In organic chemistry, which of the following substances is commonly known by the old name carbolic acid?

Introduction / Context:
Many organic chemicals have common or historical names that differ from their modern systematic names. Carbolic acid is an older name that appears in historical literature and some examination questions. Knowing the modern name that corresponds to carbolic acid helps link past and present terminology. This question asks you to identify which listed substance is also known as carbolic acid in older chemical usage.

Given Data / Assumptions:

• The term carbolic acid is the historical name to be identified.
• The options include sodium hydroxide, sulfuric acid, ethanol, and phenol.
• We assume the context of basic organic chemistry and industrial history.
• We look for the compound associated with antiseptic use and a benzene ring with a hydroxyl group.

Concept / Approach:
Carbolic acid is the traditional name for phenol, which is a compound consisting of a benzene ring directly bonded to a hydroxyl group. Phenol was once widely used as an antiseptic and disinfectant, and in that context it was referred to as carbolic acid. The word carbolic reflects its origin from coal tar and its somewhat acidic nature. Sodium hydroxide is a strong base, sulfuric acid is a mineral acid, and ethanol is an alcohol used in beverages and as a solvent. None of these correspond to carbolic acid in common chemical nomenclature.

Step-by-Step Solution:
Step 1: Recall that phenol is a simple aromatic compound with formula C6H5OH, where a hydroxyl group is attached directly to a benzene ring. Step 2: Remember that in the nineteenth century phenol was derived from coal tar and used as an antiseptic, and the name carbolic acid was widely used. Step 3: Note that carbolic refers to carbon based origin and acid refers to its weakly acidic character. Step 4: Compare this with sodium hydroxide, which is a strong base, sulfuric acid, which is a strong mineral acid, and ethanol, which is a simple aliphatic alcohol. Step 5: Conclude that the only substance among the options that matches the historical name carbolic acid is phenol.

Verification / Alternative check:
History of medicine and chemistry sources describe how Joseph Lister used carbolic acid to reduce infection in surgical wounds, and modern accounts clearly state that carbolic acid was phenol. In industrial and laboratory contexts, phenol is still sometimes referred to by the older name, although modern nomenclature prefers the term phenol. No reputable source equates carbolic acid with sodium hydroxide, sulfuric acid, or ethanol, which reinforces the identification of phenol as the correct choice.

Why Other Options Are Wrong:
Sodium hydroxide, often called caustic soda, is a strong base, not an acid, and is used in soap making and cleaning, not as carbolic acid. Sulfuric acid is a strong mineral acid used in batteries and industrial processes; it has its own specific name and is never called carbolic acid. Ethanol is a two carbon alcohol used in alcoholic beverages and as a solvent, but carbolic acid refers to a benzene derivative, not to ethanol. Therefore, none of these options match the historical name carbolic acid.

Common Pitfalls:
Some learners may guess sulfuric acid simply because the word acid appears and they recall it as a strong acid. Others might confuse carbolic with carbonic and think of carbonic acid. To avoid such confusion, remember that carbolic acid is linked with coal tar, antiseptic history, and benzene chemistry. Connecting it with the structure C6H5OH and the name phenol will help you quickly answer this type of question in exams.

Final Answer:
The substance commonly known as carbolic acid is phenol.