CDS Chemistry · Industrial and Applied Chemistry
Glass, Cement and Polymers
Kinds of glass and why glass is called a supercooled liquid, why new concrete is kept wet, and thermoplastics against thermosetting plastics, with nylon's monomers.
Why this matters
Five CDS questions, two of them from the 2024 (II) paper. The nylon-6 question is the hardest: its monomer is caprolactam, while nylon-6,6 needs two.
Concept 1 of 3: Kinds of glass
Glass is a melt cooled so fast that its particles freeze in a disordered jumble instead of forming crystals. That is why it is called an amorphous solid or a supercooled liquid. Adding different oxides gives glass for different jobs.
Definition
The facts:
- Glass is amorphous: no crystals form on cooling, so it is called a supercooled liquid.
- Soda (soft) glass: windows, bottles. Hard (potash) glass: laboratory ware.
- Pyrex (borosilicate): resists heat; cookware and lab flasks.
- Flint glass (contains lead oxide): high refractive index; optical instruments, lenses and prisms.
- Coloured glass: metal oxides added (cobalt oxide blue, chromium oxide green).
- Water glass is sodium silicate.
| Glass | Special property | Used for |
|---|---|---|
| Soda (soft) glass | Cheap, melts easily | Windows, bottles |
| Pyrex (borosilicate) | Withstands heat | Cookware, lab flasks |
| Flint glass | High refractive index (lead oxide) | Optical instrumentsQ |
| Coloured glass | Metal oxides added | Decorative and signal glass |
| Water glass | Sodium silicate (soluble) | Adhesive, fireproofing |
Practice this concept3 quick reps
The same idea in a real exam question:
CDS · 2024 · CDS (II) 2024 — General Knowledge · Q18Moderate
Example 1 · Industrial and Applied Chemistry · Glass, Cement and Polymers
Which one among the following statements is not true for glass ?
Glass does not crystallise
'Crystallinity develops on cooling molten glass' is false. Glass is cooled too fast for crystals to form; that is why it is amorphous.
Pyrex is for heat, flint for optics
Pyrex and hard glass are chosen to survive heat. Optical instruments use flint glass, chosen for its high refractive index.
Concept 2 of 3: Cement and the curing of concrete
Cement does not dry to get hard; it reacts with water. If the water evaporates too soon, the reaction stops and the concrete stays weak. So new concrete is kept wet, often under wet sacks, for days.
Definition
The facts:
- Portland cement is made by heating limestone and clay; gypsum is added to slow its setting.
- Cement sets by hydration, a reaction with water.
- Curing: new concrete is covered with wet straw or gunny bags to prevent fast evaporation until hydration has gone far enough.
- Concrete = cement + sand + gravel + water; with steel bars inside, reinforced concrete (RCC).
| Material or step | What it is |
|---|---|
| Raw materials of cement | Limestone and clay |
| Gypsum in cement | Slows down setting |
| Curing concrete | Keeping it wet so water stays for hydrationQ |
| Concrete | Cement, sand, gravel and water |
Practice this concept2 quick reps
The same idea in a real exam question:
CDS · 2024 · CDS (II) 2024 — General Knowledge · Q15Moderate
Example 2 · Industrial and Applied Chemistry · Glass, Cement and Polymers
The surface of the concrete structure of a new construction is covered with straw/gunny bags by wetting. This is done to :
Cement hardens by reacting, not by drying
Curing keeps concrete wet because cement hardens by hydration. Wet sacks are not there to keep dust or fungus off.
Concept 3 of 3: Plastics and nylon
Plastics come in two kinds. Thermoplastics have separate long chains that slide apart when heated, so they soften. Thermosetting plastics have chains cross-linked into one rigid network, so heat cannot soften them again.
Definition
The facts:
- Thermoplastic: softens on heating, can be remoulded (polythene, PVC, polystyrene).
- Thermosetting plastic: cross-linked, stays rigid when heated (bakelite, melamine).
- Nylon-6: one monomer, caprolactam.
- Nylon-6,6: two monomers, hexamethylenediamine and adipic acid.
- Bakelite: phenol + formaldehyde. Urea-formaldehyde resin: urea + formaldehyde.
| Polymer | Monomer(s) | Kind |
|---|---|---|
| Nylon-6 | Caprolactam | Thermoplastic fibreQ CDS 2019 (I), HARD: nylon-6 is made from caprolactam alone. |
| Nylon-6,6 | Hexamethylenediamine + adipic acid | Thermoplastic fibre |
| Bakelite | Phenol + formaldehyde | Thermosetting |
| Polythene | Ethene | Thermoplastic |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
CDS · 2019 · CDS (I) 2019 — General Knowledge · Q78Easy
Example 3 · Industrial and Applied Chemistry · Glass, Cement and Polymers
When hot water is placed into an empty water bottle, the bottle keeps its shape and does not soften. What type of plastic is the water bottle made from?
Nylon-6 has one monomer, nylon-6,6 two
Nylon-6 comes from caprolactam alone. Hexamethylenediamine with adipic acid gives nylon-6,6.
Rigid when hot means thermosetting
A plastic that keeps its shape in hot water is thermosetting. PVC and polythene are thermoplastics and soften.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Kinds of glass5 rows
| Glass | Special property | Used for |
|---|---|---|
| Soda (soft) glass | Cheap, melts easily | Windows, bottles |
| Pyrex (borosilicate) | Withstands heat | Cookware, lab flasks |
| Flint glass | High refractive index (lead oxide) | Optical instrumentsQ |
| Coloured glass | Metal oxides added | Decorative and signal glass |
| Water glass | Sodium silicate (soluble) | Adhesive, fireproofing |
Cement and the curing of concrete4 rows
| Material or step | What it is |
|---|---|
| Raw materials of cement | Limestone and clay |
| Gypsum in cement | Slows down setting |
| Curing concrete | Keeping it wet so water stays for hydrationQ |
| Concrete | Cement, sand, gravel and water |
Plastics and nylon4 rows
| Polymer | Monomer(s) | Kind |
|---|---|---|
| Nylon-6 | Caprolactam | Thermoplastic fibreQ CDS 2019 (I), HARD: nylon-6 is made from caprolactam alone. |
| Nylon-6,6 | Hexamethylenediamine + adipic acid | Thermoplastic fibre |
| Bakelite | Phenol + formaldehyde | Thermosetting |
| Polythene | Ethene | Thermoplastic |
Watch out for (5)
- Glass does not crystallise→ Kinds of glass
- Pyrex is for heat, flint for optics→ Kinds of glass
- Cement hardens by reacting, not by drying→ Cement and the curing of concrete
- Nylon-6 has one monomer, nylon-6,6 two→ Plastics and nylon
- Rigid when hot means thermosetting→ Plastics and nylon
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.