH- ions in their aqueous solutions.

4. Give two examples of bases.

Answer: Two examples of bases are:

(i) Sodium hydroxide (NaOH)

(ii) Calcium hydroxide [Ca(OH)₂]

5. What are indicators?

Answer: Indicators are the substances that show one colour in an acidic medium and another colour in a basic medium.

6. What is the colour of methyl orange indicator in (i) acidic medium and (ii) basic medium.

Answer: The colour of methyl orange indicator is:

(i) In acidic medium → Red
(ii) In basic medium → Yellow

7. Why do solutions of acids and bases conduct electricity?

Answer: Solutions of acids and bases conduct electricity because they dissociate in water and produce free ions. Acids produce H⁺ ions and bases produce OH⁻ ions along with other ions. These moving ions carry electric current through the solution.

8. Differentiate between strong and weak acids and give one example of each.

Answer: The difference between strong and weak acids :

Strong acids	Weak acids
1. They ionise completely in water.	1. They ionise partially in water.
2. They produce a large number of H⁺ ions.	2. They produce a small number of H⁺ ions.
Example: Strong acid – HCl (Hydrochloric acid)	Example: Weak acid – CH₃COOH (Acetic acid)

9. Write down the reaction between zinc and sulphuric acid.

Answer: When zinc reacts with sulphuric acid to form zinc sulphate (ZnSO₄) and hydrogen gas (H₂).

The reaction is: Zn+H2SO4→ZnSO4+H2

10. Which gas is evolved when an acid reacts with metal carbonates ? Which other category of compounds would produce the same gas on reacting with acids ?

Answer: When an acid reacts with metal carbonates then produces sodium chloride, water and carbon dioxide (CO₂) .
The reaction is: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

When an acid reacts with metal hydrogen carbonates then produces sodium chloride, water and carbon dioxide (CO₂) .
The reaction is : NaHCO₃ + HCl → NaCl + H₂O + CO₂

11. What type of oxides react with acids? Give one examples of this type of oxide and write down the balanced equation for the reaction.

Answer: Basic oxides (mostly metal oxides) react with acids to form salt and water.

Example: Copper(II) oxide (CuO)

The balanced equation is: CuO + 2HCl → CuCl₂ + H₂O

12. What is the name given to the reaction between an acid and a base? What are the products formed in such reactions?

Answer: The reaction between an acid and a base is called a neutralisation reaction.

In this reaction, the products formed are salt and water.

Example: HCl+NaOH→NaCl+H2O

13. “Corrosive action of acids is not related to their strength”. Justify this statement.

Answer: The corrosive action of acids is not related to their strength because corrosion depends on the presence of H⁺ ions and the concentration of the acid, not only on whether the acid is strong or weak. Even a weak acid can show corrosive action if present in high concentration.

14. Give one example each of the following (i) a strong base (ii) a weak base

Answer: (i) Strong base → Sodium hydroxide (NaOH)

(ii) Weak base → Ammonium hydroxide (NH₄OH)

15. List three categories of substances that can react with a base. Give one example of each and write the chemical reaction involved in each case.

Answer: Three categories of substances that can react with a base are:

(i) Reaction of bases with metals

Example: zinc and sodium hydroxide

Zn+2NaOH→Na2ZnO2+H2

(ii) Reaction of Bases with non-metal oxides

Example: calcium hydroxide (lime water) and carbon dioxide

Ca(OH)2+CO2→CaCO3+H2O

(iii) Reaction of bases with acids
Example: Hydrochloric acid and potassium hydroxide

HCl+KOH→KCl+H2O

16. What happens when a dry strip of each of red litmus paper and blue litmus paper is brought in contact with HCl gas? In which case a change would be observed if the strips are moistened and then brought in contact with HCl gas and what would be the change?

Answer: Dry strips: No change occurs because dry HCl gas does not furnish H⁺ ions.
Moistened strips: Blue litmus turns red; red litmus remains red. This happens because HCl gas dissolves in moisture to form H⁺ ions, showing acidic nature.

17. A small palette of NaOH is kept on dry red litmus paper. Initially, no change is observed but after some time its colour starts changing to blue around the place where the palette of NaOH is kept. Explain these observations.

