Chemistry class 12 chapter 1 Subjective Questions

SHORT ANSWER TYPE:-

1. Why are solids rigid?

Ans:- The intermolecular forces of attraction present in solids are very strong. The constituent particles of solids cannot move from their positions, that is, they have fixed positions. They can oscillate about their mean positions. This is the reason solids are rigid.

2. Why do solids have a definite volume?

Ans:- Solids have a definite volume due to the presence of the strong intermolecular forces of attraction and the constituent particles of solids have fixed positions.

3. Why are liquids and gases categorized as fluids?

Ans:- Liquids and gases are categorized as fluids because  their particles have the ability to flow.

4. Why are solids incompressible?

Ans:- The distance between the constituent particles (atoms, ions, molecules, etc.) is very less in solids. On bring- ing them still more closer, repulsion will start between electron clouds of these particles. Hence, they cannot be brought further close to each other.

5. Classify the following as amorphous or crystalline solids :  Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.

Ans:-  Amorphous solids:- Polyurethane, teflon, cellophane, polyvinyl chloride, fibre glass.

Crystalline solids :- Naphthalene, benzoic acid, potassium nitrate, copper.

6. Under which situations can an amorphoussubstance change to crystalline form?

Ans:- Anamorphous solid on heating at some temperature may become crystalline. Slow heating and cooling over a long period makes an amorphous solid tohave some crystalline character.

7. Define the term ‘amorphous’. Give a few examples of amorphous solids.

Ans:- In amorphous solids, Constituent particles are of irregular shapes and have short-range order. These solids are isotropic in nature and melt over a range of temperature. Therefore, amorphous solids are sometimes called pseudo solids or super cooled liquids. They do not have definite heat of fusion. When cut with a sharp-edged tool, they cut into two pieces with irregular surfaces. Examples of amorphous solids include glass, rubber, and plastic.

8. Why is glass considered a super cooled liquid?

Ans:- Glass has a tendency to flow, though very slowly. Therefore, glass is considered as a super cooled liquid. This is the reason that glass windows and doors are slightly thicker at the bottom than at the top.

9. What makes a glass different from a solid such as quartz? Under what conditions could quartz be converted into glass?

Ans:- The arrangement of the constituent particles makes glass different from quartz. In glass, the constituent particles have short-range order, but in quartz, the constituent particles have both long-range and short-range orders. Quartz can be converted into glass by heating and then cooling it rapidly.

10. Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this solid. Would it show cleavage property?

Ans:-  An isotropic solid has the same value of physical properties on measuring along different directions. Therefore, the given solid, having the same value of refractive index along all directions, is isotropic in nature. Hence, the solid is an amorphous solid. When an amorphous solid is cut with a sharp-edged tool, it cuts into two pieces with irregular surfaces.

11. Give an example each of a molecular solid and an ionic solid ? 

Ans:- Molecular solid : Iodine (I₂). 

Ionic soild : Sodium chloride (NaCl).

12. Ionic solids conduct electricity in molten state but not in solid state. Explain.

Ans:- In ionic compounds, electricity is conducted through jons. In solid state, ions are held together by strong electrostatic forces and are not free to move about within the solid. Hence, ionic solids do not conduct electricity in solid state. However, in molten state or in solution form, the ions are free to move and conduct electricity.

13. Solid A is a very hard electrical insulator in solid as well as in molten state and melts at extremely high temperature. What type of solid is it?

Ans:- The given properties are the properties of a covalent or network solid. Therefore, the given solid is a covalent or network solid. Examples of such solids include diamond (C) and quartz (SiO₂)

14 . Classify the following solids in different categories based on the nature of intermolecular forces operating in them :

Potassium Sulphate, Tin, Benzene, Urea, Ammonia, Water, Zinc Sulphide, Graphite, Rubidium, Argon, Silicon Carbide.

Ans:- Potassium sulphate (K2SO4) → Ionic solid

Tin (Sn) → Metallic solid

Benzene (C₆H₆) → Molecular solid (non-polar)

Urea (NH₂CON₂) → Polar molecular solid

Ammonia (NH₃) → Polar molecular solid

Water (H₂O) → Hydrogen bonded molecular solid

Zinc sulphide (ZnS) → Ionic solid

Graphite → Covalent or network solid

Rubidium (Rb) → Metallic solid

Argon (Ar) → Non-polar molecular solid

Silicon carbide (SiC) → Covalent or network solid

15. Classify each of the following solids as ionic, metallic, molecular, network (covalent) or amorphous.

(a) Tetra phosphorus decoxide (P₄O₁₀)

(b) Ammonium phosphate (NH,)3PO,

(c) Sic 

(d) I₂

(e) P₄

(f) Plastic

(g) Graphite

(h) Brass

(i) Rb

(j) LiBr

(k) Si

Ans:- Ionic solids → (b) (NH₄)₃PO₄ (j) LiBr

Metallic solids → (h) Brass, (i) Rb

Molecular solids → (a) Tetra phosphorus decoxide (P₄O₁₀), (d) I₂ (e) P₄.

Covalent (network) solids → (c) SiC, (g) Graphite, (k) Si

Amorphous solids → (f) Plastic

16. ‘Stability of a crystal is reflected in the magnitude of its melting points’. Comment. Collect melting points of solid water, ethyl alcohol, diethyl ether and methane from a data book. What can you say about the intermolecular forces between these  molecules?  

Ans:- Higher the melting point, greater is intermolecular force of attraction and greater is the stability. A substance with a higher melting point is more stable than a substance with a lower meltingthe point.

The melting points of the given substances are :

Solid water → 273 K

Ethyl alcohol → 158.8 K

Diethyl ether → 156.85 K

Methane → 89.34 K 

Now, on observing the values of the melting points, it can be said that among the given substances, the intermolecular force in solid water is the strongest and that in methane is the weakest.

Long Answer Type Question:-

1. Explain

(a) The basis of similarities and differences between metallic and ionic crystals.

(b) Ionic solids are hard and brittle.

Ans:- (a) Similarity between metallic and ionic crystals : The basis of similarities between metallic and ionic crystals are that both the crystals are held by the electrostatic forces of attraction. In metallic crystals, the electrostatic force acts between the positive ions and the electrons, In ionic crystals, it acts between the oppositely-charged ions. Hence, both have high melting points.

Differences between metallic and ionic crystals:-

The basis of differences between metallic and ionic crystals are that in metallic crystals, the electrons are free to move and so, metallic crystals can conduct electricity. However, in ionic crystals, the ions are not free to move. As a result, they cannot conduct electricity. However, in molten state or in aqueous solution, they conducts electricity 

(b) the constitutent particle of ionic crystal are ions. these ions are held together in three dimensional arrengement by the electrostatic force of attraction. since the electrostatic force of attraction is very strong, the charge ions are held in fixed positions. that is why ionic crystals are hard and brittle.