In a typical biology laboratory, which of the following chemicals is commonly used (usually as an aqueous solution called formalin) to preserve biological specimens such as animal organs and small organisms?

Introduction / Context:
This question is about preservation of biological specimens in laboratories and museums. Many students have seen jars containing preserved frogs, organs or other specimens in biology labs. The liquid in these jars usually contains a solution of a particular chemical that fixes and preserves tissues. Identifying this chemical connects practical laboratory experience with basic organic chemistry.

Given Data / Assumptions:

• The setting is a biology laboratory where specimens are preserved long term in jars.
• We are asked which chemical is used to preserve such specimens.
• Options include phenol, benzaldehyde, formaldehyde and acetic acid.
• We assume standard school or college laboratory practice where formalin is the common preservative.

Concept / Approach:
Formaldehyde is the simplest aldehyde, with formula HCHO. In laboratories, it is commonly used as an aqueous solution of about 37 to 40 percent, known as formalin. Formaldehyde reacts with amino groups in proteins and cross links them, effectively fixing tissues and preventing decay. This makes formalin a widely used chemical for preserving anatomical specimens. Phenol and acetic acid have antiseptic or preservative properties but are not the standard specimen preservative in jars, and benzaldehyde is mainly a flavour or fragrance compound.

Step-by-Step Solution:
1) Recall that preserved specimens in biology labs are often said to be stored in formalin.2) Formalin is an aqueous solution of formaldehyde, stabilised with small amounts of methanol.3) Formaldehyde reacts with proteins and nucleic acids in tissues, forming cross links that stabilise the structure and prevent decomposition by microorganisms.4) This property makes formaldehyde an effective tissue fixative and long term preservative.5) Phenol is a disinfectant and antiseptic, but it is not the usual choice for storing whole specimens in jars.6) Benzaldehyde is used mainly as an aromatic compound (almond like fragrance) and is not a standard specimen preservative.7) Acetic acid is used in some fixatives and buffering solutions but is not, by itself, the classic answer for preserving specimens in jars.

Verification / Alternative check:
Descriptions of histological and anatomical methods mention that organs and whole animals are fixed in formalin for preservation and later study. Formalin penetrates tissues and prevents autolysis and bacterial decay. School textbooks also refer to formalin jars in biology labs. Phenol based solutions may be used for disinfection, and acetic acid may be present in special fixative mixtures, but the widely known and tested preservative name is formaldehyde in the form of formalin.

Why Other Options Are Wrong:
Phenol: Used as an antiseptic and disinfectant but not as the main fluid in specimen jars for long term anatomical preservation.
Benzaldehyde: Mainly a fragrance and flavouring agent with an almond like smell, not commonly used for preserving specimens.
Acetic acid: Found in some specialised fixatives but is not the primary single chemical used in standard specimen jars.
Ethyl alcohol: Can preserve small samples and is used in some collections, but in typical school biology contexts the textbook answer is formalin (formaldehyde solution).

Common Pitfalls:
Students sometimes choose phenol or acetic acid because they are aware these chemicals can kill microbes or are used in fixative formulations. However, exam questions at this level usually expect the specific name formaldehyde (or formalin) when asking about preserved specimens in jars. A useful memory aid is the phrase “formalin jars” that many textbooks and teachers use. Remember that formalin is simply aqueous formaldehyde, and that connection will guide you to the correct answer.

Final Answer:
Biological specimens in many laboratories are preserved in an aqueous solution of formaldehyde (formalin).