Answer: Initially, no change is observed because dry NaOH does not produce OH⁻ ions without water. After some time, NaOH absorbs moisture from the air and dissolves, releasing OH⁻ ions. These ions turn the red litmus paper blue around the place where NaOH is kept.

18. How does water help in dissociation of acids and bases? Explain.

Answer: Water helps in the dissociation of acids and bases by dissolving them and separating them into ions. Acids release H⁺ ions in water, while bases release OH⁻ ions. These free ions are responsible for showing acidic or basic properties and conducting electricity.

19. What is ‘self dissociation of water’? Name the resulting species and give their concentrations at 25°C.

Answer: Water itself undergoes a very slight dissociation to furnish H⁺ and OH⁻ ions in equal numbers. This process is called the self dissociation of water (also known as autoionization).

The resulting species are the hydronium ion (H⁺) and the hydroxide ion (OH⁻).

At 25°C, the concentrations of both ions are [H⁺] = 1.0 × 10⁻⁷ M and [OH⁻] = 1.0 × 10⁻⁷ M

20. What is ionic product constant of water? Give its value at 25°C. Will the value change if an acid, base or a salt is dissolved in water?

Answer: Ionic product constant of water ( Kw) is the product of the molar concentrations of hydronium ions and hydroxide ions present in water at a particular temperature.

Kw=H+OH-

At 25°C, since H+=1.0×10-7 molL-1 and OH-=1.0×10-7 molL-1

We have,

No, the numerical value of Kw does not change upon the addition of an acid, base, or salt. Kw is a constant for a given temperature. It only changes if the temperature changes.

Effect of adding acid, base or salt :

(i) When an acid is dissolved, [H+] increases and [OH-] decreases.

(ii) When a base is dissolved, [OH-] increases and [H+] decreases.

(iii) Some salts may also change individual ion concentrations.

21. Give the relationships between the concentrations of hydrogen ions and hydroxyl ions in (i) pure water (ii) a neutral solution (iii) an acidic solution and (iv) a basic solution.

Answer: (i) Pure Water: In pure water, self-dissociation produces H⁺ and OH⁻ in exactly equal numbers.

Relationship: H+=OH-

Concentrations: H+=OH-=1.0×10-7 molL-1

(ii) A Neutral Solution:

A neutral solution is defined as one where the concentrations of H⁺ and OH⁻ are equal, just like in pure water.

Relationship: H+=OH-

Concentrations: H+=OH-=1.0×10-7 molL-1 (at 25°C)

(iii) An Acidic Solution:

In an acidic solution, the concentration of hydrogen ions is greater than that of hydroxide ions.

Relationship: H+>OH-

Concentration range: H+>1.0×10-7molL-1 and OH-<1.0×10-7 molL-1 .

(iv) A Basic Solution:

In a basic (alkaline) solution, the concentration of hydroxide ions is greater than that of hydrogen ions.

Relationship: OH->H+

Concentration range: OH->1.0×10-7molL-1 and H+<1.0×10-7 molL-1 .

22. What is pH? What happens to the pH if the hydroxyl ion concentration in the solution increases?

Answer: pH is a measure of the acidity or basicity of a solution. It is defined as the negative logarithm of the hydrogen ion concentration of the solution.

pH=-log[H+] , where [H+] is the concentration of hydrogen ions in mol/L.

If the hydroxyl ion concentration [OH-] increases in the solution, the pH will increase (the solution becomes more basic/alkaline).

23. Predict whether a given aqueous solution is acidic, basic or neutral if its pH is

(a) 7.0, (b) 11.9 and (c) 3.2

Answer: (a) pH = 7.0

Since the pH is exactly 7.0, the solution is neutral.

(b) pH = 11.9

Since the pH is greater than 7.0 (11.9 > 7.0), the solution is basic (alkaline).

Since the pH is less than 7.0 (3.2 < 7.0), the solution is acidic.

