CNA Headmaster Practice Test

A glass thermometer is primarily filled with mercury due to its specific physical properties that make it ideal for measuring temperature. Mercury expands and contracts uniformly with temperature changes, allowing for accurate readings across a wide range. Its high density and low freezing point enable it to be used in various environments without solidifying, thus making it effective for precise measurements.

In addition to the reliability of its readings, mercury's reflective surface enhances visibility against the scale, improving readability. While there are other substances that can be used in thermometers—such as alcohol, which is often used in thermometers for very low temperatures—the properties of mercury generally make it the preferred choice for traditional glass thermometers used in a variety of settings.