Thermometer capillary tube composition

Steps:

• Identify the thermometer type: liquid-in-glass, with a bulb reservoir and narrow capillary tube.
• Recall function: liquid expands with heat, rising into the capillary without resistance.
• Locate region X: space above the liquid meniscus in the capillary.
• Conclude contents: must allow free expansion, indicating no gas or liquid presence.

Why A is correct:

• In liquid-in-glass thermometers, the capillary above the liquid is evacuated (vacuum) per design principle to eliminate air pressure that would hinder uniform expansion.

Why the others are wrong:

• B: Alcohol is the expanding liquid in the bulb and lower capillary, not region X.
• C: Steam is water vapor, absent in sealed thermometers to avoid pressure buildup.
• D: Water is not used as the thermometric liquid due to high freezing point and opacity.

Final answer: A