Chemistry Class 12 NCERT Solutions: Chapter 1 The Solid State Part 3

Q: 24. What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic Justify your answer.

Ans:

Ferromagnetic substances would make better permanent magnets.

In solid state, the metal ions of ferromagnetic substances are grouped together into small regions. These regions are called domains and each domain acts as a tiny magnet. In an unmagnetized piece of a ferromagnetic substance, the domains are randomly oriented. As a result, the magnetic moments of the domains get cancelled. However, when the substance is placed in a magnetic field, all the domains get oriented in the direction of the magnetic field and a strong magnetic effect is produced.

The ordering of the domains persists even after the removal of the magnetic field. Thus, the ferromagnetic substance becomes a permanent magnet.

Exercise: 2

Q: 1. Define the term ‘amorphous’. Give a few examples of amorphous solids.

Ans:

Amorphous solids are the solids whose 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.

Q: 2. What makes a glass different from a solid such as quartz? Under what conditions quartz could 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.

Q: 3. Classify each of the following solids as ionic, metallic, molecular, network (covalent) or amorphous.

Equation

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Ans:

Equation

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Q: 4. (I) What is meant by the term ‘coordination number’?

(II) What is the coordination number of atoms:

(A) In a cubic close-packed structure

(B) In a body-centred cubic structure?

Ans:

(i) The number of nearest neighbours of any constituent particle present in the crystal lattice is called its coordination number.

(ii) The coordination number of atoms

(A)in a cubic close-packed structure is 12, and

(B) in a body-centred cubic structure is 8

Q: 5. How can you determine the atomic mass of an unknown metal if you know its density and the dimension of its unit cell? Explain.

Answer :

By knowing the density of an unknown metal and the dimension of its unit cell, the atomic mass of the metal can be determined.

Let 'a' be the edge length of a unit cell of a crystal, 'd' be the density of the metal, 'm' be the mass of one atom of the metal and 'z' be the number of atoms in the unit cell.

Now, density of the unit cell Equation

Equation

[Since mass of the unit cell = Number of atoms in the unit cell × mass of one atom][Volume of the unit cell = (Edge length of the cubic unit cell) 3]

From equation (i), we have:

Now, mass of one atom of metal (m) Equation

Therefore, Equation (iii)

If the edge lengths are different (say a, b and c), then equation (ii) becomes:

Equation (iv)

Thus, using equation (iii) and (iv), the atomic mass of an unknown metal can be determined.

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