Maharashtra Board Class 12 Chemistry Chapter 1 Solid State PDF Download

1. Solid State

As studied earlier, the solid state of matter is characterised by strong interparticle forces of attraction. As a result most solids have definite shape and volume, which change only slightly with change in temperature and pressure. The smallest constituent particles of various solids are atoms, ions or molecules. All such smallest constituent particles of solids will be referred to as particles in this chapter.

Can you recall?

What are the three most common states of matter?

How does solid state differ from the other two states? (Answer with reference to volume, shape, effect of temperature and pressure on these and the motion of constituent particles and interparticle forces.)

1.2 Types of solids

There are two types of solids, namely crystalline solids and amorphous solids.

1.2.1 Crystalline solids

Study of many crystalline solids indicates that they possess the following characteristic properties.

i. There is a regularity and periodicity in the arrangement of constituent particles in crystalline solids. The ordered arrangement of particles extends over a long range.

ii. Crystalline solids have sharp melting points, that is, they melt at a definite temperature.

iii. All crystalline substances except those having cubic structure are anisotropic. In other words their properties like refractive index, thermal and electrical conductivity, etc, are different in different directions.

Ice, salts such as NaCl, metals such as sodium, gold, copper and materials such as diamond, graphite, ceramics are examples of crystalline solids.

Observe and discuss

Collect the following solids: granular sugar, common salt, blue vitriol.

Observe a few granules of these solids under a magnifying lens or microscope.

Discuss your observations with reference to the following points: (i) Shape of the granules, (ii) Smoothness of faces of the granules and (iii) Angles between various edges of the granules.

All the above solids are crystalline solids. Name the properties of crystals that you observed in this activity.

Try this

Observe the figure carefully. The two types of circles in this figure represent two types of constituent particles of a solid.

Will you call the arrangement of particles in this solid regular or irregular?

Is the arrangement of constituent particles same or different in directions AB, CD, and EF?

Teacher's Note

In India, we can see salt crystals in salt pans near the sea. Children can observe how salt farmers make beautiful crystal patterns. Crystals are very common in our daily life.

Exam Trick

Remember: Crystalline solids = ordered and regular arrangement. Just like soldiers standing in straight lines, particles in crystals are also in neat rows.

Points to Remember

Crystalline solids have a regular and ordered arrangement of particles.
They have sharp melting points.
Their properties are different in different directions (anisotropic).
Examples are salt, sugar, and ice.

1.2.2 Amorphous solids

The particles of a liquid are in constant motion. The stop action photograph of a liquid describes the amorphous state. In fact, they are supercooled liquids. Amorphous solids have the following characteristics.

i. The constituent particles in amorphous solids are randomly arranged. The particles do not have long range ordered structure, but they do have a short range order.

ii. Amorphous solids do not have sharp melting points. They melt gradually over a temperature interval. On heating, amorphous solids gradually and continuously soften and start to flow.

iii. These solids are isotropic. In other words, their properties such as refractive index, conductivity are all independent of direction of measurement. They exhibit the same magnitude for any property in every direction.

Glass, plastic, rubber, tar, and metallic glass (metal-metalloid alloy) are a few examples of amorphous solids.

Use your brain power

Identify the arrangements A and B as crystalline or amorphous.

Do you know?

A single crystal has ordered (regular and periodic) arrangement of constituent particles throughout its bulk.

Majority of crystalline solids, including metals, are polycrystalline in nature. Single granule of a polycrystalline solid is made of many single crystals or crystallites packed together with different orientations.

Single crystals are difficult to obtain. Diamond is an example of naturally formed single crystal.

Teacher's Note

Glass and plastic are amorphous solids we use every day. When glass breaks, it breaks randomly because particles are not in order. Show children how glass and salt behave differently when broken.

Exam Trick

Remember: Amorphous = no order. Just like water that flows freely, amorphous solids also have particles that are not arranged in order.

Points to Remember

Amorphous solids have random arrangement of particles.
They do not have a sharp melting point.
They have same properties in all directions (isotropic).
Examples are glass, plastic, and rubber.

1.2.3 Isomorphism and polymorphism

Similarity or dissimilarity in crystal structure of different solids is described as isomorphism and polymorphism.

i. Isomorphism: Two or more substances having the same crystal structure are said to be isomorphous. In these substances the chemical composition has the same atomic ratio. For example (i) NaF and MgO (ii) NaNO3 and CaCO3 are isomorphous pairs, and have the same atomic ratios, 1:1 and 1:1:3, respectively, of the constituent atoms.

ii. Polymorphism: A single substance that exists in two or more forms or crystalline structures is said to be polymorphous. Polymorphs of a substance are formed under different conditions. For example: Calcite and aragonite are two forms of calcium carbonate; α-quartz, β-quartz and cristobalite are three of the several forms of silica. Polymorphism occurring in elements is called allotropy. For example: three polymorphic (allotropic) forms of carbon are diamond, graphite and fullerene.

Teacher's Note

Diamond and graphite are both made of carbon but have different structures and properties. This is like how water can be ice, liquid water, or steam - all the same thing but in different forms.

Exam Trick

Remember: Polymorphism = one element, many forms. Like how you can wear different clothes but still be the same person, one element can have different crystal forms.

Points to Remember

Isomorphism means same crystal structure for different compounds.
Polymorphism means different crystal structures for same element or compound.
Allotropy is polymorphism in elements only.
Diamond and graphite are allotropes of carbon.

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