24. Calculate the pH of 1.0×10-4 molar solution of HNO3 .

Answer: Here, H+=1.0×10-4 M

The dissociation reaction is: HNO₃ → H⁺ + NO₃⁻

We have,

pH=-log1.0×10-4=-log10-4=--4=4

25. What is the pH of 1.0×10-5 molar solution of KOH?

Answer: Here, OH-=1.0×10-5 M

The dissociation reaction is: KOH → K⁺ + OH⁻

We have,

pOH=-log1.0×10-5=-log10-5=--5=5

Again, pH+pOH=14

⇒pH=14-pOH=14-5=9

⇒pH=9

26. What is the pH of 1.0×10-2 mol/L solution of NaCl?

Answer: A 1.0×10-2 mol/L solution of NaCl (sodium chloride) has pH = 7 (at 25°C).

[ NaCl is a salt formed from a strong acid (HCl) and a strong base (NaOH). It does not undergo hydrolysis in water, so it does not affect the H+ or OH- concentrations. The solution remains neutral, with H+=OH-=1.0×10-7 M.

pH=-logH+

pH=-log1.0×10-7=-log10-7=--7=7

27. What do you understand by the term ‘universal indicator’?

Answer: A universal indicator is a mixture of several different indicators. It is designed to show a different but characteristic colour at each pH value across a wide range (typically from pH 0 to 14).

28. What is acid rain?

Answer: Acid rain is rain in which the pH of rainwater falls below 5.6.

29. What is the importance of pH for humans and animals, and our digestive system?

Answer: Our stomach produces hydrochloric acid (HCl) which helps in the digestion of food. When we eat spicy food, the stomach may produce excess acid, causing acidity, irritation and pain. To reduce this, we use antacids, which are bases. One common antacid is Milk of Magnesia (magnesium hydroxide suspension), which neutralises excess acid and provides relief.

30. Which chemical causes pain and burning sensation when somebody accidentally touches ‘nettle plant’?

Answer: When somebody accidentally touches a nettle plant, the methanoic acid (formic acid) present in its stinging hairs causes pain and a burning sensation.

31. What is a salt? Give two examples.

Answer: Salts are ionic compounds made of a cation other than H+ ion and an anion other than OH– ion.

For example: NaCl and KNO₃

32. How are salts obtained from an acid? Mention four types of substances that can be used for it.

Answer: Salts are obtained from an acid by replacing the hydrogen ion (H⁺) of the acid with a metal ion or ammonium ion through chemical reactions. This process is called neutralisation or salt formation.

Four types of substances that can be used to obtain salts from an acid are:

1. Active Metals : When reactive metals are added to dilute acids, they displace hydrogen gas and form the corresponding metal salt.

General Word Equation: Metal + Dilute Acid → Salt + Hydrogen Gas

Chemical Equation: Zn+H2SO4→ZnSO4+H2

2. Metal Bases (Alkalis / Metal Hydroxides) : Soluble or insoluble metal bases react with acids in a classic neutralization reaction to produce a salt and water only.

General Word Equation: Base + Acid → Salt + Water

Chemical Equation: NaOH+HCl→NaCl+H2O

3. Metal Oxides (Basic Oxides) : Metal oxides are basic in nature. When they react with acids, they undergo a neutralization reaction similar to bases, forming salt and water.

General Word Equation: Metal Oxide + Acid →Salt + Water

Chemical Equation: CuO+2HCl→CuCl2+H2O

4. Metal Carbonates (or Bicarbonates) : When metal carbonates or hydrogen carbonates react with dilute acids, they decompose to produce a salt, water and carbon dioxide gas .

General Word Equation: Metal Carbonate + Acid → Salt + Water + Carbon Dioxide

Chemical Equation: Na2CO3+2HCl→2NaCl+H2O+CO2

33. Give chemical formula of (i) baking soda and (ii) washing soda.

Answer: The chemical formula are:

(i) Baking soda→ NaHCO3

(ii) Washing soda → Na2CO3.10H2O

34. List the raw materials required for the manufacture of baking soda and describe the process with the help of suitable chemical equations.

Answer: The raw materials required to manufacture washing soda are:

(i) Lime stone which is calcium carbonate, CaCO3

(ii) Sodium chloride (NaCl) in the form of brine( Conc. NaCl Solution)

(iii)Ammonia ( NH3)

In Solvey’s process, carbon dioxide is obtained by heating limestone strongly,

CaCO3→CaO+CO2

It is then passed through cold brine (a concentrated solution of NaCl in water) which has previously been saturated with ammonia,

NaCl+CO2+NH3→NaHCO3+NH4Cl

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35. Distinguish between baking powder and baking soda. Why is baking powder preferred for making cakes?

