CDS Chemistry · Industrial and Applied Chemistry

Industrial Gases and Paper-Making

What liquid nitrogen, argon, helium, oxygen and hydrogen sulphide are used for or known by, and the two chemicals behind paper: caustic soda to pulp, chlorine to bleach.

All Industrial and Applied Chemistry notes

Why this matters

Six CDS questions. Liquid nitrogen for storing blood and tissue came twice (2020 and 2022). The 2024 (II) paper-bleaching question is defective as printed: none of its options is a bleaching agent.

Concept 1 of 2: Uses of industrial gases

Each gas is chosen for one property. Liquid nitrogen is very cold, cheap and unreactive, so it preserves living tissue. Argon is unreactive and heavy, so it protects a bulb filament. Hydrogen sulphide is known by its smell.

Definition

The uses:

Liquid nitrogen (boils at about −196 °C): cryopreservation of blood, semen, embryos and organs. Solid CO₂ only reaches about −78 °C.
Nitrogen gas replaces air in food packets to stop the food oxidising.
Argon (often with nitrogen) fills electric bulbs; helium is not used in bulbs. Helium fills balloons and is used in divers' breathing mixtures.
Liquid oxygen is the oxidiser that burns rocket fuel.
Hydrogen has three isotopes (protium, deuterium, tritium).
Hydrogen sulphide smells of rotten eggs; ammonia is pungent; sulphur dioxide is suffocating.

Gas	Use or property
Liquid nitrogen	Storing blood, semen and organs at about −196 °CQ Asked in CDS 2020 (II) and 2022 (II).
Nitrogen gas	Fills food packets to prevent oxidation
Argon	Fills electric bulbsQ CDS 2024 (II): 'helium is commonly used in electric bulbs' was the incorrect statement.
Liquid oxygen	Burns fuel in rocket engines
Hydrogen sulphide	Smell of rotten eggsQ

Practice this concept4 quick reps

The same idea in a real exam question:

CDS · 2022 · CDS (II) 2022 — General Knowledge · Q62Easy

Example 1 · Industrial and Applied Chemistry · Industrial Gases and Manufacturing

Storage of biological tissues such as blood, organs, semen requires low temperature which is achieved upon the usage of

Bulbs use argon, not helium

Electric bulbs are filled with argon (with some nitrogen) because it is inert and heavy enough to slow the filament wearing away. Helium is for balloons and diving gas.

Dry ice is not cold enough for tissue

Solid CO₂ reaches only about −78 °C. Biological tissue is stored in liquid nitrogen at about −196 °C.

Drill 3 more on uses of industrial gases

Concept 2 of 2: Chemicals in paper-making

Paper is cellulose fibre. To get it, wood or bamboo is cooked with an alkali that dissolves the gums and lignin holding the fibres together. The brown pulp is then bleached white with chlorine compounds.

Definition

The two steps:

Pulping (degumming): raw material is digested with caustic soda (NaOH), which dissolves lignin and gums and frees the cellulose.
Bleaching: chlorine and its compounds (chlorine dioxide, bleaching powder) oxidise the coloured matter; hydrogen peroxide is also used.
A chloride ion, chlorobenzene or hydrochloric acid does not bleach.

Step	Chemical
Degumming / pulping	Caustic soda (NaOH)Q
Bleaching	Chlorine, chlorine dioxide, bleaching powderQ CDS 2024 (II) printed 'chloride', which cannot bleach; the question is defective as printed.

Practice this concept2 quick reps

The same idea in a real exam question:

CDS · 2016 · CDS (II) 2016 — General Knowledge · Q8Moderate

Example 2 · Industrial and Applied Chemistry · Industrial Gases and Manufacturing

In paper manufacturing, degumming of the raw material is done using

Pulping uses an alkali, not an acid

Paper pulp is digested with caustic soda. Sulphuric and nitric acids are distractors.

Chlorine bleaches; chloride does not

Bleaching needs an oxidiser: chlorine or chlorine dioxide. The chloride ion (as in common salt) is not an oxidiser and does not bleach.

Drill 1 more on chemicals in paper-making

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 (2)

Uses of industrial gases5 rows

Gas	Use or property
Liquid nitrogen	Storing blood, semen and organs at about −196 °CQ Asked in CDS 2020 (II) and 2022 (II).
Nitrogen gas	Fills food packets to prevent oxidation
Argon	Fills electric bulbsQ CDS 2024 (II): 'helium is commonly used in electric bulbs' was the incorrect statement.
Liquid oxygen	Burns fuel in rocket engines
Hydrogen sulphide	Smell of rotten eggsQ

Chemicals in paper-making2 rows

Step	Chemical
Degumming / pulping	Caustic soda (NaOH)Q
Bleaching	Chlorine, chlorine dioxide, bleaching powderQ CDS 2024 (II) printed 'chloride', which cannot bleach; the question is defective as printed.

Watch out for (4)

Bulbs use argon, not helium
→ Uses of industrial gases
Dry ice is not cold enough for tissue
→ Uses of industrial gases
Pulping uses an alkali, not an acid
→ Chemicals in paper-making
Chlorine bleaches; chloride does not
→ Chemicals in paper-making

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.

Sit a CDS past paper Drill all 6 questions