Q.Give the uses of freon 12, DDT, carbon tetrachloride and iodoform.

Concept: Organic Compound Applications — each compound has a specific use tied to its chemical properties (stability, toxicity, reactivity).

Reasoning:

1. Freon 12 ($\text{CCl}_2\text{F}_2$) is a non-toxic, non-flammable chlorofluorocarbon with a low boiling point, making it ideal as a refrigerant in air conditioners and refrigerators.
2. DDT ($\text{C}_{14}\text{H}_9\text{Cl}_5$) is a persistent insecticide that disrupts nerve function in insects; it was widely used in agriculture and for malaria control.
3. Carbon tetrachloride ($\text{CCl}_4$) is a dense, non-flammable solvent that dissolves oils and fats, used in dry cleaning and as a fire extinguisher (now restricted due to toxicity). …

The key to answering such questions in exams is to recall the specific, well-known uses of each compound — not just general properties. Let's go through them one by one, with the reasoning behind each use.

1. Freon 12 (Dichlorodifluoromethane, $\text{CCl}_2\text{F}_2$)

Freon 12 is a chlorofluorocarbon (CFC). Its uses stem from its unique physical properties: it is a gas at room temperature but easily liquefies under pressure, it is non-toxic, non-flammable, and chemically stable.

• Refrigerant in air conditioners and refrigerators: Because it can absorb heat when it evaporates and release it when compressed, it was widely used in cooling systems.
• Propellant in aerosol sprays: Its low boiling point and inertness made it ideal for dispensing products like deodorants and paints.
• Foam-blowing agent: Used in manufacturing insulating foams.

2. DDT (Dichlorodiphenyltrichloroethane, $\text{C}_{14}\text{H}_9\text{Cl}_5$)

DDT is an organochlorine compound. Its primary use is as an insecticide, but its story is complex.

• Insecticide in agriculture: It was massively used after World War II to control crop pests like mosquitoes, beetles, and moths.
• Disease vector control: It was sprayed to kill mosquitoes that transmit malaria and typhus, saving millions of lives.
• Household pest control: Used in powders and sprays for flies, cockroaches, and lice.

3. Carbon Tetrachloride ($\text{CCl}_4$)

Carbon tetrachloride is a dense, non-flammable liquid. Its uses are based on its ability to dissolve oils and fats, and its fire-suppressing property.

• Solvent for oils, fats, and greases: Used in dry cleaning and as a degreasing agent in industries.
• Fire extinguisher (under the name Pyrene): Its non-flammable nature and ability to form a heavy vapour that smothers flames made it a common fire extinguisher for electrical fires. …

Here is a clear, concept-first solution method for this type of question.

Method: Functional Group & Property Mapping

This method works by linking the chemical structure (specifically the functional group or key element) of a compound to its physical/chemical properties, and then mapping those properties to a real-world application.

Core Concept: A compound's use is almost always a direct consequence of one of its dominant properties (e.g., non-flammability, toxicity to insects, antiseptic action).

Steps to Solve

Step 1: Identify the Key Structural Feature

Look at the compound's formula or name and identify the atom or group that makes it special.

• Freon 12 ($CCl_2F_2$): Contains Chlorine (Cl) and Fluorine (F) atoms. It is a halogenated hydrocarbon (a CFC).
• DDT ($C_{14}H_9Cl_5$): Contains multiple Chlorine (Cl) atoms attached to two benzene rings. It is a polychlorinated hydrocarbon.
• Carbon tetrachloride ($CCl_4$): Contains four Chlorine (Cl) atoms. It is a non-polar, dense liquid.
• Iodoform ($CHI_3$): Contains Iodine (I) atoms. It is a trihalomethane.

Step 2: Link Structure to a Dominant Property

Connect the structural feature to its most important chemical or physical behavior.

• Freon 12: The C-F and C-Cl bonds are very strong and stable. This makes the compound non-toxic, non-flammable, and a good heat transfer agent (it evaporates easily, absorbing heat).
• DDT: The chlorine atoms make the molecule highly stable (persistent) and toxic to the nervous system of insects.
• Carbon tetrachloride: It is a non-polar solvent (dissolves oils, fats, grease) and is non-flammable. It is also denser than water.
• Iodoform: The iodine atom is easily released. Iodine is a powerful antiseptic (kills bacteria).

Step 3: Map Property to Application

Match the dominant property to a real-world need.

• Freon 12 (Property: Non-toxic, non-flammable, good heat transfer) …

Here are the common mistakes students make when answering questions on the applications of freon 12, DDT, carbon tetrachloride, and iodoform, along with clear strategies to avoid them.

1. Freon 12 (Dichlorodifluoromethane, $\text{CCl}_2\text{F}_2$)

Common Mistake:

• Writing that Freon 12 is used as a fuel or solvent.
• Confusing it with Freon 11 or other CFCs.

Why it happens:

Students often memorise "CFCs are refrigerants" but don't distinguish between specific numbers. They also confuse "refrigerant" with "fuel" because both involve gases.

How to avoid:

• Remember: Freon 12 is a chlorofluorocarbon (CFC). Its primary use is as a refrigerant in air conditioners and refrigerators.
• It is non-flammable and non-toxic, so it was ideal for cooling — never a fuel.
• Key exam point: It is also used as a propellant in aerosol sprays (though now banned in many countries due to ozone depletion).

Correct use: Refrigerant and aerosol propellant.

2. DDT (Dichlorodiphenyltrichloroethane)

Common Mistake:

• Writing that DDT is used as a medicine or insecticide for crops (without mentioning its ban).
• Forgetting to mention its environmental impact (bioaccumulation).

Why it happens:

Students recall "DDT kills insects" but don't read the question carefully — many exam questions expect you to note its historical use and current status.

How to avoid:

• Always mention: DDT is an insecticide used to control malaria-carrying mosquitoes and agricultural pests.
• Crucial addition: It is banned in many countries because it is non-biodegradable and causes bioaccumulation in food chains.
• Do not say it is used as a medicine — it is not a drug.

Correct use: Insecticide (now restricted/banned due to environmental persistence).

3. Carbon Tetrachloride ($\text{CCl}_4$)

Common Mistake:

• Writing that it is used as a fire extinguisher (without noting the danger).
• Saying it is used as a cleaning agent without mentioning toxicity.

Why it happens:

Students remember "carbon tetrachloride puts out fires" but forget that it produces phosgene gas ($\text{COCl}_2$) when heated, which is highly toxic.

How to avoid:

• Correct use: It was used as a solvent for oils, fats, and as a dry cleaning agent.
• Fire extinguisher use: Mention it was formerly used in fire extinguishers, but now avoided because it forms toxic phosgene gas on contact with hot surfaces.
• Key exam point: It is non-flammable but toxic — never say it is safe.

Correct use: Solvent, dry cleaning agent, formerly in fire extinguishers (now discontinued due to toxicity).

4. Iodoform ($\text{CHI}_3$)

Common Mistake:

• Writing that it is used as a general antiseptic (without specifying its limited use).
• Confusing it with iodine ($\text{I}_2$) or tincture of iodine.

Why it happens:

Students know "iodoform is an antiseptic" but don't realise it is not used on open wounds because it is irritating and stains.

How to avoid: …