Formaldehyde is the first member of the aldehyde series. It is present in green leaves of plants where its presence is supposed to be due to the reaction of \(CO_2\) with water in the presence of sunlight and chlorophyll. Commercially, formaldehyde is obtained by the controlled oxidation of methane in the presence of molybdenum oxide \((MoO)\) acting as a catalyst. Formaldehyde is a constituent of a number of polymers also called resins. These are non volatile solids or semi solids which are obtained directly from certain plants and can be synthesized in the laboratory.

What is formalin?

Formalin is \(40\%\) aqueous solution of formaldehyde.

The correct answer is option 3. Formalin is \(40\%\) aqueous solution of formaldehyde.

Formalin is a solution of formaldehyde gas dissolved in water, and it is commonly used for various purposes, including as a disinfectant, preservative, and in embalming fluids. Formaldehyde (H₂CO) is a simple aldehyde that is a gas at room temperature. To handle it safely and conveniently, it is typically dissolved in water to form formalin. Formalin is typically prepared as an aqueous solution of formaldehyde, with a concentration of about 37-40% by weight. This concentration ensures the solution is effective for its applications while being manageable for use.

Formalin Composition: The solution generally contains around 37% formaldehyde gas by weight, with the remainder being water. This concentration is often reported as "37% formaldehyde solution" or just "37% formalin."

Specifics

Formalin vs. Formaldehyde:

Formalin: An aqueous solution containing 37-40% formaldehyde gas. It also contains a small amount of methanol to prevent polymerization of formaldehyde.

Formaldehyde: The pure gas or its anhydrous form.

Uses of Formalin:

Preservation: Commonly used in embalming and preserving biological specimens.

Disinfection: Used as a disinfectant in laboratories and medical settings.

Manufacturing: Utilized in the production of various chemicals and resins.

Standard Concentration: The 37-40% concentration is standard in industry and laboratory settings. This concentration is chosen for its effectiveness in preservation and disinfection while being relatively stable.

Among the given options:

1. Formalin is a 20% aqueous solution of formaldehyde - This is incorrect; it is too diluted for standard applications.

2. Formalin is a 10% aqueous solution of formaldehyde - This is also incorrect; it is much less concentrated than formalin.

3. Formalin is a 40% aqueous solution of formaldehyde - This is the closest to the standard concentration of formalin, which is typically around 37-40%.

4. Formalin is a 35% aqueous solution of formaldehyde - While close, 35% is slightly lower than the standard range.

The best answer, based on standard industry practice and concentration, is 3. Formalin is a 40% aqueous solution of formaldehyde.