Answer: The difference between baking powder and baking soda :

Baking soda	Baking powder
It is sodium hydrogen carbonate (NaHCO3)	It is a mixture of baking soda and a mild edible acid (such as tartaric acid).
It alone releases CO2 only under suitable conditions.	It releases CO2 easily during baking.
May leave a bitter taste if used alone.	Does not leave a bitter taste.

Baking powder is preferred because it produces carbon dioxide gas, which makes the cake soft and spongy, and the acid present neutralises any bitter taste.

36. Give any two uses of baking soda.

Answer: Uses of baking soda :

(i) Used for cooking of certain foods.

(ii) In soda acid fire extinguishers.

37. What is washing soda? Give its chemical formula. How is it manufactured by Solvey’s method?

Answer: Washing soda is the common name for sodium carbonate decahydrate. It is a crystalline white solid used as a cleansing agent for domestic and industrial purposes.

Chemical Formula: Na₂CO₃·10H₂O (Sodium carbonate decahydrate)

Manufacture by Solvay's method:

First, sodium hydrogen carbonate is prepared by passing carbon dioxide through ammoniated brine:

NaCl+NH3+CO2+H2O→NaHCO3+NH4Cl
On heating sodium hydrogen carbonate:

2 NaHCO3→Na2O3+CO2+H2O

The sodium carbonate obtained is crystallised with water to form washing soda:

Na2O3+10H2O→ Na2O3.10H2O

38. Give two uses of washing soda.

Answer: Uses of washing soda :

(i) It is used in the manufacture of caustic soda, glass, soap powders, borex and in paper industry.

(ii) For removing permanent hardness of water.

(iii) As a cleansing agent for domestic purpose.

39. What is the chemical formula of ‘plaster of paris’? How is it manufactured? What precaution is taken during its manufacture?

Answer: Chemical formula of Plaster of Paris : CaSO4.12H2O

(Calcium sulphate hemihydrate)

Plaster of Paris is prepared by heating gypsum CaSO4.2H2O at about 373 K (100°C).

CaSO4.2H2O heat CaSO4.12H2O+32 H2O

Heating should be done carefully and the temperature should not exceed 373 K, otherwise Plaster of Paris loses water completely and forms dead burnt plaster, which does not set properly.

40. List any four uses of ‘plaster of paris’.

Answer: Uses of ‘plaster of paris’.

(i) In making casts for manufacture of toys and statues.

(ii) In medicine for making plaster casts to hold fractured bones in place while they set. It is also used for making casts in dentistry.

(iii) For making the surface of walls and ceiling smooth.

(iv) For making decorative designs on ceilings, walls and pillars.

41. What is bleaching? Chemically, what is bleaching powder? Give its any four uses.

Answer: Bleaching is a process of removing colour from a cloth to make it whiter.

Bleaching powder is calcium oxychloride with the chemical formula CaOCl₂.

Use of bleaching powder :

(i) In making wool unshrinkable.

(ii) Used as disinfactant and germicide for sterilization of water.

(iii) For the manufacture of chloroform.

(iv) Used as an oxidizing agent in chemical industry

42. List the raw materials required and the method of manufacture of bleaching powder. Write the equation for the reaction involved.

Answer: Raw materials required for manufacture of bleaching powder:

(i) Dry slaked lime [ Calcium hydroxide →Ca(OH)₂ ]

(ii) Chlorine gas (Cl₂)
Bleaching powder is manufactured by passing chlorine gas over dry slaked lime in a special chamber called Hasenclever plant (bleaching powder chamber). Chlorine reacts with slaked lime and forms bleaching powder.

Chemical equation: Ca(OH)2+Cl2→CaOCl2+H2O

Thus, calcium oxychloride (CaOCl₂), called bleaching powder, is produced.

Posted 3 weeks ago

Chapter 8: ACIDS, BASES AND SALTS

Chapter 8: Acid , Bases and Salts Class 10 Science and Technology (212) NIOS Textbook Solutions

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