Analyzing Pairs of Physical Quantities and Units/Instruments

The question asks us to identify the pair of words that is different from the others. Let's examine each pair presented in the options:

• Volume : Litre: This pair relates a physical quantity, Volume, to one of its common units of measurement, Litre. The Litre (L) is a metric unit often used for measuring volumes of liquids or gases.
• Time : Second: This pair connects the physical quantity, Time, to its standard unit of measurement in the International System of Units (SI), the Second (s).
• Resistance : Ohm: This pair links the physical quantity, Electrical Resistance, to its SI unit of measurement, the Ohm ($\Omega$). Resistance is a measure of how much a material opposes the flow of electric current.
• Pressure : Barometer: This pair associates the physical quantity, Pressure, with the Barometer. A Barometer is an instrument used to measure atmospheric pressure. It is not a unit of pressure. Units of pressure include Pascal (Pa), Bar, Standard Atmosphere (atm), etc.

Comparing the Pairs

Let's compare the nature of the relationship between the words in each pair:

From the table, we can see a clear pattern in the first three pairs: they all consist of a physical quantity followed by its unit of measurement. The fourth pair, however, consists of a physical quantity (Pressure) followed by an instrument used to measure that quantity (Barometer).

Identifying the Odd Pair

Based on the analysis, the pair "Pressure : Barometer" is different from the other three pairs because it links a physical quantity to a measuring instrument, while the other pairs link a physical quantity to its unit of measurement.

Revision Table: Physical Quantities, Units, and Instruments

Additional Information on Measurement Concepts

Understanding the difference between a physical quantity, its unit, and the instrument used to measure it is fundamental in science. A physical quantity is a property of a material or system that can be quantified by measurement. A unit of measurement is a standard amount of a physical quantity, used as a basis for measurement. A measuring instrument is a device used to measure a physical quantity.

• Physical Quantity: Something that can be measured, like length, time, mass, temperature, pressure, volume, resistance, etc.
• Unit: A standard size or amount that tells you how much of a physical quantity you have. Examples: metre (m), second (s), kilogram (kg), Kelvin (K), Pascal (Pa), Litre (L), Ohm ($\Omega$).
• Instrument: A tool or device used to perform the measurement. Examples: ruler, stopwatch, balance, thermometer, barometer, measuring cylinder, ohmmeter.

In the given options, pairs 1, 2, and 3 follow the pattern Quantity : Unit, while pair 4 follows the pattern Quantity : Instrument. Therefore, Pressure : Barometer is the odd pair.