The instrument used to measure the pressure of a gas is

Analysis

The question asks for the specific scientific instrument designed to measure the pressure of a gas, particularly in a laboratory or enclosed system setting.

Concept Explanation

To identify the correct instrument, we must understand the function of each device listed:

• Barometer: Used primarily to measure atmospheric pressure. It is commonly used in weather forecasting.
• Thermometer: Used to measure temperature (the degree of hotness or coldness of a body).
• Hydrometer: Used to measure the relative density (specific gravity) of liquids. It is often used to check the state of charge of a lead-acid battery.
• Manometer: A device consisting of a U-shaped tube containing a liquid (usually mercury or water). It is specifically designed to measure the pressure of a gas or vapor by comparing the difference in liquid levels between the two arms of the tube.

Step-by-Step Solution

1. Evaluate the options: We eliminate the thermometer (measures temperature) and the hydrometer (measures density).
2. Distinguish between Barometer and Manometer: While both measure pressure, a barometer is fixed to measure the ambient atmospheric pressure. A manometer is the versatile tool used to measure the pressure of a gas trapped in a container or flowing through a pipe.
3. Conclusion: Since the question asks for the instrument used to measure the pressure of a gas (often in a closed system), the manometer is the correct scientific instrument.

Exam Tip

Think of the "M" in Manometer as standing for "Measurement of gas pressure." Don't confuse it with the Barometer, which is strictly for the "B"ig atmosphere!