Which of the following gases is widely used as an anaesthetic in medicine and dentistry?

Introduction / Context:
This general science question focuses on a medical application of a gas, specifically which gas is used as an anaesthetic. Anaesthetics are substances that cause loss of sensation or consciousness during surgery and dental procedures. Recognising nitrous oxide as laughing gas and an anaesthetic is a common exam point at school level.

Given Data / Assumptions:

• We are asked which gas is used as an anaesthetic.
• Options: nitrogen, nitrous oxide, methane, carbon dioxide and oxygen.
• Assume standard medical practice for minor procedures and historical context.

Concept / Approach:
Nitrous oxide, N2O, is commonly called laughing gas. It has mild anaesthetic and analgesic properties and has been used in dentistry and surgery, often in combination with oxygen. Other gases in the list have important roles but are not primary anaesthetics. Nitrogen is largely inert, oxygen is essential for respiration, methane is a fuel gas, and carbon dioxide is a respiratory gas and fire extinguisher component.

Step-by-Step Solution:
Step 1: Recall that nitrous oxide is known as laughing gas because it can cause a feeling of euphoria when inhaled. Step 2: In medical practice, nitrous oxide mixed with oxygen is used to provide pain relief and mild anaesthesia, especially in dentistry and during childbirth. Step 3: Nitrogen is mainly used as an inert gas and does not have anaesthetic properties in the same way at normal conditions. Step 4: Methane is a flammable gas used as a fuel and would be dangerous to use as an anaesthetic. Step 5: Carbon dioxide is involved in breathing regulation and is used in fire extinguishers, not as an anaesthetic. Step 6: Oxygen is vital for life and often supplied together with anaesthetic gases but is not itself the anaesthetic agent. Step 7: Therefore the gas used as an anaesthetic in the options is nitrous oxide, N2O.

Verification / Alternative check:
Many science texts mention nitrous oxide as laughing gas and note its use in dental clinics. Historical discussions of anaesthesia also highlight nitrous oxide as one of the earliest anaesthetic gases used in surgery. Remembering this association of N2O with both laughter and anaesthesia is a simple way to verify the answer.

Why Other Options Are Wrong:
Nitrogen does not have the required anaesthetic effect in typical medical concentrations. Methane and other fuel gases would be explosive and harmful if inhaled. Carbon dioxide in high concentration can cause suffocation, not anaesthesia. Oxygen is essential for respiration and is used to support patients during anaesthesia, but it is not the primary anaesthetic agent. Hence, these options do not match the role described in the question.

Common Pitfalls:
Students may mix up nitrous oxide with nitrogen or assume that oxygen, because it is administered in operating theatres, must be the anaesthetic. It is crucial to differentiate between the life supporting role of oxygen and the pain relieving or sedative role of anaesthetic gases. Another pitfall is confusing nitrous oxide with nitric oxide (NO), which is a different gas with different biological roles.

Final Answer:
The gas widely used as an anaesthetic in medicine and dentistry is nitrous oxide (N2O).