PHYSICS S.S ONE 3RD TERM WEEK 3

 

Topic:                    CRYSTALLINE AND AMORPHOUS SUBSTANCE   

Sub topic:

Reference materials:

(1) ESSENTIAL PHYSICS, TONALD PUBLISHERS, EWELUKWA (2017)

(2) NEW SCHOOL PHYSICS, AFRICAN FIRST PUBLISHERS, ANYAKOHA M.W (2011)

(3) INTERNET

Instructional materials:

Entry behavior: The students have been taught expansion

Behavioural objective: At the end of the lesson the students should be able to:

i.                     State the two classes of solid

ii.                   Define a unit cell and state the types of unit cell

iii.                  Explain amorphous substance

iv.                 Differentiate between crystalline and amorphous substance

CONTENT

CRYSTALLINE AND AMORPHOUS SUBSTANCE

Solid are usually divided into two classes. They are the crystalline and the non – crystalline or amorphous solid.

A Crystal is a piece of solid matter in which the atoms, molecules or ions are arranged in a highly regular repeating pattern or lattice.

Unit cells: simple basic units or building blocks that form crystal. There are 3 types of unit cells. Viz:

i.                     simple cubic lattice:- the particles are placed at the corners of the imaginary cubes stacked side by sid,up and down like building block.eg NaCl

ii.                     ii. face-centred cubic lattice:- the unit cell has identical particles at each of the corners plus another at the centre of each face. Eg ZnS; Cu; Ag; Al; Pb

iii.                  iii. Body centred cubic:- unit cell has identical particles at each corner plus one in the centre. Eg Cr, Fe, Pt salts. The crystal patterns are not actually seen by human eye but through X- ray diffraction.

Non-crystalline or Amorphous substances

They are substance without form. They are usually liquids than solid. Amorphous substances soften gradually when heated. It is for this reason that you can heat glass tubing in a flame to soften it and bend it. Eg plastics; glass.

Differences between crystalline and Amorphous substances

Crystalline  substances

Amorphous substances

It is with form

It is without form

Mostly solid

Mostly liquid

Melted when heated

Soften when heated

Soluble in water

Insoluble in water

Have definite melting point

Have no definite melting point

Cooling curve: Amorphous solids show smooth cooling curve while crystalline solids show two breaks in cooling curve. In the case of crystalline solids two break points ‘a’ and ‘b’ appear. 

These points indicate the beginning and the end of the process of crystallization. In this time interval temperature remains constant. This is due to the fact that during crystallization process energy is liberated which compensates for the loss of heat, thus the temperature remains constant.

PRESENTATION

Step I: The teacher explains crystalline substance

Step II: The teacher explains a unit cell with their types

Step III: The teacher explains amorphous substance

Step IV: The teacher leads the students to differentiate between crystalline and amorphous substance.

EVALUATION

The teacher evaluates the lessons by asking the following questions

i.                     State the two classes of solid

ii.                   Define a unit cell and state the types of unit cell

iii.                  Explain amorphous substance

iv.                 Differentiate between crystalline and amorphous substance

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