Which of the following common substances is also known in chemistry as carbolic acid?

Introduction / Context:
This simple chemistry question checks knowledge of an older common name for a widely used organic compound. Carbolic acid is a historical name that appears in many texts, especially in discussions of antiseptics and the history of surgery. Knowing that carbolic acid refers to phenol helps learners recognise the same compound under different names and connect past and present terminology.

Given Data / Assumptions:
- The options are phenol, hydroxide, sulphuric acid, and ethanol.
- The task is to identify which of these is also called carbolic acid.
- We assume standard nomenclature as used in general chemistry and history of science.

Concept / Approach:
Carbolic acid is the common historical name for phenol, which has the formula C6H5OH. It is an aromatic hydroxy compound derived from benzene with one hydroxyl group. Phenol was famously used by Joseph Lister as an antiseptic in surgery. Hydroxide refers generally to the OH minus ion or to metal hydroxides, not to a specific organic compound. Sulphuric acid is a strong mineral acid with formula H2SO4 and bears no relation to the term carbolic acid. Ethanol is an alcohol used in beverages and as a solvent. Only phenol matches the name carbolic acid.

Step-by-Step Solution:
Step 1: Recall that phenol is an aromatic compound consisting of a benzene ring and one hydroxyl group, historically called carbolic acid due to its acidic nature and carbon rich aromatic ring.
Step 2: Remember that the term carbolic was used in the nineteenth century and early twentieth century when phenol was widely used as an antiseptic and disinfectant.
Step 3: Consider hydroxide. This term generally refers to the hydroxide ion OH minus or compounds like sodium hydroxide and does not describe a specific organic acid.
Step 4: Sulphuric acid is a strong mineral acid with formula H2SO4 and is never called carbolic acid.
Step 5: Ethanol is a simple alcohol, C2H5OH, and is also not associated with the term carbolic acid.
Step 6: Therefore, the substance also known as carbolic acid is phenol.

Verification / Alternative check:
Historical accounts of antiseptic surgery describe the use of carbolic acid, later identified as phenol, to clean wounds and sterilise instruments. Modern safety data sheets and chemical dictionaries list phenol with the synonym carbolic acid. No such synonym exists for hydroxide, sulphuric acid, or ethanol. These references confirm that phenol is uniquely associated with the name carbolic acid.

Why Other Options Are Wrong:
- Hydroxide: This term denotes the OH minus ion or its metal salts and is not the name for a specific organic compound known as carbolic acid.

Why Other Options Are Wrong (continued):
- Sulphuric acid: A strong inorganic acid with formula H2SO4, commonly called oil of vitriol historically, but never referred to as carbolic acid.
- Ethanol: A simple alcohol used in beverages and fuel, sometimes called ethyl alcohol, but not carbolic acid.

Common Pitfalls:
Some learners may confuse different historical names such as oil of vitriol, spirit of wine, and carbolic acid. It is helpful to link each old name with the modern IUPAC related compound: oil of vitriol with sulphuric acid, spirit of wine with ethanol, and carbolic acid with phenol. Making such associations helps prevent mixing up these old terms in exams.

Final Answer:
Phenol is the substance that is also known as carbolic acid.