In chemistry and medicine, the substance popularly known as "laughing gas" is which of the following compounds?

Introduction / Context:
Some chemical compounds have popular names that reflect their effects or common uses. Laughing gas is one such name that students often encounter in basic chemistry and general science. It is associated with mild anaesthetic and euphoric effects when inhaled in controlled doses. This question asks the learner to match the informal name laughing gas with its correct chemical compound.

Given Data / Assumptions:

• The popular name given is laughing gas.

• Options include nitrous oxide, carbon monoxide, sulphur dioxide, hydrogen peroxide and ozone.

• The learner is expected to recognise which gas is used as a mild anaesthetic and can cause laughter.

• The question is conceptual and factual rather than numerical.

Concept / Approach:
Laughing gas is the common name for nitrous oxide, with the formula N₂O. It has been used historically in dentistry and minor medical procedures for its pain relieving and sedative effects, often leading to a sensation of light headedness and uncontrollable laughter in some people. The other listed compounds are important industrial or environmental gases but do not match this particular property or nickname.

Step-by-Step Solution:
Step 1: Recall that laughing gas is associated with dentistry and minor surgery as a mild anaesthetic. Step 2: Remember that the chemical name for laughing gas is nitrous oxide, N₂O. Step 3: Check the options provided and locate nitrous oxide among them. Step 4: Confirm that other options, such as carbon monoxide or sulphur dioxide, are toxic gases with very different effects. Step 5: Select nitrous oxide as the correct answer.

Verification / Alternative check:
Standard chemistry textbooks and basic medical references state that nitrous oxide is commonly called laughing gas. Historical notes on anaesthesia discuss its early use and describe the euphoric effect that led to this nickname. On the other hand, carbon monoxide is known as a deadly poisonous gas, and sulphur dioxide is associated with air pollution and respiratory irritation. These distinctions confirm that nitrous oxide is the only option that fits the popular name.

Why Other Options Are Wrong:
Carbon monoxide is a colourless, odourless gas that binds strongly to haemoglobin and reduces oxygen transport, causing poisoning rather than laughter. Sulphur dioxide is a pungent gas released from burning sulphur compounds and contributes to acid rain. Hydrogen peroxide is a liquid used as a disinfectant and bleach, not a gaseous anaesthetic. Ozone is a reactive form of oxygen found in the upper atmosphere and near some electrical equipment, again with no connection to the term laughing gas. Thus, none of these options matches the description given.

Common Pitfalls:
Sometimes learners confuse nitrous oxide with nitrogen dioxide or nitric oxide because the names sound similar. Others may select carbon monoxide because they recognise it as a gas discussed frequently in safety contexts. To avoid confusion, it is important to memorise a few famous common names: laughing gas for nitrous oxide (N₂O), tear gas for certain lacrimatory agents and so on. These one to one associations help answer such direct factual questions quickly.

Final Answer:
The compound popularly known as laughing gas is nitrous oxide.