Is Solid State part of the EAPCET Chemistry syllabus?

Yes. Solid State is taught in Intermediate second year Chemistry, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.

How many Chemistry questions are there in EAPCET?

40 of the 160 questions in the Engineering paper are Chemistry, spread across 24 chapters.

What mistakes do students make in Solid State?

Leaving the edge length in pm or converting it to metres while wanting g cm⁻³ — use 1 pm = 10⁻¹⁰ cm before cubing. Mixing up the radius relations — √3a = 4r is for bcc (body diagonal) and √2a = 4r is for fcc (face diagonal). Saying Frenkel defects lower density — only Schottky defects do, since Frenkel defects merely shift ions within the crystal. Counting tetrahedral voids equal to the number of atoms — in close packing there are 2N tetrahedral and N octahedral voids.

EAPCET Chemistry · Intermediate second year

Solid State

Solid State covers crystal lattices and unit cells, packing efficiency, density calculations, voids and crystal defects.

Subject: Chemistry
Taught in: Intermediate second year
Chemistry in the paper: 40 of 160 questions

Key formulas and results

Atoms per unit cell: simple cubic 1, body-centred 2, face-centred 4
Packing efficiency: simple cubic 52.4%, bcc 68%, fcc and hcp 74%
Density $\rho = \frac{ZM}{a^3 N_A}$
fcc: $a = 2\sqrt{2}\,r$ ; bcc: $\sqrt{3}\,a = 4r$
In close packing there are N octahedral and 2N tetrahedral voids
Schottky defects lower density; Frenkel defects do not

What this chapter covers

Crystalline and amorphous solids
Unit cells
Packing efficiency
Density of a unit cell
Voids
Defects in solids

How it is usually asked

Density or edge-length calculations
Packing efficiency
Defects
Voids

Understanding Solid State

Crystalline solids have a regular, repeating arrangement of particles over long distances, so they have sharp melting points and are anisotropic (properties differ with direction). Amorphous solids such as glass have only short-range order, soften over a range of temperatures and are isotropic. The smallest repeating block of a crystal is the unit cell. There are seven crystal systems and fourteen Bravais lattices, but most calculations use only the three cubic cells: simple, body-centred and face-centred.

Each particle is shared between neighbouring cells: a corner counts 1/8, an edge centre 1/4, a face centre 1/2 and a body centre 1. That gives Z = 1 for simple cubic, 2 for bcc and 4 for fcc. Touching atoms fix the radius–edge relation: a = 2r (simple), √3a = 4r (bcc, along the body diagonal) and √2a = 4r (fcc, along the face diagonal). These lead to packing efficiencies of 52.4%, 68% and 74%. Density is then ρ = ZM/(a³Nₐ), with a in cm for g cm⁻³.

In close packing (ccp = fcc, ABCABC, and hcp, ABAB), N spheres leave N octahedral and 2N tetrahedral voids, which small ions can occupy. Real crystals have defects. A Schottky defect removes equal numbers of cations and anions and lowers density (NaCl, KCl, CsCl); a Frenkel defect moves a cation into an interstitial site and leaves density unchanged (ZnS, AgCl); AgBr shows both. Metal-excess defects with F-centres colour crystals, such as NaCl turning yellow when heated in sodium vapour.

Worked example

EAPCET-style question

A metal of molar mass 63.5 g mol⁻¹ crystallises in a face-centred cubic lattice with edge length 400 pm. Its density is approximately ( $N_A = 6.022 \times 10^{23}\ \text{mol}^{-1}$ ):

(1)1.65 g cm⁻³
(2)3.30 g cm⁻³
(3)6.59 g cm⁻³
(4)13.18 g cm⁻³

Solution

For fcc, $Z = 8 \times \tfrac{1}{8}\ (\text{corners}) + 6 \times \tfrac{1}{2}\ (\text{faces}) = 4$ .
Convert the edge: $a = 400\ \text{pm} = 4.00 \times 10^{-8}\ \text{cm}$ , so $a^3 = 6.40 \times 10^{-23}\ \text{cm}^3$ .
$a^3 N_A = 6.40 \times 10^{-23} \times 6.022 \times 10^{23} = 38.54\ \text{cm}^3\,\text{mol}^{-1}$ .
$\rho = \frac{ZM}{a^3 N_A} = \frac{4 \times 63.5}{38.54} = \frac{254}{38.54} \approx 6.59\ \text{g cm}^{-3}$ .
Using $Z = 2$ (bcc) or $Z = 1$ (simple cubic) by mistake gives the 3.30 and 1.65 distractors.

Answer: 6.59 g cm⁻³

Common mistakes to avoid

Leaving the edge length in pm or converting it to metres while wanting g cm⁻³ — use 1 pm = 10⁻¹⁰ cm before cubing.
Mixing up the radius relations — √3a = 4r is for bcc (body diagonal) and √2a = 4r is for fcc (face diagonal).
Saying Frenkel defects lower density — only Schottky defects do, since Frenkel defects merely shift ions within the crystal.
Counting tetrahedral voids equal to the number of atoms — in close packing there are 2N tetrahedral and N octahedral voids.

How to prepare it

Learn Z and the radius–edge relation for each cell type; most calculations use only those.

Once the formulas feel familiar, test them the way the exam does: practise Solid State questions chapter-wise, then attempt a full previous year paper to see whether it holds up under a 3-hour timer.

Related Chemistry chapters

See every chapter in the EAPCET Chemistry syllabus.

Practise Solid State chapter-wise

Chapter-wise quizzes show exactly which topics in this chapter you still get wrong — before a full paper does.

Frequently asked questions

Is Solid State part of the EAPCET Chemistry syllabus?: Yes. Solid State is taught in Intermediate second year Chemistry, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.
How should I prepare Solid State for EAPCET?: Learn Z and the radius–edge relation for each cell type; most calculations use only those.
How many Chemistry questions are there in EAPCET?: 40 of the 160 questions in the Engineering paper are Chemistry, spread across 24 chapters.
What mistakes do students make in Solid State?: Leaving the edge length in pm or converting it to metres while wanting g cm⁻³ — use 1 pm = 10⁻¹⁰ cm before cubing. Mixing up the radius relations — √3a = 4r is for bcc (body diagonal) and √2a = 4r is for fcc (face diagonal). Saying Frenkel defects lower density — only Schottky defects do, since Frenkel defects merely shift ions within the crystal. Counting tetrahedral voids equal to the number of atoms — in close packing there are 2N tetrahedral and N octahedral voids.