Carbon compounds containing more than one halogen atom are usually referred to as polyhalogen compounds. Many of these compounds are useful in industry and agriculture. Some polyhalogen compounds are described in this section.
-- NCERT Class 12 Chemistry, Ch. 6, p. 187Halogen Uses Environment
8 MCQs9-step worked example
Source: NCERT Organic Compounds Containing HalogensOfficial key: NTA-verifiedLast updated: 7 Oct 2026
Try this first
Which polyhalogen compound does NCERT say should be stored in closed, dark coloured bottles that are completely filled?
- A.Carbon tetrachloride
- B.Dichloromethane
- C.Chloroform
- D.Iodoform
Tap to see the answer
Answer: C. Chloroform is slowly oxidised by air in the presence of light to poisonous carbonyl chloride (phosgene), so air and light are both kept out (NCERT Class 12 Chemistry, Chapter 6, page 187).
A is wrong: A is wrong because NCERT describes carbon tetrachloride through its uses (refrigerants, cleaning fluid, fire extinguisher) and its ozone-depleting effect, not through air-and-light storage precautions.
B is wrong: B is wrong because dichloromethane is described as a solvent, paint remover and propellant that harms the nervous system; no air-oxidation storage rule is given for it.
D is wrong: D is wrong because iodoform is described as a former antiseptic that acts by liberating free iodine; the dark-bottle rule belongs to chloroform.
Halogen Uses Environment, explained for NEET
A bottle of chloroform sits half full on a sunny shelf for six months. What has it turned into, and what should the label have said?
NCERT answers it in the polyhalogen-compounds section: chloroform is slowly oxidised by air in the presence of light to carbonyl chloride, an extremely poisonous gas also called phosgene. That is why it is stored in closed, dark coloured bottles completely filled, so that air is kept out (NCERT Class 12 Chemistry, Chapter 6, page 187).
Polyhalogen compounds are carbon compounds with more than one halogen atom. The uses and hazards NCERT attaches to each one are the whole topic:
- Dichloromethane: solvent, paint remover, aerosol propellant, drug manufacture; it harms the central nervous system (page 187).
- Chloroform: solvent for fats, alkaloids and iodine; today its major use is making the refrigerant freon R-22. It was once an anaesthetic and was replaced by ether (page 187).
- Iodoform: once an antiseptic, but only because it liberates free iodine; replaced because of its objectionable smell (page 187).
- Carbon tetrachloride: refrigerants, aerosol propellants, feedstock for chlorofluorocarbons; once a cleaning fluid and fire extinguisher. Released into air it rises and depletes the ozone layer (page 187).
- Freons: chlorofluorocarbons of methane and ethane; stable, unreactive, non-toxic, non-corrosive, easily liquefiable; used as aerosol propellants and in refrigeration and air conditioning. They reach the stratosphere unchanged and start radical chain reactions that upset the ozone balance (page 188).
- DDT: the first chlorinated organic insecticide, effective against malaria mosquitoes and typhus lice. Insects became resistant, it is toxic to fish, and because it is stable and fat soluble it is stored in fatty tissues and builds up. It was banned in the United States in 1973 (page 188).
NEET watch-out: do not swap the hazards. Ozone depletion belongs to carbon tetrachloride and freons; DDT's problems are resistance, toxicity to fish and build-up in fat; iodoform's "antiseptic" action is free iodine, not the compound itself.
How do you solve a Halogen Uses Environment question? A worked example
- 1
Given
A school lab buys chloroform and keeps it in a clear glass bottle, half full, on a window shelf. The bottle is opened often. After some months a test shows an extremely poisonous gas in the bottle.
- 2
Required
Name the gas, explain how it formed, and state how the chloroform should have been stored.
- 3
Concept
NCERT says chloroform is slowly oxidised by air in the presence of light to carbonyl chloride, also called phosgene (NCERT Class 12 Chemistry, Chapter 6, page 187). Two things drive the change: air and light.
