In ordinary thermometers, which substance is most commonly used as the thermometric liquid to measure temperature accurately?

Introduction / Context:
This question is from basic physics and asks about the liquid commonly used inside thermometers. Understanding why a particular substance is chosen as the thermometric liquid helps you remember its properties and see how instruments are designed for accurate and reliable temperature measurement.

Given Data / Assumptions:

• The question refers to commonly used thermometric substances in ordinary thermometers.
• Options include mineral oil, alcohol, mercury, and petrol.
• Standard school level knowledge about thermometer construction is assumed.

Concept / Approach:
A thermometric substance is a material whose physical property changes predictably with temperature and can be used to measure that temperature. In many traditional clinical and laboratory thermometers, mercury is used as the thermometric liquid. Mercury remains liquid over a convenient temperature range, expands uniformly with temperature, has a shiny appearance that makes readings easy, and does not wet glass, so it forms a clear meniscus. Alcohol is also used in some thermometers, especially at very low temperatures, but mercury has historically been the most common choice in many standard thermometers. Mineral oil and petrol are not used as standard thermometric liquids in basic equipment because of issues such as flammability, colour, and non uniform expansion.

Step-by-Step Solution:

Verification / Alternative check:
Physics textbooks and diagrams show traditional thermometers labelled as mercury in glass thermometers. They describe the advantages of mercury: it has a wide liquid range, good thermal conductivity, uniform expansion, and a visible meniscus. Alcohol in glass thermometers are discussed as alternatives for low temperatures but are usually introduced after mercury. These explanations confirm that mercury is widely recognised as the common thermometric substance in many ordinary thermometers.

Why Other Options Are Wrong:

Mineral oil as the standard thermometric liquid is incorrect because mineral oil is not the typical choice for precise thermometers in basic physics education and would pose design challenges.

Alcohol used in every type of thermometer is wrong because, although alcohol is used in some thermometers, it is not the universal or most iconic thermometric liquid in general textbooks.

Petrol used as a precise thermometric liquid in laboratories is incorrect due to high flammability and unsuitable physical properties; petrol is not used as a standard thermometric liquid.

Common Pitfalls:
Some learners may think of alcohol thermometers used in very cold climates and generalise that alcohol must be the main substance in all thermometers. Others may be confused by coloured liquids and not realise that traditional clinical thermometers usually use mercury. Another pitfall is to be distracted by the term mineral oil and assume it is a neutral, safe choice without considering the actual properties needed in a thermometric substance. Staying close to textbook examples helps avoid these errors.

Final Answer:
The most commonly used thermometric liquid in many ordinary thermometers is Mercury used as a common thermometric liquid in many thermometers.