- 4
Formula
No numerical formula applies. The relation is: chloroform + air + light gives carbonyl chloride (phosgene).
- 5
Substitution
Map each fact in the setup onto a driver. A half-full bottle leaves air in the bottle. A clear bottle on a window shelf lets light in. Frequent opening lets in fresh air.
- 6
Calculation
Remove each driver in turn. To keep air out, the bottle must be closed and completely filled. To keep light out, the bottle must be dark coloured. Removing only one driver leaves the oxidation possible. No numerical constants are used, exact or measured, so there is nothing to count for significant figures.
- 7
Final answer
The gas is carbonyl chloride (phosgene), formed by slow air oxidation of chloroform in light. Correct storage is a closed, dark coloured bottle completely filled so that air is kept out.
- 8
Common trap
Believing a tightly closed bottle is enough. A closed but half-full bottle still holds air, and a closed clear bottle still admits light; NCERT asks for dark colour and a completely filled bottle together.
- 9
Similar NEET-style question
A student stores chloroform in a dark coloured, closed bottle that is only half full. Is this storage correct? (Answer: No. Light is excluded, but the half-filled bottle still contains air, and air plus any light reaching it on opening can oxidise chloroform to poisonous carbonyl chloride; the bottle must be completely filled so that air is kept out.)
Can you answer these Halogen Uses Environment MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which polyhalogen compound does NCERT say should be stored in closed, dark coloured bottles that are completely filled?
Show answer and why every option is right or wrong
Answer: C. Chloroform is slowly oxidised by air in the presence of light to poisonous carbonyl chloride (phosgene), so air and light are both kept out (NCERT Class 12 Chemistry, Chapter 6, page 187).
Why A is wrong: A is wrong because NCERT describes carbon tetrachloride through its uses (refrigerants, cleaning fluid, fire extinguisher) and its ozone-depleting effect, not through air-and-light storage precautions.
Why B is wrong: B is wrong because dichloromethane is described as a solvent, paint remover and propellant that harms the nervous system; no air-oxidation storage rule is given for it.
Why D is wrong: D is wrong because iodoform is described as a former antiseptic that acts by liberating free iodine; the dark-bottle rule belongs to chloroform.
MCQ 2Easy RecallPractice
According to NCERT, what is the major use of chloroform today?
Show answer and why every option is right or wrong
Answer: A. NCERT states that the major use of chloroform today is in the production of the freon refrigerant R-22 (NCERT Class 12 Chemistry, Chapter 6, page 187).
Why B is wrong: B is wrong because chloroform was once used as a general anaesthetic but has been replaced by less toxic, safer anaesthetics such as ether (page 187).
Why C is wrong: C is wrong because the antiseptic use belongs to iodoform, and even there the action is due to free iodine (page 187).
Why D is wrong: D is wrong because the fire-extinguisher use is listed for carbon tetrachloride (page 187).
MCQ 3Direct ApplicationPractice
Chloroform kept for months in a half-filled clear bottle in sunlight is found to contain an extremely poisonous gas. Which gas is it?
Show answer and why every option is right or wrong
Answer: D. Air and light slowly oxidise chloroform to carbonyl chloride, also called phosgene, an extremely poisonous gas (NCERT Class 12 Chemistry, Chapter 6, page 187).
Why A is wrong: A is wrong because freon R-22 is a refrigerant manufactured from chloroform industrially, not the product of its slow oxidation in a bottle.
Why B is wrong: B is wrong because iodoform is a different polyhalogen compound, a former antiseptic that acts through free iodine.
Why C is wrong: C is wrong because methylene chloride is dichloromethane, a separate compound used as a solvent and paint remover; it is not formed from chloroform.
MCQ 4Easy RecallPractice
Why was iodoform once used as an antiseptic, according to NCERT?
Show answer and why every option is right or wrong
Answer: B. NCERT says the antiseptic properties are due to the liberation of free iodine and not due to iodoform itself (NCERT Class 12 Chemistry, Chapter 6, page 187).
Why A is wrong: A is wrong because NCERT explicitly says the effect is not due to iodoform itself.
Why C is wrong: C is wrong because iodoform contains iodine, not chlorine; the active agent named is free iodine.
Why D is wrong: D is wrong because NCERT says iodoform was replaced because of its objectionable smell.
MCQ 5Direct ApplicationPractice
Which statement about freons is correct?
Show answer and why every option is right or wrong
Answer: A. NCERT defines freons as the chlorofluorocarbon compounds of methane and ethane and lists them as extremely stable, unreactive, non-toxic, non-corrosive and easily liquefiable gases (NCERT Class 12 Chemistry, Chapter 6, page 188).
Why B is wrong: B is wrong because freons are described as unreactive and non-toxic and are used as aerosol propellants and in refrigeration; the insecticide is DDT.
Why C is wrong: C is wrong because NCERT says most freon makes its way into the atmosphere and diffuses unchanged into the stratosphere.
Why D is wrong: D is wrong because freons are chlorofluorocarbons, not brominated compounds, and Freon 12 is manufactured from tetrachloromethane.
MCQ 6Direct ApplicationPractice
A classmate says DDT cannot accumulate because animals clear it quickly. What does NCERT say about DDT in animals?
Show answer and why every option is right or wrong
Answer: D. NCERT says DDT is not metabolised very rapidly by animals; it is deposited and stored in fatty tissues and builds up if ingestion continues at a steady rate (NCERT Class 12 Chemistry, Chapter 6, page 188).
Why A is wrong: A is wrong because the problem NCERT describes is fat solubility and storage in fatty tissue, not quick excretion in water.
Why B is wrong: B is wrong because NCERT states DDT is not metabolised very rapidly, which is exactly why it builds up.
Why C is wrong: C is wrong because ozone depletion is attached to carbon tetrachloride and freons, not to DDT.
MCQ 7Concept TrapPractice
Which pair of compounds does NCERT link to depletion of the ozone layer?
Show answer and why every option is right or wrong
Answer: B. Carbon tetrachloride released into air rises to the atmosphere and depletes the ozone layer (page 187), and freons diffuse unchanged into the stratosphere and upset the natural ozone balance (NCERT Class 12 Chemistry, Chapter 6, page 188).
Why A is wrong: A is wrong because DDT's listed problems are insect resistance, toxicity to fish and build-up in fatty tissue, and iodoform is described only as a former antiseptic.
Why C is wrong: C is wrong because the harms listed for dichloromethane and chloroform are effects on the nervous system, liver and kidneys, not ozone depletion.
Why D is wrong: D is wrong because neither iodoform nor dichloromethane is linked to the ozone layer in this section.
MCQ 8Concept TrapPractice
Which pairing of compound and use is correct as described in NCERT?
Show answer and why every option is right or wrong
Answer: C. Dichloromethane is widely used as a solvent, as a paint remover, as an aerosol propellant and in drug manufacture (NCERT Class 12 Chemistry, Chapter 6, page 187).
Why A is wrong: A is wrong because DDT is the chlorinated organic insecticide (page 188); refrigerants are made from freons and related compounds.
Why B is wrong: B is wrong because the anaesthetic use belongs to chloroform, and it was replaced by ether (page 187).
Why D is wrong: D is wrong because the fire-extinguisher and cleaning-fluid uses belong to carbon tetrachloride; iodoform was an antiseptic (page 187).
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What to remember before solving Halogen Uses Environment questions
15 NCERT lines
Key Fact
Dichloromethane (methylene chloride): solvent, paint remover, aerosol propellant, drug manufacture
Dichloromethane is widely used as a solvent as a paint remover, as a propellant in aerosols, and as a process solvent in the manufacture of drugs. It is also used as a metal cleaning and finishing solvent.
-- NCERT Class 12 Chemistry, Ch. 6, p. 187Key Fact
Methylene chloride harms the central nervous system (impaired hearing and vision at low levels)
Methylene chloride harms the human central nervous system. Exposure to lower levels of methylene chloride in air can lead to slightly impaired hearing and vision.
-- NCERT Class 12 Chemistry, Ch. 6, p. 187Chemically, chloroform is employed as a solvent for fats, alkaloids, iodine and other substances. The major use of chloroform today is in the production of the freon refrigerant R-22. It was once used as a general anaesthetic in surgery but has been replaced by less toxic, safer anaesthetics, such as ether.
-- NCERT Class 12 Chemistry, Ch. 6, p. 187Chloroform is slowly oxidised by air in the presence of light to an extremely poisonous gas, carbonyl chloride, also known as phosgene. It is therefore stored in closed dark coloured bottles completely filled so that air is kept out.
-- NCERT Class 12 Chemistry, Ch. 6, p. 187Key Fact
Iodoform was an antiseptic only because it liberates free iodine; replaced because of its smell
It was used earlier as an antiseptic but the antiseptic properties are due to the liberation of free iodine and not due to iodoform itself. Due to its objectionable smell, it has been replaced by other formulations containing iodine.
-- NCERT Class 12 Chemistry, Ch. 6, p. 187It is produced in large quantities for use in the manufacture of refrigerants and propellants for aerosol cans. It is also used as feedstock in the synthesis of chlorofluorocarbons and other chemicals, pharmaceutical manufacturing, and general solvent use. Until the mid 1960s, it was also widely used as a cleaning fluid, both in industry, as a degreasing agent, and in the home, as a spot remover and as fire extinguisher.
-- NCERT Class 12 Chemistry, Ch. 6, p. 187When carbon tetrachloride is released into the air, it rises to the atmosphere and depletes the ozone layer.
-- NCERT Class 12 Chemistry, Ch. 6, p. 187ozone layer is believed to increase human exposure to ultraviolet rays, leading to increased skin cancer, eye diseases and disorders, and possible disruption of the immune system.
-- NCERT Class 12 Chemistry, Ch. 6, p. 188The chlorofluorocarbon compounds of methane and ethane are collectively known as freons. They are extremely stable, unreactive, non-toxic, non- corrosive and easily liquefiable gases. Freon 12 (CCl2F2) is one of the most common freons in industrial use. It is manufactured from tetrachloromethane by Swarts reaction. These are usually produced for aerosol propellants, refrigeration and air conditioning purposes.
-- NCERT Class 12 Chemistry, Ch. 6, p. 188Freon 12 (CCl2F2) is one of the most common freons in industrial use. It is manufactured from tetrachloromethane by Swarts reaction.
-- NCERT Class 12 Chemistry, Ch. 6, p. 188Most freon, even that used in refrigeration, eventually makes its way into the atmosphere where it diffuses unchanged into the stratosphere. In stratosphere, freon is able to initiate radical chain reactions that can upset the natural ozone balance.
-- NCERT Class 12 Chemistry, Ch. 6, p. 188The use of DDT increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus.
-- NCERT Class 12 Chemistry, Ch. 6, p. 188However, problems related to extensive use of DDT began to appear in the late 1940s. Many species of insects developed resistance to DDT, and it was also discovered to have a high toxicity towards fish. The chemical stability of DDT and its fat solubility compounded the problem. DDT is not metabolised very rapidly by animals; instead, it is deposited and stored in the fatty tissues. If ingestion continues at a steady rate, DDT builds up within the animal over time.
-- NCERT Class 12 Chemistry, Ch. 6, p. 188The use of DDT was banned in the United States in 1973, although it is still in use in some other parts of the world.
-- NCERT Class 12 Chemistry, Ch. 6, p. 188Halogen Uses Environment: NEET previous year questions (PYQs) with answers
7 questions in Organic Compounds Containing Halogens
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Sources
NCERT refs: Class 12 Chemistry Chapter 6, p.187
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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